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Ibis (2004), 146, 197– 226
Review
Blackwell Publishing, Ltd.
Population limitation in migrants
IAN NEWTON*
Centre for Ecology and Hydrology, Monks Wood, Abbots Ripton, Huntingdon, Cambs. PE28 2LS, UK
Unlike resident bird species, the population sizes of migratory species can be influenced by
conditions in more than one part of the world. Changes in the numbers of migrant birds, either
long-term or year-to-year, may be caused by changes in conditions in the breeding or wintering
areas or both. The strongest driver of numerical change is provided in whichever area the
per capita effects of adverse factors on survival or fecundity are greatest. Examples are given of
some species whose numbers have changed in association with conditions in breeding areas,
and of others whose numbers have changed in association with conditions in wintering areas.
In a few such species, the effects of potential limiting factors have been confirmed locally
by experiment. In theory, population sizes might also be limited by severe competition at
restricted stopover sites, where bird densities are often high and food supplies heavily depleted,
but (with one striking exception) the evidence is as yet no more than suggestive. In some species,
habitats occupied in wintering and migration areas, and their associated food supplies, can
influence the body condition, migration dates and subsequent breeding success of migrants.
Body reserves accumulated in spring by large waterfowl serve for migration and for subsequent
breeding, and females with the largest reserves are most likely to produce young. Hence, the
conditions experienced by individuals in winter in one region can affect their subsequent breeding success in another region. Such effects are apparent at the level of the individual and at
the level of the population. Similarly, the numbers of young produced in one region could,
through density-dependent processes, affect subsequent overall mortality in another region.
Events in breeding, migration and wintering areas are thus interlinked in their effects on bird
numbers. Although in the last 30–40 years the numbers of some tropical wintering birds have
declined in western Europe and others in eastern North America, the causes seem to differ. In
Europe, declines have mainly involved species that winter in the arid savannas of tropical Africa,
which have suffered from the effects of drought and increasing desertification. In several
species, annual fluctuations in numbers and adult survival rates were correlated with annual
fluctuations in rainfall, and by implication in winter food supplies. In North America, by contrast,
numerical declines have affected many species that breed and winter in forest, especially
those eastern species favouring the forest interior. Declines have been attributed ultimately
to human-induced changes in the breeding range, particularly forest fragmentation, which
have led to increases in the densities of nest predators and parasitic cowbirds. These in turn are
thought to have caused declines in the breeding success of some neotropical migrants, which
is now too low to offset the usual adult mortality, but as yet convincing evidence is available
for only a minority of species. The breeding rates and population changes of some migratory
species have been influenced by natural changes in the availability of defoliating caterpillars.
In other species, tropical deforestation is likely to have played the major role in population
decline, and if recent rates of tropical deforestation continue, it is likely to affect an increasing
range of migratory species in the future. Not all such species are likely to be affected adversely
by deforestation, however, and some may benefit from the resulting habitat changes.
*Email: [email protected]
© 2004 British Ornithologists’ Union
198
I. Newton
In the limitation of their populations, migratory birds
differ from residents in at least one important respect.
Their population sizes may be influenced by conditions
in more than one part of the world: in areas that
are used for breeding, as well as in areas that are visited
at other times of year. For many migrant species, the
breeding and wintering ranges are widely separated
geographically, and might differ greatly in the numbers
of birds they can support. Hence, factors operating
in the migration or wintering range might limit the
numbers that can occur in the breeding range, or vice
versa. This is most clearly apparent where changes
in the numbers of a species over a period of years
are associated with changes in conditions in one area,
but not in the other. In this review, I address some
of the issues involved in the population limitation of
migrants, using as examples species whose long-term
or year-to-year numerical changes can be clearly
linked to events in breeding or non-breeding areas.
Special attention is given to landbird migrants that
breed in Eurasia and winter in Africa (Moreau 1972,
Newton 1994), or that breed in North America and
winter in Central and South America (Terborgh
1989, Rappole 1995, Sherry & Holmes 1995). Most
such species spend less of each year in their breeding
areas than in their migration and wintering areas.
In understanding population changes, it is helpful
to separate long-term trends from year-to-year
fluctuations about the trend, because different factors
might be involved (Newton 1998). For example, some
species may undergo long-term change in numbers,
resulting from progressive habitat change, but may also
continue to fluctuate from year to year in response to
different factors, such as annual variations in rainfall
and associated food supplies. Moreover, within species,
population trends and limiting factors may vary across
the range, and also at different time periods, so that local
research findings cannot necessarily be extrapolated
over wider areas or longer periods (see Peterjohn
et al. 1995 for analysis of recent regional trends in
North American species). Because of the problems
of studying birds that routinely occupy two or more
different areas each year, it has seldom been possible
to examine the same individuals throughout the year,
and most of the evidence on population limitation in
migrants is indirect.
Spacing and movement patterns
The spacing behaviour of birds in the non-breeding
season can vary from solitary and territorial to
gregarious and flocking, depending on the species,
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
the habitat, the distribution of food supplies, predation
pressures and other factors, as well as on the social
status of the individuals concerned (Newton 1998).
In addition, whereas some migrant birds remain
in the same place for the whole non-breeding period
from arrival to departure, others occupy two or more
sites in succession, at different points on the migration
route, or they may remain continually on the move,
again depending largely on the spatial and temporal
occurrence of food supplies. In the northern continents,
such itinerancy is shown by boreal finches and
others, whereas in the drier parts of both Africa and
South America, some migrant species follow rainbelts for the food sources they promote. Hence, if
migrants are limited by conditions in wintering areas,
for some species there may be more than one such
area involved, and these areas may differ from year
to year. Some birds may spend different amounts
of time in the same localities each year, and reach
localities in some years that remain vacant in others.
The bulk of the population may thus be concentrated
in different areas in different years, depending on the
distribution of food. African examples include the
Lesser Spotted Eagle Aquila pomarina and White
Stork Ciconia ciconia ( both studied by radiotracking,
Meyburg et al. 1995, Berthold et al. 2002), and
South American examples on a smaller spatial scale
include the Eastern Kingbird Tyrannus tyrannus and
Swainson’s Hawk Buteo swainsoni (Brown & Amadon
1968, Morton 1971). They illustrate a major difference
between the breeding and non-breeding seasons: in
the former, breeding birds are tied for up to several
months to the localities in which they nest, but in the
non-breeding season, individuals of at least some
species are free to move around, concentrating
wherever food is plentiful. Those migrants that
‘winter’ in the southern hemisphere, while the local
birds are breeding, thus have an advantage that the
local birds then lack, and in theory this could reduce
competition between the two groups.
SOME GENERAL PRINCIPLES
If we accept that, for various reasons, the total
carrying capacities of the breeding and non-breeding
habitats of particular populations need not necessarily
correspond, then two scenarios are possible when birds
return to their nesting areas each year and compete
for territories or nest-sites (Fig. 1):
1 Too few birds are left at the end of the non-breeding
season to occupy all nesting habitat fully, so that
practically all individuals of appropriate age and
Population limitation in migrants
Figure 1. Model showing seasonal changes in total bird
numbers in relation to the carrying capacity of the breeding area
(thick line). In the lower curve (A) numbers left at the end of the
non-breeding season are fewer than the nesting habitat could
support, and in the upper curve (B) numbers are greater than the
nesting habitat could support, leading to a surplus of nonterritorial, non-breeders. In (A), breeding numbers are limited by
conditions in the area occupied in the non-breeding season, and
in (B) by conditions in the area occupied in the breeding season.
From Newton (1998).
condition can breed. In this case, breeding numbers
would be limited by whatever factors operate in the
non-breeding season, and alleviation of the factors
that influence the extent or carrying capacity of nonbreeding habitats would be needed before breeding
numbers could rise. For convenience, the breeding
numbers in such populations could be described as
‘winter-limited’.
2 More birds are left at the end of the non-breeding
season than the available nesting habitat can support,
producing a surplus of non-territorial, non-breeders.
In this case, alleviation of the factors that influence
the extent or carrying capacity of the nesting habitat
would be needed before breeding numbers could
rise. For convenience, the breeding numbers of such
populations could be described as ‘summer-limited’.
The same two types of scenario could hold at the
end of the breeding season, as birds return to their
non-breeding quarters. At that time, the total numbers of adults and young might be insufficient to use
the resources of non-breeding areas to the full, leading
to good survival through the non-breeding season;
alternatively, the total numbers may exceed the
carrying capacity of non-breeding areas, leading to
intense competition and poor survival.
Because bird reproduction is by definition confined
to the breeding range, numbers there might be ‘summer
limited’ in a different way, namely if breeding success
were so poor that subsequent breeding numbers
could not reach the level necessary to fill either the
available breeding habitat or the available non-breeding
199
habitat. This situation could arise even with relatively
good year-round survival. In other words, whereas
failure to occupy all wintering habitat must be due
to events in breeding or migration areas, failure to
occupy all breeding habitat could be due to events
in either breeding, migration or wintering areas.
Understanding the limiting mechanisms in any one
population may not be easy, especially if the situation
changes from year to year. There is no reason why a
species might not be summer-limited in one year or
area and winter-limited in another year or area (for
examples see Newton 1998).
Effects of habitat loss on migrants
It is a matter of observation that, when areas of
habitat are lost or added through human action,
bird numbers often change accordingly. To take some
recent examples from Britain, Siskins Carduelis spinus
have increased in numbers and expanded in range as
new conifer plantations have matured and provided
additional breeding habitat (Gibbons et al. 1993).
By contrast, Redshanks Tringa totanus have declined
and contracted as summer nesting habitat has been
destroyed by land drainage (Norris et al. 2004), and
Twite Carduelis flavirostris have declined as areas of
winter salt marsh have been lost to reclamation
projects (Atkinson et al. 2004). Many other examples
have been described in the literature from Europe
and North America, including the massive declines
in numbers of waterbirds that followed the drainage
of marshes. Although many such examples may
represent causal relationships, some may be due to
coincidence between population decline (caused
by some other factor) and habitat loss. Clearly, not all
bird population changes occur in response to habitat
changes, and some species seem to have far more
potential habitat than they currently occupy. In the
case of migrants, this again raises the possibility that
numbers may be limited at one end of the migratory
terminal at a level lower than habitat at the other
end would support.
In recent years, much thought has been given to
predicting the effect of habitat loss (equivalent to food
loss) in both resident and migratory bird species. For
resident birds, in which breeding and wintering
areas are the same, population declines should
be roughly proportional to habitat loss, if habitat
were of uniform quality and fully occupied throughout. In other words, if half the habitat (or food
supplies) were lost, we would in general expect the
population to be roughly halved ( but see later). The
© 2004 British Ornithologists’ Union, Ibis, 146, 197– 226
200
I. Newton
Figure 2. Model of the relationship between per capita mortality
and reproduction (continuous lines), and equilibrium population
size (E ). Per capita winter mortality increases, and per capita
reproduction decreases, with increasing population size. The
equilibrium population size is where the two lines intersect.
The model shows how loss of habitat (or food supply) results
in population decline. If wintering habitat (or food supply) is
reduced by 50%, this results in a displacement of the relationship
between mortality and total population size in direct proportion to
the degree of habitat loss (dashed line). The equilibrium population
size is reduced accordingly (E1). A 50% loss of breeding habitat
(or food supply) similarly results in a displacement of the
relationship between net breeding output and total population
size (dashed line) and a reduced equilibrium population (E2). In
this example, 50% loss of breeding habitat results in a smaller
reduction in equilibrium population size than does 50% loss of
wintering habitat, because density-dependence is stronger in
winter than in summer. Modified from Sutherland (1996).
situation is more complicated for migrant birds because
they occupy separate areas in winter and summer,
and habitat loss may occur in one area or both
(Fig. 2). The actual population change following loss
of breeding or wintering habitat would be expected
to depend on where the tightest bottleneck occurred;
that is, the relative strengths of density-dependent
constraints in the two areas (Sherry & Holmes 1995,
Sutherland 1996). Such constraints include those
various pressures, such as competition for space and
food, that can affect an increasing proportion of individuals as their density rises, resulting in an increased
per capita mortality or decreased per capita reproduction. In the wintering area, the strength of densitydependence is measured by the per capita rate of
increase in mortality that occurs as a result of rising
population size (or decreasing area) (slope d ). In the
breeding area, the strength of density-dependence is
measured by the per capita rate of decrease in reproduction with rising population size (or decreasing
area) (slope b). If slope b > slope d, loss of breeding
habitat would have most impact on overall population
size, and if d > b, loss of wintering habitat would
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
have most impact. If the two density-dependent
relationships were known, the effect of loss of habitat
(or food supply) on equilibrium population size
could in theory be calculated as b/(b + d ) for the
breeding area, or as d/(b + d ) for the wintering area.
If, in an extreme case, all the density-dependence
occurred in winter habitat (say), with no densitydependence in breeding habitat (which at prevailing
population levels was present in excess, as in Fig. 1a),
then loss of winter habitat would cause a matching
reduction in population size. In this situation, loss
of breeding habitat would have no effect up to the
point at which density-dependent decline in breeding success set in. Although as yet there can be few
species for which enough information is available
to test the model in Figure 2, or to judge the form
of density-dependent relationships over a wide range
of densities at both seasons, attempts have been made
for the Oystercatcher Haematopus ostralegus in Britain
(Goss-Custard et al. 1995, Sutherland 1996).
The above considerations lead to a number of
conclusions regarding the effects of habitat (or food)
loss on the equilibrium population sizes of migrants:
(1) knowledge of the density-dependent response
within just the wintering or breeding area cannot
be used to predict precisely the effects of habitat or
food loss in either, because it is the ratio of densitydependence in the two areas that is important;
(2) unless there is no density-dependence acting during
one of the seasons, a loss of habitat or food supply in
either summer or winter areas could result in population decline; and (3) the consequence of habitat or
food loss is greatest for the season in which densitydependence is strongest (winter in Fig. 2). In practice,
all migrants are likely to be affected more by changes
in one area than the other, although whether breeding
or wintering areas are most important in this respect
may change through time. They could also change
from year to year in species subject to large annual
fluctuations in habitat, food supplies or other conditions.
The above generalizations on the role of summer
and winter conditions hold most clearly for populations limited by resources – by the available habitats
and food supplies. They could also hold for populations limited below the levels that resources would
permit by factors such as parasitism, predation and
human persecution. However, in some circumstances,
the latter factors can also kill an unsustainably large
number of individuals each year, sending populations
into decline, and leaving a surplus of unused habitat
and food. For example, if for some reason the predation pressure on eggs and chicks in the breeding areas
Population limitation in migrants
increased so much that loss of annual production
could not be offset by improved annual survival,
the population would decline below the levels that
both breeding and wintering habitats would support.
Similarly, if shooting pressure on full-grown birds
increased in winter quarters, so that the loss could
not be offset by improved reproduction or natural
survival, the population could again decline below the
carrying capacities of both breeding and wintering
habitats. In both these examples, decline would
continue while that situation held (eventually to
extinction), the trend being driven primarily in
whichever area the per capita effects of adverse factors
on reproduction or survival were greatest.
The buffer effect and density dependence
All habitats seem to vary from place to place in
quality and attractiveness to the birds they support. One
known mechanism through which density dependence
in mortality or reproduction could occur during a
period of population growth involves the ‘buffer
effect’. This occurs when birds occupy the best habitat
areas first, and as they fill these areas to capacity, they
spread increasingly to poorer areas as their numbers
continue to rise. As survival or reproduction is lower
in the poorer areas, the mean per capita performance
in the population as a whole declines as overall numbers
grow, in a density-dependent manner. Sequential
habitat fill of this type is seen: (1) as birds arrive in
their breeding areas in spring, or their wintering
areas in autumn, when they occupy the best places
first, so that later arrivals are relegated to poorer places
(e.g. Brooke 1979, Lundberg et al. 1981, GossCustard et al. 1984); (2) in the annual fluctuations of
populations, where numbers remain more stable from
year to year in the preferred habitats (or territories)
than in the secondary habitats (Kluijver & Tinbergen
1953, Zimmerman 1982, Rodenhouse et al. 2003);
and (3) in the progressive occupation of habitat
areas (or territories) of different quality, as a population
grows over a period of several years (e.g. Mearns &
Newton 1988, Ferrer & Donázar 1996, Löhmus 2001).
All these processes, which result from habitat variation, can help to regulate bird populations (for further
examples of each type see Newton 1998). Food and
other resources are involved in the regulation because
they influence the quality and carrying capacity of
habitats.
The Icelandic population of the Black-tailed
Godwit Limosa l. islandica wintering in Britain has risen
four-fold since the 1970s, but rates of increase within
201
individual estuaries have varied from zero to sixfold (Gill et al. 2001). In accordance with the buffer
effect, rates of increase were greatest on estuaries
with low initial numbers, and Godwits on these sites
were found to have lower prey intake rates and lower
survival rates than Godwits on longer-occupied sites
with stable populations. Godwits from the poorer,
more recently occupied, wintering sites also arrived
later in spring on their Icelandic breeding areas.
Their breeding has not been studied, but by analogy
with other species, later arrival usually means relegation to poorer habitat and poorer breeding success.
In this species, therefore, population growth could
have resulted in a progressively larger proportion
of the population wintering in poorer habitat, with
measured consequences on feeding rates and migration dates, and possible consequences on breeding
success. The buffer effect, acting on a large spatial
scale, could therefore have been a major densitydependent process acting to constrain population
growth in this migratory species.
A similar spread to poor sites during a period of
population growth was earlier noted in Grey Plovers
Pluvialis squatarola wintering in different parts of
Britain (Moser 1988), in Brent Geese Branta bernicla
wintering in the Netherlands (Ebbinge 1992) and
in Great Cormorants Phalacrocorax carbo wintering
in Switzerland (Suter 1995). In the last of these, the
process was stepwise, and each category of habitat
experienced a rapid build-up in numbers, followed
by stabilization, before the next type of habitat was
occupied. In each of these studies, the first-filled
habitat was assumed to be better, in which case
survival would have been poorer in the secondary
habitats, leading to progressive decline in mean per
capita survival as the population grew, although this
was not confirmed (but for effects on reproduction
of Brent Geese see Ebbinge 1992). Hence, although
the presence of secondary habitat would permit a
population to attain a higher level than it could in
the primary habitat alone, the poorer performance
of individuals in secondary habitat would put ever
increasing constraints on further population growth.
EXAMPLES OF SPECIES AFFECTED
BY EVENTS IN BREEDING OR
WINTERING AREAS
In some species, year-to-year changes in overall
population levels have been clearly driven largely by
conditions in breeding areas. On the North American
prairies, rainfall varies greatly from year to year,
© 2004 British Ornithologists’ Union, Ibis, 146, 197– 226
202
I. Newton
Figure 3. Relationship between the numbers of American
Coots Fulica americana shot each year in the United States
(reflecting total population size) and the number of ponds on the
prairies in the preceding summer (reflecting the habitat and
feeding conditions). Significance of relationship: b = 0.21, r 2 = 0.54,
P < 0.001. Redrawn from Alisauskas and Arnold (1994).
and this influences the amount of wetland habitat
available to nesting waterfowl. In wet periods,
populations increase, and in dry periods they decline.
So important are these prairie wetlands as nesting
habitat that they influence the entire continental
wintering populations of several species, including
the American Coot Fulica americana (Fig. 3). They
show how year-to-year conditions in the breeding
areas can largely determine year-to-year fluctuations
in total populations.
By contrast, the numbers of several migrant songbird species counted each spring on their European
breeding areas have fluctuated according to rainfall
(and hence food supplies) in their African wintering
areas. Examples include the Sedge Warbler Acrocephalus schoenobaenus, Sand Martin Riparia riparia
and Purple Heron Ardea purpurea (den Held 1981,
Peach et al. 1991, Bryant & Jones 1995, Szép 1995).
They show how conditions in wintering areas can largely
determine year-to-year fluctuations in total populations.
In some migratory species, breeding density declined,
and it was not immediately obvious whether the
causal factors lay in breeding or wintering areas.
However, where fecundity and survival rates were
monitored in the same population during periods
of both increase (or stability) and decrease, the comparison provided useful pointers to where the cause
of the decline might lie. Thus, where annual reproduction declined while annual survival stayed the
same, the problem lay in the breeding areas, but
where survival had decreased, the problem lay in
either breeding or wintering areas, depending on the
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
time of year the extra deaths occurred. Decline in a
European Golden Plover Pluvialis apricaria population
was associated with a decline in survival rate but
no change in reproduction, whereas a decline in a
Northern Lapwing Vanellus vanellus population
was associated with a decline in reproduction but no
change in survival (Parr 1992, Peach et al. 1994,
Yalden & Pierce-Higgins 1997).
In yet other species, long-term population declines
were associated with reductions in both breeding and
survival. The White Stork, for example, has suffered
from reduced reproduction on its European breeding
grounds, caused by drainage and pesticide-induced
food shortages, and also from reduced survival on its
West African wintering grounds, caused mainly by
drought-induced and pesticide-induced food shortages
(mainly locust control) (Dallinga & Schoenmakers
1989, Kanyamibwa et al. 1993, Bairlein 1996). Thus,
in any avifauna we can expect to find species whose
numbers are changing because of events in breeding or
non-breeding areas or both, and routine monitoring
of fecundity or survival rates can often be enlightening.
Other examples of summer-influenced and winterinfluenced population changes in migrants, involving
more than 53 different populations of 44 species, are
given in Table 1. In most populations, the evidence
is entirely circumstantial, and based on long-term or
year-to-year correlations between changes in breeding numbers and changes either in (1) conditions
in breeding or wintering areas, or in (2) associated
breeding or mortality rates. In some populations,
however, potential causal relationships were subsequently confirmed by experiments involving
manipulation of likely limiting factors (see later).
In some of the bird populations mentioned in
Table 1, breeding numbers increased in years that
followed a good breeding season and decreased in
years that followed a poor breeding season (Fig. 4).
In these populations, spring–summer conditions
in breeding areas evidently had most influence on
subsequent year-to-year changes in breeding numbers,
and such populations were therefore below the limit
imposed by winter habitat (at least in most years).
The same was true for other populations whose survival
rates changed over the years, according to conditions
on breeding areas. In yet other populations, breeding
numbers varied from year to year according to previous
winter conditions, implying that such populations
were close to the limit imposed by winter habitat
(or food supplies).
If we can assume that breeding rates were influenced primarily by conditions in breeding areas and
Location
B. Associated with change in summer conditions /survival/ breeding rate
Chaffinch Fringilla coelebs
Russia
Willow Warbler Phylloscopus trochilus
Russia
Icterine Warbler Hippolais icterina
Russia
Kirtland’s Warbler Dendroica kirtlandii
Michigan
Black-throated Blue Warbler
New Hampshire
Dendroica caerulescens
Prairie Warbler Dendroica discolor
Indiana
Wilson’s Warbler Wilsonia pusilla
California
Bell’s Vireo Vireo bellii
California
Missouri
Black-capped Vireo Vireo atricapillus
Texas
A. Associated with change in winter conditions/survival
Twite Carduelis flavirostris
Germany–Netherlands
Snow Bunting Plectrophenax nivalis
Germany–Netherlands
Shorelark Eremophila alpestris
Germany–Netherlands
Sedge Warbler Acrocephalus schoenobaenus
England
Netherlands
Blackcap Sylvia atricapilla
Britain
Whitethroat Sylvia communis
Britain
Sweden
Britain
Willow Warbler Phylloscopus trochilus
Loggerhead Shrike Lanius ludovicianus
Minnesota
Barn Swallow Hirundo rustica
Denmark
Britain
Sand Martin Riparia riparia
Britain
Hungary
Avocet Recurvirostra avosetta
England
Golden Plover Pluvialis apricaria
Scotland
Puffin Fratercula arctica
Scotland
Night Heron Nycticorax nycticorax
France
Purple Heron Ardea purpurea
Netherlands
Barnacle Goose Branta leucopsis
Svalbard
Dark-bellied Brent Goose Branta bernicla
Netherlands
Lesser Snow Goose Chen caerulescens
Canada
Species
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Long-term
upward or
downward
trend
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Annual
fluctuations
continued
Nolan (1978)
Chase et al. (1997)
Griffith and Griffith (2000)
Budnick et al. (2000), Rothstein and Robinson (1994)
Hayden et al. (2000)
Sokolov (1999)
Sokolov (1999)
Sokolov (1999)
DeCapita (2000)
Holmes et al. (1991, 1996), Rodenhouse et al. (2003)
Dierschke (2002)
Dierschke (2002)
Dierschke (2002)
Peach et al. (1991)
Foppen et al. (1999)
Baillie and Peach (1992)
Winstanley et al. (1974), Baillie and Peach (1992)
Hjort and Lindholm (1978)
Baillie and Peach (1992), Peach et al. (1995)
Brooks and Temple (1990)
Møller (1989)
Robinson et al. (2003)
Cowley (1979), Bryant and Jones (1995)
Szép (1995)
Hill (1988)
Parr (1992), Yalden and Pearce-Higgins (1997)
Harris and Wanless (1991)
den Held (1981)
den Held (1981), Cavé (1983)
Owen (1984)
Ebbinge (1991)
Francis et al. (1992)
Source
Table 1. Migratory bird species in which temporal changes in breeding density have been linked with changes in previous winter conditions (A), with changes in summer conditions
(B), or with changes in both winter and summer conditions (C). Updated from Newton (1998).
Population limitation in migrants
203
© 2004 British Ornithologists’ Union, Ibis, 146, 197– 226
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
Finland
England
Russia
New Hampshire
California
Illinois
Delaware
Britain
Britain
Fennoscandia
South Africa
South Africa
Britain
England
North America
Iceland
Britain
Svalbard
Britain
Location
+
+
+
+
+
+
Long-term
upward or
downward
trend
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Annual
fluctuations
Virolainen (1984)
Stenning et al. (1988)
Sokolov (1999)
Sherry & Holmes (1992)
Johnson & Geupel (1996)
Robinson (1992)
Roth and Johnson (1993)
Green et al. (1997), Green (1999)
Hollands and Yalden (1991)
Soikkeli (1970), Jönsson (1991)
Summers and Underhill (1987)
Summers and Underhill (1987)
Peach et al. (1994)
Aebischer et al. (2000)
Reynolds (1987)
Gardarsson and Einarsson (1994)
Summers and Underhill (1987)
Madsen et al. (2002)
Suddaby and Ratcliffe (1997)
Source
C. Associated with change in both winter conditions (overwinter survival) and summer conditions (previous breeding rate)
White Stork Ciconia ciconia
France–Germany
+
+
Dallinga and Schoenmakers (1989),
Kanyamibwa et al. (1993), Bairlein (1996)
Great Skua Stercorarius skua
Scotland
+
+
Klomp and Furness (1992)
Corncrake Crex crex
Common Sandpiper Actitis hypoleucos
Dunlin Calidris alpina
Curlew Sandpiper Calidris ferruginea
Little Stint Calidris minuta
Lapwing Vanellus vanellus
Stone Curlew Burhinus oedicnemus
Mallard Anas platyrhynchos
Various duck species
Brent Goose Branta bernicla
Pink-footed Goose Anser brachyrhynchus
Arctic Tern Sterna paradisaea
American Redstart Setophaga ruticilla
Swainson’s Thrush Catharus ustulatus
Wood Thrush Hylocichla mustelinus
Pied Flycatcher Ficedula hypoleuca
Species
Table 1. continued.
204
I. Newton
Population limitation in migrants
205
Figure 4. Demography and population change. (a) Relationship between annual survival of adult Barn Swallows and annual change
in breeding density. From Møller (1989). (b) Relationship between annual breeding output of Pied Flycatchers and annual change
in breeding density. From Virolainen (1984). Significance of relationships: Barn Swallow, b = 3.44, r 2 = 0.93, P < 0.001; Pied Flycatcher,
b = 0.15, r 2 = 0.74, P < 0.001.
mortality rates primarily by conditions in wintering
areas (unless otherwise specified), then 22 of the
populations in Table 1 were winter-limited, another
29 or more were summer-limited, and two were
influenced by both summer and winter conditions.
However, because both the extent and carrying
capacities of habitats vary from year to year, and from
area to area, we can expect that the same species
might be winter-limited in some years or areas
and summer-limited in other years or areas, as in
the different Willow Warbler Phylloscopus trochilus
populations given in Table 1. Each case must be
judged on its particular circumstances.
Over several years, the same population might
change from one state to another, as its status with
respect to available habitat changed. Several species
of geese increased during the latter half of the 20th
century in response to reduced shooting pressure in
their wintering areas, but then came up against food
shortage in the breeding areas, as growing numbers
competed for favoured food plants. This increased
competition resulted in reduced chick survival in
Lesser Snow Geese Chen caerulescens in the central
Canadian arctic (Francis et al. 1992), and in reduced
summer survival among adult Pink-footed Geese
Anser brachyrhynchos on Svalbard (Madsen et al. 2002).
The major constraint to further population growth
thus shifted from the wintering to the breeding areas
as the populations grew. In some other goose populations, studied in less detail, increasing competition
was manifest chiefly in declining proportions of young
in wintering flocks (Fig. 5). In Brent Geese wintering
in western Europe, total numbers fluctuated from
year to year around the long-term upward trend,
according to annual variations in predation rates on
eggs and chicks in Siberian breeding areas (Summers
& Underhill 1987).
The pattern in which breeding density fluctuated
from year to year in parallel with the previous year’s
breeding success was recorded only in short-lived
species, in which individuals breed in their first
year of life (passerines and dabbling ducks). It would
not be expected in longer-lived species, in which
individuals do not breed until they are two or more
years old, and in which annual recruitment rates are
naturally low. In such species, breeding success would
need to be poor over several years before any effect on
breeding numbers became obvious, unless accompanied by a simultaneous increase in mortality or
emigration (for Arctic Tern Sterna paradisaea see
Suddaby & Ratcliffe 1997).
Another indication of the importance of winter
conditions in influencing population changes comes
when closely related species that breed in the same
area show different trends according to where they
winter. Of the various waterfowl species that breed
in Siberia, those that migrate to western Europe have
all increased in numbers in recent decades, following
their greater protection from winter hunting. Examples
include the western population of the Greater
White-fronted Goose Anser albifrons and the Darkbellied Brent Goose Branta b. bernicla. By contrast,
all those populations that winter in south-east
Asia have continued to decline, in association with
© 2004 British Ornithologists’ Union, Ibis, 146, 197– 226
206
I. Newton
Figure 5. Reduced production of young by Greylag Geese Anser anser and Barnacle Geese Branta leucopsis, as their numbers have
grown. The populations concerned breed in Iceland (Greylag) and Svalbard (Barnacle) and winter in Britain, where counts were made.
The graphs show 5-year moving mean values. Adapted from Owen et al. (1986).
rising persecution in that region (Syroechkovski &
Rogacheva 1994). Examples include the eastern
subspecies of the Bean Goose Anser fabalis serrirostris
and A. f. middendorffii and the Baikal Teal Anas formosa.
These various waterfowl species share similar nesting
habitats, which are largely undisturbed by people,
and their divergent population trends have been
attributed to conditions in their different wintering
areas (Syroechkovski & Rogacheva 1994).
Another divergent pattern, evident in temperate
regions after a hard winter, is that resident species
are often found to have declined greatly, whereas
summer visitors from tropical wintering areas
have not (Dobinson & Richards 1964, Graber &
Graber 1979, Cawthorne & Marchant 1980, Holmes
& Sherry 2001). This difference provides another
indication of the importance of winter conditions, at
least for the resident species (Newton 1998). Some
summer visitors may in fact benefit from the scarcity
of resident competitors. The unusually large numbers
of European Pied Flycatchers Ficedula hypoleuca that
bred in Britain in 1917 and 1947 were attributed to
the greater availability of nest cavities in those years,
occasioned by the scarcity of resident tits caused
by preceding hard winters (Elkins 1983). This early
observation also suggested that Flycatcher numbers
were limited in some areas by shortages of nest-sites,
an inference supported by subsequent experiments
involving either the removal of competing tit species
(Gustafsson 1988), or the provision of additional
nestboxes (e.g. von Haartman 1971, Currie & Bamford
1982, Newton 1998). Many other species of cavitynesting migrants (excluded from Table 1) also increased
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
in breeding density and distribution following
the provision of artificial nest-sites, as did various
hirundines and swifts that use buildings (Erskine
1979, Newton 1994, 1998, Evans et al. 2003). The
implication is that nest-site availability (and hence, a
feature of the breeding area) is important in limiting
the overall population levels of such species.
Aseasonal weather and other events
Where population declines affect both residents and
migrants simultaneously, this has often been attributed to late cold springs that affect the migrants on
return passage as well as the residents. Such declines
affecting both groups were recorded on the Courish
Spit on the southern Baltic coast during a succession
of cold springs in 1958–77 and 1985–98 (Sokolov
et al. 2000). Storm-induced mortality among migrating birds is outside the scope of this paper, but spring
weather events, killing large numbers of migrants
soon after their return to breeding areas, have been
recorded infrequently in many species, especially
insectivores (e.g. Buss 1942, Ligon 1968, Vespäläinen
1968, Whitmore et al. 1977, Brown & Brown 1998).
Among Common Sandpipers Actitis hypoleucos, which
migrate from Africa to breed in Europe, annual survival
fluctuated in one area according to the weather in
April when they arrived (Hollands & Yalden 1991).
The mean annual survival over 13 years was 79%, but
following late snowstorms in 1981 and 1989, survival
fell to 39% and 50%, respectively, and breeding
pairs from 21 to 14 and from 20 to 12. In general,
losses that occur in spring, when numbers are near
Population limitation in migrants
their seasonal low, are much more likely to affect
subsequent breeding density than are losses that occur
in late summer or autumn, when numbers are near
their seasonal high and subject to density-dependent
effects over winter. Recoveries in numbers from
serious spring events would normally be expected to
take at most a few years.
Carry-over effects
Most discussion in the literature of population
limitation in migrants carries the implicit assumption
that conditions in wintering areas have no effects on
subsequent breeding performance, and that conditions in breeding areas have no effect on subsequent
winter survival. However, these assumptions are not
always justified. Among geese and other waterfowl,
foraging conditions in wintering and migration sites
have long been known to affect reproductive success
in the subsequent breeding season, through the effect
of food supplies on body condition (for Snow Goose
see Ankney & MacInnes 1978, Bêty et al. 2003, for
Canada Goose Branta canadensis see Hanson 1962,
Raveling 1979, for Mallard Anas platyrhynchos see
Krapu 1981, Pattenden & Boag 1989), and in a few
such species the mechanisms have been studied.
Thus in Brent Geese in the Netherlands, the favoured
spring staging habitat is saltmarsh whose plants allow
the geese to fatten rapidly. However, the number
of geese that can feed in saltmarsh is limited, so as
the population grew over a period of years, increasing
proportions of birds were relegated to less nutritious
agricultural grassland. The geese used body reserves
accumulated in spring for migration and subsequent
nest defence, egg production and incubation, and
individuals that had fed on saltmarsh showed better
breeding success than those that had fed on grassland
(Ebbinge 1992). Females that had accumulated the
greatest body reserves at a spring stopover site were
more likely to return with young in the following
autumn than were females that accumulated smaller
reserves, whereas males, which accumulated smaller
reserves than females, showed no such relationship
(Ebbing & Spaans 1995).
In some other species of geese smaller proportions
of females laid, and clutch-sizes were smaller, in
years when feeding conditions in staging areas were
poor than in years when they were good (for Barnacle
Geese Branta leucopsis see Cabot & West 1973, for
Lesser Snow Geese see Davies & Cooke 1983). In
Whooper Swans Cygnus cygnus, the proportions of
young each year in wintering flocks in Sweden were
207
correlated with the mean temperature in the preceding
winter, implying that winter temperatures influenced
subsequent breeding success (presumably through
effects on feeding conditions and body reserves) (Nilsson
1979). From these and other studies, it is clear that
the breeding success of some migratory waterfowl
depends partly on the body reserves accumulated
in wintering and staging areas, and that the effects of
such reserves are evident at the level of the individual
bird and at the level of the population. By contrast, in
some shorebird species, formerly thought to depend
for egg production partly on similar reserves, isotope
analysis revealed that eggs were formed from terrestrial
rather than coastal foods, and hence were produced
from food eaten after arrival in breeding areas (Klaassen
et al. 2001).
One important factor contributing to reproductive
success among migrant birds is date of spring arrival
and commencement of nesting. Within populations,
individuals that arrive and start nesting early in the
season do better in terms of habitat quality, territory
acquisition and number of young raised, than those
that arrive and nest late (for Willow Ptarmigan Lagopus lagopus see Moss 1972; Common Wheatear
Oenanthe oenanthe see Brooke 1979, Currie et al.
2000; Pied Flycatcher see Lundberg et al. 1981;
Painted Bunting Passerina ciris see Lanyon &
Thompson 1986; Great Reed Warbler Acrocephalus
arundinaceus see Bensch & Hasselquist 1991; Barn
Swallow Hirundo rustica see Møller 1994; Savi’s
Warbler Locustella luscinioides see Aebischer et al.
1996). In some such species, the same sequence of
territory settlement held from year to year, even
though the occupants changed, and even though
some early settlers were displaced by former owners
that arrived later. Variations in arrival dates sometimes
exceeded a month, and late arriving individuals were
often in poorer condition.
In a few studies, poor body condition and late
arrival have been associated with winter habitat.
Marra et al. (1998) used analyses of carbon isotopes
in muscle tissue of American Redstarts Setophaga
ruticilla to make this connection. Individuals that
wintered in moist forest in Jamaica had lower
tissue 13C values than those found in poorer, secondary
scrub habitat, a pattern mirrored in the available
insect prey. Also, those individuals occupying the
poorer scrub habitats were in poorer body condition
than those in better habitats. Muscle tissue 13C
values provided a tracer to link newly arrived birds
in North America with the quality of the habitat
they had occupied in winter. Those birds that arrived
© 2004 British Ornithologists’ Union, Ibis, 146, 197– 226
208
I. Newton
earliest in the North American breeding areas had
lower 13C values than birds arriving later, indicating that
the earlier birds had come from the best wintering
habitat. This work thus provided another link between
the conditions experienced by individuals in winter,
their subsequent migration dates and breeding
success.
Whatever the date of departure from wintering
areas, adverse weather encountered en route may
delay arrival in breeding areas and thus affect reproduction (Johnson & Herter 1990, Richardson 1990).
In 1997, many White Storks were late in leaving
their African wintering areas; this was attributed to
poor food supply (Berthold et al. 2002). In addition,
some individuals (including a radiotagged bird)
were delayed for another week en route, as they hit
a severe cold spell. These circumstances led to late
arrival in European breeding areas, and depressed
breeding success over wide areas, providing another
link between conditions in wintering and migration
areas and subsequent breeding success.
Although I know of no specific examples of events
in breeding areas affecting overwinter (post-migration)
survival in migrants, at the level of the population,
good breeding success is likely to result in large
populations. In species limited in wintering areas,
mortality is likely to be density-dependent, resulting
in high mortality following good breeding years,
and lower mortality following poor breeding years.
Among migrants, overwinter loss has been shown to
be density-dependent in populations of Sedge Warbler,
Blackcap Sylvia atricapilla, Common Whitethroat
S. communis, Willow Warbler, European Pied Flycatcher, Common Redstart Phoenicurus phoenicurus,
Barn Swallow, Redshank, Mallard, Northern Shoveler
Anas clypeata and Barnacle Goose (Mihelsons et al.
1985, Järvinen 1987, Kaminski & Gluesing 1987,
Stenning et al. 1988, Owen & Black 1991, Baillie &
Peach 1992, Whitfield 2003). In most of these species,
overwinter loss was also the key factor governing
year-to-year change in breeding numbers (reviewed
by Newton 1998).
CAUSAL FACTORS
Over the past several decades, various species of
intercontinental landbird migrants have declined
in Europe and North America, but the main causal
factors seem to have differed between the two regions.
The European species winter in the Afrotropics,
and most of those that have declined occupy the arid
scrub habitats prone to drought. Their declines have
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
been widely attributed to events on wintering areas,
notably reductions in rainfall and associated food
supplies. The North American species winter in the
tropics of Central and South America, and most of
those that have declined occupy forest, with smaller
numbers in scrub and grassland habitats. Their declines
have been widely attributed to events in breeding areas,
notably forest fragmentation and the associated
increases in densities of predators and parasitic
Brown-headed Cowbirds Molothrus ater. The lines
of evidence proposed in favour of these various
hypotheses are summarized below, along with some
alternative explanations applicable to at least a
minority of species.
Eurasian–Afrotropical migrants
In much of Africa north of the equator, rainfall
during the northern summer largely determines the
state of the vegetation for the overwintering migrants,
and hence may be assumed to influence the food
supplies of many birds, whether they consume
plant or animal matter. Rainfall varies greatly from
year to year, but over much of the region since the
late 1960s has in most years been well below former
levels, owing to a failure of the rain-belts to extend
so far to the north. Hence, drought conditions have
been most severe along the northern edge of the
Sahel zone, lying immediately south of the Sahara
desert, and have diminished southwards across the
savannah zones, towards the equator. In the Sahel
zone, rainfall deficits were particularly marked in
1968, 1973, 1983 and 1984, and again in 1990. It
was in 1969 (after the 1968 drought) that the
importance of conditions in African wintering areas
was first impressed upon European bird-watchers,
when massive declines were apparent in the
returning numbers of Common Whitethroats and
Common Redstarts, two species that are strongly
represented in the northern Sahel (Winstanley
et al. 1974). In Britain, where the Whitethroat was
among the species monitored by the Common Bird
Census of the British Trust for Ornithology, numbers
dropped by about 70% between 1968 and 1969
(Fig. 6). The Sand Martin and Sedge Warbler were
also badly hit, even though most individuals spend
only 4–6 weeks in the Sahel in October–November,
before moving south, and an even shorter period
on return passage in March (Cowley 1979, Jones
1985).
Other species that winter in the Sahel zone, or
pass through in autumn and spring, showed lower
Population limitation in migrants
209
Figure 6. Above: rainfall trends in the Sahel region of Africa expressed as departures from the long-term mean (- - -) for 1940 – 88. Data
from Grainger (1990). Below: population trend (log scale) of Common Whitethroats Sylvia communis breeding in Britain, as revealed by
the Common Bird Census of the British Trust for Ornithology, 1962–88. Data from Marchant et al. (1990).
population levels in Britain in 1984, 1985 or 1991 than
in any previous year since 1962 (when the Common
Bird Census started). They included, besides the two
species just mentioned, the Barn Swallow, Grasshopper Warbler Locustella naevia, Eurasian Chiffchaff
Phylloscopus collybita and Spotted Flycatcher Muscicapa striata. The Chiffchaff subsequently recovered
but the Willow Warbler continued to decline, as
did the Spotted Flycatcher (Peach et al. 1995, 1998).
Moreover, annual fluctuations in the numbers of
some species followed annual fluctuations in the
preceding year’s rainfall in the Sahel zone, as shown
for the Sedge Warbler in Britain and the Netherlands
(Table 1; Peach et al. 1991, Foppen et al. 1999).
With the lapse of further years, it has become
apparent that rainfall in the western Sahel has a
major impact on the breeding populations of several
species that nest in Europe, including some that
overwinter there and others that pass through. Of
the 25 species of Palaearctic–African migrants that
were adequately monitored in Britain, 11 showed
substantial net declines over the period 1962–88
(Marchant et al. 1990). The pattern of decline
differed between species, the Common Whitethroat
suffering a catastrophic crash mainly in one year,
and others showing more gradual declines over ten or
more years, with some subsequently recovering and
others not. These differences may reflect differences in
wintering areas between species, or in the mechanisms
involved, and in some species may be confounded by
simultaneous changes in breeding areas. Most of the
remaining European–Afrotropical migrant species
showed no marked net change over the 27-year period,
but five showed a substantial rise. The latter winter
well south of the Sahel zone, however, and some
extend south of the equator to the austral summer.
Further long-term data on population trends of
migrants were obtained at the Mettnau trapping
station in southern Germany, where birds were caught
in mist-nets operated annually between late June
and early November (Berthold et al. 1993). In the
20 years from 1972, 12 out of 21 trans-Saharan
migrant species showed significant overall declines,
and only one (Common Nightingale Luscinia megarhynchos) showed an overall increase. By contrast,
only two of 14 other species (residents and shortdistance migrants) showed significant declines and
none showed a significant increase. Declines were
evidently more widespread among the long-distance
migrants that winter in Africa than among the residents and short-distance migrants that winter in
Europe. However, as the trapping occurred after the
breeding season, the totals included adults and young,
and so reflected both breeding numbers and breeding
success. Comparisons of count data from different
countries have shown that year-to-year changes in
numbers are often correlated over large parts of the
European breeding range but, as expected, the counts
from some regions show different trends, and some
show no evidence for links with Sahel droughts
(Svensson 1985, Marchant 1992, Sokolov et al. 2001).
Range-wide trends would perhaps not be expected in
© 2004 British Ornithologists’ Union, Ibis, 146, 197– 226
210
I. Newton
species whose west European populations winter in
West Africa and east European populations in East or
southern Africa, and are thereby exposed to different
climatic regimes.
It is not just counts that point to the importance
of wintering areas in influencing numbers. In keyfactor analyses, variation in overwinter loss was
found to account for most of the variation in total
annual loss in seven migrant passerine species monitored in Britain or Denmark (Baillie & Peach 1992,
Møller 1989). In addition, annual survival rates of
Sedge Warblers and Common Whitethroats trapped
each summer at specific sites in Britain were also
correlated with rainfall in the Sahel zone (Fig. 4;
Peach et al. 1991, Baillie & Peach 1992), as were
survival rates of Sand Martins trapped at sites in
Scotland and Hungary (Bryant & Jones 1995, Szép
1995). Major crashes in numbers occurred in 1968/
69, 1983/84 and 1990/91, all following years of poor
rainfall. Similarly, annual survival rates of Barn Swallows breeding in an area of Denmark were correlated
with March rainfall in their southern African wintering
areas (Møller 1989). In British Swallows, breeding
numbers were not correlated with rainfall in their
South African wintering areas, but with rainfall
in the western Sahel, at its driest during spring
migration (Robinson et al. 2003). Although factors
causing long-term trends in bird numbers are not
necessarily the same as those causing annual fluctuations, they appear to be the same in these cases.
These studies provided further circumstantial support
for the proposed causal chain: low rainfall → low
winter food supplies → lower overwinter survival →
low breeding population.
It is not only insectivorous passerines that have
shown such links. The numbers of Purple Herons
nesting in the Netherlands over a 19-year period,
and their annual survival rates, were correlated with
wetland conditions in their West African wintering
areas, as measured by water discharge through the
Niger and Senegal Rivers (den Held 1981, Cavé
1983). The wetter the conditions in West Africa, the
better was the overwinter survival, and the greater
the number of herons that returned to Holland to
breed each year. A similar relationship was apparent
among Night Herons Nycticorax nycticorax counted
at colonies in southern France, and to a lesser extent
among Squacco Herons Ardeola ralloides in the same
localities (den Held 1981). All these species, and the
Little Bittern Ixobrychus minutus, which also winters
in drought-prone areas of West Africa, have decreased
in western Europe since the 1960s (Marion et al. 2000).
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
Similarly, the numbers of White Storks that breed
in western Europe have declined greatly since 1960,
but those in eastern Europe much less so. Over this
period annual adult survival was lower in the western
population, and annual variations (measured from
ringed birds) were closely linked to rainfall in the western
Sahel zone where the birds winter (Kanyamibwa
et al. 1993, Barbraud et al. 1999). The eastern population, which winters mainly in East Africa, showed
no such relationship with rainfall, perhaps because
rainfall has declined less there, and because East
Africa offers better conditions and greater variety of
habitats than West Africa (Dallinga & Schoenmakers
1989). Moreover, the mean numbers of young
raised per pair of White Storks in western Europe
was correlated with rainfall in West Africa in the
preceding winter, presumably because the winter
food supplies influenced the condition of adult
Storks in spring, and not only their survival (Bairlein
& Henneberg 2000). Differences in rainfall patterns
may also explain the differences in population
dynamics of some other conspecific populations of
birds wintering in different parts of Africa, such as
western and eastern populations of the White Wagtail
Motacilla alba (Svensson 1985). Overall, however,
drought in African wintering areas has been suggested as the major causal factor in the regional
population declines of at least 17 Eurasian breeding
species.
Other possible factors in population declines
Decreasing rainfall is not the only factor that
may have caused declines in the numbers of some
European birds that winter in West Africa. The burgeoning human population, and associated overgrazing,
burning and woodcutting, have all accentuated the
process of desertification, so that even in relatively
wet years many areas now hold little habitat for birds
(Jones 1985). So even if rainfall returns to its earlier
levels, some migrant species may not regain their
former numbers because of the habitat degradation
that has occurred in the interim. With the increasing
use of pesticides, locusts and other insects eaten
by birds are likely to have become less available, and
in various wintering areas many birds have themselves been killed directly by pesticide use (e.g.
Mendelssohn & Paz 1977, Mullié & Keith 1993,
Goldstein et al. 1996). At least four Palaearctic species,
which feed extensively on locusts and grasshoppers
on their African wintering grounds, have declined in
recent decades, including the White Stork discussed
above, the Lesser Kestrel Falco naumanni, the Montagu’s
Population limitation in migrants
Harrier Circus pygargus and the Pallid Harrier
C. macrourus. In all these species, however, factors in
the breeding range may also have been involved (for
Lesser Kestrel see Donázar et al. 1993, for Montagu’s
Harrier see Arroyo et al. 2002).
Unusually, the felling of rainforest in West Africa
may have affected very few migrant species adversely,
because the forest is replaced by more open secondary
habitats (‘derived savanna’), which many species
favour (Morel & Morel 1992). The same is true for
miombo woodland in southern Africa, and is simply
a consequence of the small number of Eurasian
migrant species that winter in closed-canopy woodland
compared with those that favour more open areas.
Not all Eurasian–African migrant species are
necessarily limited primarily by factors acting on the
wintering range, and for any one species the situation
may differ from one part of the breeding-wintering
range to another. Examples of species whose changes
in breeding numbers have been linked with events in
breeding areas include the Pied Flycatcher for yearto-year changes (Fig. 4; Virolainen 1984), and the
Turtle Dove Streptopelia turtur for long-term decline
(Browne & Aebischer 2001). In some species, as
mentioned above, the relative importance of breeding and wintering conditions in influencing breeding
numbers may well change from one period of years
to another or from one region to another.
Nearctic–Neotropical migrants
Notwithstanding the ongoing debate about the
number of species involved, and the regions affected,
most studies in North America have attributed declines
in tropical migrants to events in breeding areas (Askins
et al. 1990, Hagen & Johnston 1992, Sherry &
Holmes 1992, Martin & Finch 1995, Latta & Baltz
1997). For some eastern forest species, problems
are thought to stem from forest fragmentation – the
division of the once extensive deciduous forest into
small urban and rural woodlots. Such fragmentation
could have caused population declines much greater
than expected from the areas of habitat lost. The
evidence consists partly of the apparent contrast
in population trends between birds in small forest
fragments and those in extensive forest areas, and partly
from current species distributions, which show that
many species are now almost absent from small
forest fragments. Relevant studies span a wide area
from the Atlantic to the Mississippi (Bond 1957,
Robbins 1979, 1980, Whitcomb et al. 1981, Ambuel
& Temple 1983, Howe 1984, Lynch & Whigham 1984,
211
Hayden et al. 1985, Freemark & Merriam 1986,
Blake & Karr 1987, Robbins et al. 1989, Askins
et al. 1990).
Many studies have entailed assessment of the
numbers and kinds of species breeding in forest
fragments of different sizes in the same region. The
main finding was as expected, namely that total
species numbers increased with increasing size of
forest. Whereas certain species were found in both small
and large forest fragments, others were found entirely
or almost entirely in large ones, mainly in the forest
interior. More specifically, larger forests tended
to have more species per unit area of neotropical
migrants, some species of which tended to be scarce
or absent in small forests.
In most studies, forest area accounted for most of
the variation in species richness and total density of
birds, and especially of neotropical migrants (Askins
et al. 1990, Blake & Karr 1987). Other variables,
such as the degree of isolation from other forests,
or vegetation structure and heterogeneity, explained
relatively little additional variance in species numbers,
as judged from the findings of multiple regression
analyses. However, vegetation variables influenced
the kinds of species that were present, and their
local densities (Ambuel & Temple 1983, Lynch &
Whigham 1984). As expected, edge species occurred
at greatest density in small forests.
The relative importance of forest area and
isolation to species occurrence seemed to vary
between studies (Lynch & Whigham 1984, Robbins
et al. 1989, Askins et al. 1990), probably because both
features depended on the distribution of woodland
in the landscapes concerned, on the particular bird
species represented, as well as on the way that
isolation was measured: whether by the distance to
the nearest wood, by the proportion of woodland
in the surrounding area or by some combination of
these measures. Similar relationships between species
numbers and area have been found in forest and
other habitat areas in other parts of the world, but
European woods seem to hold few species that could
be considered as forest-interior specialists (Newton
1998).
Predation
Forest fragmentation is held to have promoted
increases in the numbers of predators and broodparasites. In eastern North America, as in Europe,
many small generalist predators, which eat eggs and
chicks, reach much higher densities in suburban and
farming areas than in more natural areas (Wilcove
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212
I. Newton
et al. 1986, Small & Hunter 1988, Hoover et al. 1995),
benefiting from the additional food provided by
human activities. Such predators include mammals,
notably Grey Squirrels Sciurus carolinensis, Raccoons
Procyon lotor and feral cats, and birds such as
American Crows Corvus brachyrhynchos and Blue Jays
Cyanocitta cristata. All these species tend to concentrate their hunting along woodland edges (Gates &
Gysel 1978, Whitcomb et al. 1981, Paton 1994).
Several studies have shown that the nesting success
of songbirds was higher in large forests than in small
woods, and that in large forests nest success increased
from edge to interior. In total, overall predation rates
decreased with increasing area of forest patch in all
of eight studies, and decreased from centre to edge
in ten of 14 studies involving artificial nests, and
in four of seven involving natural nests (Paton 1994).
In each case, predators that entered the woods from
nearby farmed or suburban areas accounted for most
of the losses. In one study in southern Wisconsin, for
example, the overall nest success of 13 species was
only 18% within 100 m of forest edge (n = 96), compared with 58% at 100–200 m within forest (n = 98)
and 70% at more than 200 m into forest (n = 82)
(Temple & Cary 1988). The entire area of most small
woods lay within the most vulnerable zone. It is
not hard to imagine therefore that as forest becomes
more fragmented, overall nest success could decline,
eventually to the point at which insufficient young
were produced to offset the annual adult mortality,
leading to regional population declines (Brawn &
Robinson 1996). Other (but not all) studies using
artificial or natural nests have given similar results
elsewhere in North America (Gates & Gysel 1978,
Wilcove 1985, Small & Hunter 1988, Yahner & Scott
1988, Donovan et al. 1995, Hoover et al. 1995,
Robinson et al. 1995, Weinburg & Roth 1998, Manolis
et al. 2002), as have studies with artificial nests in
Europe (Andrén & Angelstam 1988, Sandström 1991,
Andrén 1992).
If nest predation has increased with forest
fragmentation, why should it have affected some
neotropical migrants more than other birds? Three
reasons have been commonly suggested. First, with
few exceptions, the neotropical migrants that breed
in the eastern United States construct open nests,
either on the ground or in low bushes, in sites that are
most vulnerable to predators (Wilcove & Whitcomb
1983). By contrast, none of the residents or shortdistance migrants that breed commonly in suburban
settings places its nest on the ground, and many use
tree-cavities, which provide some of the safest sites.
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
Secondly, long-distance migrants start breeding later
in the year (mostly June) and usually raise only one
brood per year, occasionally two (Greenberg 1980,
Whitcomb et al. 1981). By contrast, many resident
species and short-distance migrants start in April
or May and routinely attempt second and even third
broods. Thirdly, many neotropical migrants are small
and unable to drive off nest predators in the way that
some larger resident species can. For these various
reasons, then, neotropical migrants are supposedly
disadvantaged with respect to nest predators, which
could together cause a disproportionate reduction in
their breeding rates, and contribute to the selective
disappearance of tropical migrants from fragmented
eastern woodlands. Through nest predation alone,
Wood Thrushes Catharus mustelinus in Pennsylvania
produced insufficient young to offset the usual
annual mortality (Hoover et al. 1995), and through
a combination of predation and cowbird parasitism
the same was true for Wood Thrushes in Illinois
(Robinson 1992).
In an analysis of the population trends of different
species in relation to various features of their biology,
Böhning-Gaese et al. (1993) found that nest location
and nest-type were the best predictors of population
trends. Species with high nest locations did better
than those with low, and species with closed nests
did better than those with open nests. These are both
aspects of nesting biology that influence rates of
predation and probably also of parasitism, as discussed
below.
Parasitism
The Brown-headed Cowbird lays its eggs in the nests
of a wide range of passerine host species, thereby
reducing their own production of young (Mayfield
1977). Originally confined to the grasslands of mid
continent, following deforestation the species gradually spread to occupy most of the continent north
to the boreal forest (Mayfield 1977, Brittingham &
Temple 1983). Within this newly occupied terrain,
its numbers continued to rise at least to the 1960s,
as it benefited from the increased food supplies
provided by waste grain in cereal fields, cattle feedlots
and garden feeders. Its overall numbers probably
increased more than ten-fold during the 20th
century (Brittingham & Temple 1983).
Forest fragmentation increased the amount of
edge, along which Cowbirds have access to the nests of
forest species previously immune to attack. Several
studies have shown greater parasitism of nests in
small than large woods, and near the edges of woods
Population limitation in migrants
than in the interior (Gates & Gysel 1978, Brittingham
& Temple 1983, Temple & Cary 1988, Donovan
et al. 1995, Robinson et al. 1995). Many of these
new hosts, including neotropical migrants, have no
innate defences, and will not desert or eject strange
eggs, but raise the resulting young. Because Cowbirds use many host species, moreover, they are not
vulnerable to decline in any one, and could in theory
parasitize favoured species to extinction, while being
maintained at high density by other common hosts
(May & Robinson 1985). The same features that
make neotropical migrants nesting near the edges of
woods more vulnerable to nest predators than residents also make them more vulnerable to parasitism,
namely their use of accessible nest-sites and their
predominant single-broodedness.
During the 1960s, Cowbirds parasitized up to
70% of nests of the rare Kirtland’s Warbler Dendroica
kirtlandii, reducing the production of young
Warblers to fewer than one per nest (Mayfield 1983,
Walkinshaw 1983). This was fewer than needed to
maintain the population level, and during 1961–71
the Warbler population declined by 60%. Then, with
the start in 1972 of a programme of Cowbird removal,
parasitism dropped to 6%, Warblers fledged about
three young per nest and their breeding numbers
stabilized (Kelly & DeCapita 1982, DeCapita 2000).
Later, they increased more than threefold when
the habitat expanded as a result of a forest fire
(Rothstein & Robinson 1994), but to facilitate this
increase, and to maintain Kirtland’s Warbler in
the long term, Cowbird control was continued. The
historical suppression of wild fires had clearly not
helped this species, which prefers young stands.
Cowbird removal also resulted in large increases
in both breeding success and subsequent breeding
numbers of the Least Bell’s Vireo Vireo bellii pusillus
in southern California and the Black-capped Vireo
Vireo atricapillus in central Texas (Griffith & Griffith
2000, Hayden et al. 2000). On the other hand,
removal of Cowbirds from Mandarte Island off
British Columbia led to an improvement in the nest
success of Song Sparrows Melospiza melodia, but to no
increase in subsequent breeding density, which was
determined by other factors (Smith 1981). The same
held for South-western Willow Flycatchers Empidonax traillii in California (Whitfield 2000). Thus, of
five experimental removals of Cowbirds, all five led
to improved nest success in the host species, but only
three led to an unequivocal response in host breeding
numbers. Nonetheless, some other species seem no
longer to be producing enough young to offset known
213
adult mortality. In a mainly agricultural landscape in
central Illinois during 1985–89, nests of forest-nesting
neotropical migrants contained an average of 3.3
Cowbird eggs per nest. End-of-season juvenile to adult
ratios averaged 0.1 in neotropical migrants compared
with > 1.0 for year-round residents and short-distance
migrants (Robinson 1992). The neotropical migrants
could not have maintained their numbers under this
level of parasitism without large-scale immigration.
Other populations, which may have declined
following greater contact with this generalized broodparasite, include the Golden-cheeked Warbler Dendroica chrysoparia in parts of Texas, the Black-capped
Vireo in parts of Oklahoma (Grzybowski et al.
1986), and the Eastern Warbling Vireo Vireo gilvus in
parts of California and British Columbia (Verner &
Ritter 1983, Ward & Smith 2000), White-crowned
Sparrow Zonotrichia leucophrys in parts of California
(Trail & Baptista 1993), and Bell’s Vireo Vireo bellii
in Missouri (Budnik et al. 2000). At present, Cowbird removal forms part of the management action
for several endangered North American migratory
birds, including the Kirtland’s Warbler, Black-capped
Vireo, Least Bell’s Vireo and South-western Willow
Flycatcher.
It thus seems, then, that many songbirds in eastern
North America which nest in forest fragments in
suburban and agricultural areas are now experiencing
the dual effects of increased nest predation and
parasitism, particularly near forest edges, and that
for various reasons these effects fall most strongly on
many neotropical migrants. So far, attempts to assess
these impacts on population levels have been made
only in a minority of species, such as the Kirtland’s
Warbler and Wood Thrush in which survival, as well
as breeding success, have been measured. Studies on
a greater range of species, and in a greater range of
areas, are needed before the generality of these findings can be assessed. However, many other species
are suffering such high rates of nest failure in some
areas that population decline seems inevitable in
these areas without continuing immigration. Although
the predisposing fragmentation of the eastern deciduous forest occurred mainly in the 19th century, the
associated increase in generalist predators and
cowbirds has occurred mainly in the latter half of
the 20th century. Decline through excess predation
and parasitism would be expected to leave increasing
amounts of breeding habitat unoccupied. In this
scenario, it is not shortage of nesting habitat as such
that limits numbers, but shortage of ‘source’ habitat
in which annual per capita reproduction matches or
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I. Newton
exceeds annual per capita mortality. Habitat where
the reverse conditions hold acts as a sink.
Forest fragmentation may also reduce the food
available per unit area. This aspect has received much
less attention than predation or parasitism, but could
be important in some species. For example, among
Ovenbirds Seiurus aurocapillus nesting in different
sized forest fragments in Ontario, the density and
pairing success of territorial males increased with
area of the woodlot (Burke & Nol 1998). Within
Ovenbird territories, the biomass of invertebrate
prey was 10–26 times greater in large woods than
in small ones, associated with the deeper leaf litter
in larger woods.
The relationship between forest area and species
numbers was found to vary regionally: deficiencies
in species numbers in small forests tended to be more
marked in agricultural than in more forested landscapes (Freemark & Collins 1992). These regional
differences were in turn associated with similar
regional variations in songbird nest success. In
nine migratory species, predation and parasitism
rates, measured in standardized ways, were found to
increase with reduction in the proportion of forest
present. Landscapes with little forest acted as sinks
for some species, in that reproduction was insufficient to offset annual mortality, so that breeding
numbers could not be maintained in the absence of
continual net immigration from more forested areas
(notably Wood Thrush, Hooded Warbler Wilsonia
citrina, Ovenbird and Kentucky Warbler Oporornis
formosus). There were some exceptions to the
trends, as local factors – including those influencing
Cowbird abundance – modified the overall patterns
(compare the studies on Wood Thrushes in Illinois,
Indiana, Pennsylvania and Delaware: Robinson 1992,
Roth & Johnson 1993, Hoover et al. 1995, Weinberg
& Roth 1998, Fauth 2000).
Other factors operating in breeding areas
Other evidence points to events in breeding areas (as
opposed to wintering areas) as being a major cause of
population fluctuations in some neotropical migrants.
For many years, some migrant warblers have undergone
long-term changes in breeding densities in association
with changes in caterpillar abundance. Most outbreaks of defoliating caterpillars in eastern North
American hardwood forests are sporadic, occurring
in one location in one year and somewhere else
the next. The probability that any particular stand
will experience an outbreak in any given year is low.
Every now and then, however, one caterpillar species
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
of boreal regions, the Spruce Budworm Choristoneura
fumiferana, increases over a period of years to reach
plague proportions over wide areas, causing extensive defoliation. Several migrant bird species show
strong numerical responses to this insect, including
the Bay-breasted Warbler Dendroica castanea, Blackpoll Warbler D. striata, Tennessee Warbler Vermivora
peregrina, Cape May Warbler D. tigrina, Black-billed
Cuckoo Coccyzus erythropthalmus and Yellow-billed
Cuckoo C. americanus (Kendeigh 1947, Morris et al.
1958, Crawford & Jennings 1989). In one study,
Bay-breasted Warblers increased in abundance from
2.5 pairs per 10 ha in uninfested stands to 300 pairs
per 10 ha during an outbreak, Blackburnian Warblers
D. fusca increased from 25–30 pairs to 100–125
pairs per 10 ha, and Tennessee Warblers from zero
to 125 pairs per 10 ha (Morris et al. 1958). Because
in each outbreak the numbers of the birds rose over
several years in parallel with the caterpillars, it was
hard to tell how much the increase was due to immigration and how much to the high local breeding
success, though both were involved.
The role of previous breeding success in the
annual population changes of some neotropical
migrants was confirmed in studies in the Hubbard
Brook Experimental Forest in New Hampshire
(Holmes et al. 1991, 1996, Rodenhouse & Holmes
1992, Sherry & Holmes 1992). Over a period of
years, and in different forest plots, warbler breeding
densities fluctuated in parallel with caterpillar biomass the previous summer. There was thus a correlation between the production of young in one year
and the numbers of breeders returning the next. In
other words, breeding success rather than overwinter
survival had most influence on the annual changes in
breeding numbers, and increases in breeding density
were marked by greater recruitment of yearlings
(Sherry & Holmes 1988, 1992). These findings were
apparent in the American Redstart setophaga
ruticilla, Black-throated Blue Warbler Dendroica
caerulescens, Black-throated Green Warbler D. virens,
Red-eyed Vireo Vireo olivaceus and others, so in
these species the year-to-year changes in breeding
success and in subsequent breeding numbers were
attributable to fluctuations in summer food supplies.
Elsewhere, a relationship between the annual
recruitment of yearlings to a breeding population
and fledgling production the previous year was
noted in another neotropical migrant, the Prairie
Warbler Dendroica discolor (Nolan 1978), paralleling
the Pied Flycatcher and others in Europe (Fig. 4;
Virolainen 1984, Stenning et al. 1988).
Population limitation in migrants
The abundance of foliage-gleaning birds in the
Hubbard Brook Forest, especially of warblers and
vireos, increased in the early 1970s, coincident with
a major outbreak of defoliating caterpillars (Holmes
1990). Furthermore, natural and experimentally
induced declines in food (mainly caterpillar) abundance were shown to reduce the frequency of
re-nesting and second-brood attempts, as well as
growth-rates, hatching and fledging success, and to
increase the frequency of nestling starvation. The
general pattern seemed to be that food was abundant
for birds in this forest only during lepidopteran
irruptions, but such occasions occurred infrequently,
perhaps once every 10–20 years in any one forest
stand. Between these outbreaks birds probably experienced prolonged periods of food limitation, as
caterpillars were scarce for several years in succession.
The neotropical migrants thus showed population
trends in this forest that differed from those of
residents and short-distance migrants, whose breeding
numbers fluctuated from year to year mainly in line
with winter severity.
The importance of these findings in our present
context is twofold. First, they show that events in
breeding areas that influence breeding success can
influence subsequent breeding densities, apparently
over-riding any effects of events in wintering areas.
Secondly, they show that food supply can affect
breeding success and breeding numbers in addition
to any effects of predation and parasitism. The question then arises as to what extent recent declines
in the numbers of certain migrant species in North
America were due to recent declines in caterpillar
densities. Budworm caterpillars were abundant over
wide areas in the late 1940s and again in the 1970s.
One would therefore expect a decline in the numbers
of some species in the 1950s−60s and again in the
1980s−90s. In addition, since the 1950s, insecticides
have been used widely in eastern Canada to suppress
caterpillar outbreaks. This practice is likely not only
to have obliterated the ‘good food years’, but also to
have suppressed all insect populations in the affected
areas in the years of spraying. The influence of food
supply on migrant population trends could thus be
substantial and may have contributed to the declines
in some species witnessed in the 1980s−90s.
Another factor thought to operate on breeding
areas to reduce migrant numbers is summer drought,
again through its influence on insects and other food
supplies. Severe declines in the numbers of several
neotropical migrant species in Wisconsin and Michigan
during 1986–88 coincided with drought (Blake et al.
215
1992), as did similar declines in Illinois (Robinson
1992). Because the migrants do not start nesting
until June, they are more vulnerable to the effects
of drought than are residents and short-distance
migrants that start in April or May. Such effects
would be expected to be short-lived, however, and
followed by population recoveries as the weather
returned to normal. In addition, long-term changes
in the species composition of particular forest areas
would be expected from the successional changes
that occur as the forest matures, making the habitat
less suitable for some species and more suitable
for others (for examples see Litwin & Smith 1992,
Holmes & Sherry 2001).
Further evidence for the importance of events in
breeding areas for declines in Nearctic–Neotropical
migrants has come from studies in wintering areas.
For example, between 1972 and 1990, birds were
systematically netted in the Guanica forest in Puerto
Rico (Faaborg & Arendt 1992). Over the period concerned, the numbers of wintering migrants steadily
declined. Some species, such as Northern Parula Warbler
Parula americana and Prairie Warbler Dendroica
discolor, were common at the start, but virtually
absent in later years, whereas other migrants, such as
Black-and-white Warbler Mniotilta varia, and various
resident species maintained their numbers. As no
obvious change occurred in the forest itself during
the study, the declines were attributed to events in
breeding areas.
Most of the evidence thus points to changes in the
eastern North American breeding areas as being the
main cause of recent declines in the numbers of some
forest-dwelling neotropical migrants. Such changes
may not have occurred over the entire range or in
every wood, but they have clearly occurred in a
sufficiently large proportion of small woods to put
the regional populations of some species into decline.
Only time will tell how long the declines will
continue.
Factors operating in wintering areas
Over the same period, much tropical forest has been
destroyed. The highest rates of deforestation have
occurred in Central America and the Caribbean, the
very regions where forest migrants are most concentrated (Myers 1980, Terborgh 1989). On present
evidence, it is impossible to say how much tropical
deforestation has contributed to the declines of
forest migrants, but if it continues at the recent rate, it
may soon overtake events in the breeding areas as
the major cause of declines. However, not all species
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I. Newton
are likely to have been affected adversely because,
as mentioned above, some thrive in the secondary
habitats that replace the forest.
One species already affected by tropical deforestation, according to Terborgh (1989), is Bachman’s
Warbler Vermivora bachmanii, now probably extinct.
At the turn of the century, this species bred widely
in damp broadleaved woods across the southern
United States, but by the 1950s it could be seen reliably only at a few places in coastal South Carolina.
All wintering records came from Cuba, from the dense
evergreen thicket that once covered large parts of
the island, and which has since been largely replaced
by sugar cane. Almost certainly, this warbler
has succumbed primarily through destruction of its
wintering habitat, for substantial areas of breeding
habitat still remain.
All migrants are affected by conditions in both
breeding and wintering areas, and in recent years
species have presumably been adjusting to changes
in one or other area. However, the habitats occupied
by some species, in both breeding and wintering
areas, are so vulnerable to human activity that
such species seem destined to decline markedly in
the coming years. The Cerulean Warbler Dendroica
cerulea has suffered extensive loss of breeding habitat
in the past 200 years (Robbins et al. 1992). Its favoured
nesting habitat, mature floodplain forest with tall
trees, has become scarce over most of its original
nesting range, and its apparent sensitivity to fragmentation of remaining tracts gives it an additional
disadvantage. In winter, the species is restricted to
primary humid evergreen forest along a narrow
elevation zone at the base of the Andes. This zone is
among the most intensively logged and cultivated
regions of the Neotropics. From 1966 to 1989, the
Cerulean Warbler showed the most precipitous
decline of any North American Warbler (3.4% per
year nationwide).
Non-forest species
Migrants dependent on natural grassland, such as the
Sprague’s Pipit Anthus spragueii and Upland Sandpiper
Bartramia longicauda, declined greatly after the
ploughing of the prairies, but species that can live
in agricultural habitats, such as the Indigo Bunting
Passerina cyanea, greatly expanded after the felling of
the eastern deciduous forest. Of more recent changes,
some have been attributed to changes in breeding
areas and others to changes in wintering areas.
Thus the Bobolink Dolichonyx oryzivorus has been
declining in the eastern United States since the early
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
1900s. It nests mainly in hayfields, and decline has
been attributed to procedural changes that make
hayfields less suitable as habitat (Bollinger & Gavin
1992). By contrast, an earlier decline of the Dickcissel
Spiza americana, which inhabits brushy pastures,
was put down to destruction of wintering habitat,
resulting from overgrazing by cattle (Fretwell 1980).
Most arctic nesting shorebirds that winter on
neotropical coastlines have so far been spared the
effects of large-scale habitat destruction. Their breeding
habitats are still largely intact, whereas their wintering
habitats may have expanded as a result of the soil
erosion following deforestation, and the resulting
expansion of coastal mud. During part of the 20th
century, some shorebird species increased in numbers
in response to lessened shooting pressure, which had
reduced their numbers in the past. Some raptors,
notably Swainson’s Hawks, suffered from pesticideinduced mortality in wintering areas in Argentina.
Around 5000 individuals were picked up under their
communal roosts, after the spraying of their food
organisms (grasshoppers) with the organophosphate
compound, monocrotophos, now banned in this region
(Goldstein et al. 1996). Earlier declines in Peregrine
Falcon Falco peregrinus numbers were attributed to
the use of organochlorine pesticides in both breeding
and wintering areas, and recent recoveries in Peregrine numbers have followed reductions in organochlorine use (Cade et al. 1988). It is more difficult to
explain recent declines in wetland and coastal species,
such as Least Tern Sterna antillarum, which in eastern
North America declined by 14% per year during
1978–88 (Sauer & Droege 1992).
Comparison between Old and New World
systems
Although in the last 30–40 years some tropical
migrant birds have declined in western Europe and
others in eastern North America, the causes seem
to have differed. In Europe, declines have mainly
involved species that winter in the arid savannas of
tropical Africa, which have suffered from the effects
of drought and increasing desertification. In several
species, annual fluctuations in numbers and adult
survival rates were correlated with annual fluctuations in winter rainfall, and by implication in winter
food supplies. There were no obvious changes in
breeding success that could be linked with population changes.
In North America, by contrast, declines have affected
many species that breed and winter in forest. In
Population limitation in migrants
eastern forest regions, declines have been attributed
to human activities on the breeding range, particularly forest fragmentation and associated agricultural
and suburban developments, which have led not
only to loss of forest, but to increases in the densities
of nest predators and parasitic cowbirds. Declines in
the numbers of some migrants are thought to result
from reduced breeding success, which is now too
low to offset the usual adult mortality, though as
yet convincing evidence is available for only a few
species, such as Kirtland’s Warbler and Wood Thrush
in some areas. In other species, such as Bachman’s
Warbler, tropical deforestation seems to have played
the major role in population decline, and is likely
to affect an increasing range of species in the future.
Whereas for the Palaearctic–Afrotropical system, a
considerable consensus exists on the causes of declines,
for the North American system, much of the evidence
is as yet little more than suggestive, and no one explanation can account for all the facts (for discussion see
James & McCulloch 1995).
Experimental evidence on population
limitation in migrants
Most of the evidence discussed above on the causes
of population changes, whether for Old World or for
New World species, is based on correlation and inference. Experiments are almost impossible to perform
on regional scales, but widespread changes in landuse or legislation have sometimes been followed
by widespread changes in bird populations. Examples
include the increases in many waterfowl and shorebird populations that followed the introduction of
protective legislation or other conservation measures,
or the increases of bird-of-prey populations that
followed reductions in the use of organochlorine
pesticides (Newton 1979, 1998, Cade et al. 1988).
In addition, local population increases have followed
experimental manipulations of potential limiting
factors or the introduction of specific conservation
measures. Examples include increases in breeding
success and breeding density that were associated
with: (1) removal of predators (for various ducks see
Duebbert & Kantrud 1974, Duebbert & Lokemoen
1980, for Sandhill Crane Grus canadensis see Littlefield 1995); (2) removal of Brown-headed Cowbirds
(for Bell’s Vireo see Griffith & Griffith 2000, for
Black-capped Vireo see Hayden et al. 2000); (3)
provision of extra nest-sites for species with special
needs (for many species of cavity-nesting and other
birds see Newton 1994, 1998); (4) removal or exclusion
217
of competing species (for effects of tit Parus removal
on Pied Flycatcher densities (via access to nest-sites)
see Gustafsson 1988); and (5) change or intervention
in destructive agricultural procedures (for Corncrake
Crex crex see Green et al. 1997, Green 1999). Similar
measures have led to population increases in some
resident bird species too, as has the provision of
additional winter food (Newton 1998). Restrictions
in hunting pressure and pesticide use have affected
wintering populations, as well as local breeding
populations, but the remaining types of experiments
were restricted to breeding populations. They confirm that many migrant species were at that time and
location limited by conditions in breeding areas.
IMPOR TANCE OF STOPOVER SITES
Although most studied declines in the numbers of
migratory bird species have been attributed to events
on the breeding or wintering areas (or both), some
might have been caused by events at localities that lie
on the migration routes, in crucial staging areas that
individual birds may visit only for days or weeks each
year, but nevertheless provide essential refuelling
points. In order to accumulate the body fat necessary for migration, birds need to obtain more food
per day than is usual. Moreover, because the same
stopping sites can be used by large numbers of birds
at a time, competition is often intense, and as wave
after wave of migrants passes through, food supplies
can be severely depleted. The potential for limitation on staging areas is perhaps especially acute in
shorebirds and waterfowl, which in many regions
have only a limited number of possible refuelling
sites. The quality of any stopover site depends, of
course, not just on the available food supplies, and
levels of competition, but also on the security that the
site offers against predation, disturbance and other
threats. For example, in springs when Greater Snow
Geese Chen caerulescens atlantica were exposed
to hunting on their main staging area in eastern
Canada, they migrated in poorer condition, and laid
later and smaller clutches than in years when hunting
at this crucial site was banned (Bêty et al. 2003).
Marked declines in food supplies, mainly through
depletion, have been measured at stopover sites during the migration season. This was done by excluding
birds from some places, and comparing the trends in
prey populations inside and outside the exclosures.
For example, a 60% decline in the total standing crop
of macro-invertebrates and 7–90% declines in different
prey species were recorded during the shorebird
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I. Newton
passage (July–September) in Massachusetts (Schneider
& Harrington 1981). Similarly, some passerine
migrants were found to depress food supplies rapidly
by up to 67% at coastal stopover sites in spring
following their migration over the Gulf of Mexico
(Moore & Yong 1991).
The rate of fattening in particular species has
varied between sites or years according to local food
supplies, and was assumed to influence the overall
speed of migration. At particular sites, individual
birds remained longer and accumulated fat more
slowly at times when food was scarce, or at times when
the density of competing birds was high (Bibby &
Green 1981, Cherry 1982, Moore & Yong 1991, Kelly
et al. 2002, Whalen & Watts 2002). For example, at
two different stopover sites in France, Sedge Warblers
gained weight at 0.42 g (se = ±0.07) per day and at
0.05 g (se = ±0.10) per day (Bibby & Green 1981).
Similarly, the rate at which migrants gained weight
varied from year to year at the same site: Sedge
Warblers gained 0.40 g, 0.05 g and 0.55 g per day in
1973, 1974 and 1975, respectively (Bibby et al. 1976).
In 1974, when the rate of weight gain was low, 84%
of Sedge Warblers stayed more than 2 days. But in
1973 and 1975, when the rate of gain was greatest,
only 46% stayed two or more days. Similarly, Dunlin
Calidris alpina stayed on a Moroccan estuary for an
average of 16 days when food was scarce, but only
11 days the following spring when food was abundant (Piersma 1987). Slow rates of fattening, which
delay migration at early staging sites, could affect
the rates of fattening at later sites if birds arrive after
food supplies have been depleted (the domino effect
of Piersma 1987), and migrants that reach breeding
areas late or in poor condition may fail to breed that
year (Newton 1977, Johnson & Herter 1990, Berthold
et al. 2002).
Density-dependent patterns of habitat settlement
during stopovers (e.g. Veiga 1986) also implied
competition, as did observations revealing that
dominants gained weight faster than subordinates
(Lindström et al. 1990). Some species behaved
territorially at stopover sites, and individuals were
unable to gain weight until they had acquired a territory,
with its associated food supplies (Rappole & Warner
1976, Mehlum 1983). Competition may thus result
in a longer stopover, or cause a migrant to leave with
low fat reserves, reducing the distance it could travel
before the next stop. In many species, adults trapped
at stopover sites were heavier, with greater body
reserves, and stayed for shorter periods than first-year
birds; and within age-groups, males were heavier,
© 2004 British Ornithologists’ Union, Ibis, 146, 197–226
with greater body reserves, and stayed for shorter
periods than females (Veiga 1986, Ellegren 1990,
Morris et al. 1996, Woodrey & Moore 1997, Yong
et al. 1998). Such differences could influence the
travel speeds, departure and arrival dates, and survival
chances of individuals during migration.
A link between the body condition achieved at
migration sites and the subsequent performance
of individual birds has emerged in several studies.
The body weight achieved at stopover sites has been
found in some species to correlate with subsequent
survival or, more strictly, return rates (for Great
Reed Warbler see Nisbet & Medway 1972; for
Semipalmated Sandpipier Calidris pusilla see Pfister
et al. 1998; for Red Knot Calidris canutus see Baker
et al. 2004). The assumption is that return rates do
indeed reflect survival, and not simply the movement of poor-performing birds to other staging or
wintering sites, but this remains uncertain. No such
doubt hangs over the relationship between spring
body condition and subsequent reproductive success,
as shown in Brent Geese and others (see above).
In general, geese in good condition in spring were
more likely to return in autumn with young than
were geese in poor condition in spring (Ebbinge &
Spaans 1995). These various findings indicate that
poor conditions on wintering and stopover sites can
reduce the survival and reproduction of individuals;
but whether these effects influence subsequent
population size depends on the extent to which
they are offset by improved survival or breeding in
the remaining birds.
Where a food supply is depleted by passage
migrants, latecomers arriving after most of the food
has been eaten could be penalized by reduced rates
of fat accumulation, and eventually by the inability
to form reserves. This situation is exemplified by
Bewick’s Swans Cygnus columbianus bewickii studied
at a staging site in the White Sea, the last stop on
spring migration before arrival on the Siberian
breeding grounds (Nolet & Drent 1998). In this
locality, the Swans could feed on their main food,
tubers of Fennel Pondweed Potamogeton pectinatus,
only during low tide. In the course of the staging
period, the Swans tended to forage at progressively
lower water levels, indicating that they gradually
depleted this food supply, and exploited increasingly
deeper parts of the tuber bank as the days went by.
This depletion reduced the Swans’ main foraging
period from 6.0 h per tide on 20 May to 3.3 h per tide
on 28 May. The authors calculated that this must
have greatly reduced the rate of refuelling during
Population limitation in migrants
the staging period. In accordance with this, Swans
arriving early stayed for shorter periods than those
arriving late. It seemed of paramount importance for
the Swans to arrive at the stopover site as soon after
ice break-up as possible, because a month later the
tubers were greatly depleted and those remaining
began to sprout. The first Swans to arrive could also
leave the site first, and (in theory) reach the breeding
grounds earliest, get the best territories and achieve
the highest reproductive success. The latest Swans to
leave would have arrived on the breeding grounds too
late to breed that year. In such a situation, the more
Swans that fed there, the greater the proportion
likely to have been excluded from breeding.
Hence, this swan study provided an example of how
competition for limited food supplies at a stopover
site, used for no more than a few weeks each year,
could have helped to regulate the population. The
White Sea provides the only sizeable stopover site
for swans on this part of the spring migration route,
so with such severe competition at this crucial
site, the birds would be limited in how much they
could respond to any improvements in conditions that
might occur in their breeding or wintering areas.
The above seven paragraphs illustrate the main
types of evidence proposed to support the view that
events at stopover sites could act to limit migrant
populations. Food supplies at staging sites can be
heavily depleted, slowing rates of fattening, which in
turn delays migration, in some individuals in spring
to the extent that it reduces breeding success, or prevents breeding altogether. So stopover events clearly
affect individuals, but whether these effects are
sufficient to reduce population sizes below what they
would otherwise achieve remains, for most species,
an open question. In only one study known to me has
a marked decline in a bird population been firmly
tied to a change in conditions at a major stopover
site. This is the recent catastrophic decline over three
years (from 51 000 to 27 000 individuals between
the years 2000 and 2002) of the Red Knot C. c. rufa
population that breeds in arctic Canada and winters
in Tierra del Fuego. Decline coincided with collapse
(through human overfishing) of the Horseshoe Crab
Limulus polyphemus population, the eggs of which form
the food of Knots at their stopover site in Delaware
Bay (Baker et al. 2004). This locality is the last refuelling site of these birds en route to their arctic
breeding areas. From 1997 to 2002, increasing proportions of Knots studied in the Bay failed to reach
the threshold departure mass of 180–200 g. Survival
of adults fell by 37%, and the proportions of second-
219
year birds in wintering flocks by 47%. Of birds
caught in the Bay, known survivors were heavier
at initial capture than were birds not seen again. In
view of this example, the situation for spring migrants
passing through the Sahel zone of Africa in drought
years might well repay more study.
CONCLUDING REMARKS
In this brief review, I have been concerned mainly
with changes in the numbers of migratory birds, and
with the relative importance of events in the breeding
and wintering areas in influencing the changes. In
the longer term, however, population limitation in
migrants is probably a dynamic process, involving both
areas. If conditions in the wintering range permit
increased survival, a species could become more
numerous, and may expand its breeding distribution,
perhaps into places where the production of young
is lower. Similarly, if conditions in the breeding range
permit increased production of young, the species
could become numerous enough to expand its
wintering range, perhaps into areas where survival
is lower. In this way, the summer and winter ranges
of migrants will tend to expand until reproduction
and mortality balance (see also Cox 1985). It is only
during a period of change, as in recent decades, that the
breeding or wintering range may emerge as providing
the stronger limitation. These speculations assume
that there are indeed vacant areas into which migrants
could expand if their numbers rose. For most species
this is probably true; either in other habitats within
the existing range or in other suitable areas outside it.
Various populations of geese, which have increased
greatly in the past 50 years, provide some instructive
examples. As their numbers have grown, they have
expanded both breeding and wintering ranges, and in
some populations, constraints to further population
growth have shifted increasingly from wintering to
breeding areas. This is shown in various ways, notably
by the gradually decreasing proportions of young in
some wintering populations (Fig. 5).
Migratory birds depend on encountering suitable
conditions at all staging points on their routes. If
conditions deteriorate at any one point, a bottleneck
might develop that could begin to limit the population. When conditions are deteriorating everywhere
at once, it becomes hard to pinpoint that bottleneck
except in the most obvious cases. But the fact that
migrants use two or more essential areas each year
means that they are inevitably more susceptible to
the effects of habitat destruction than are resident
© 2004 British Ornithologists’ Union, Ibis, 146, 197– 226
220
I. Newton
birds. Residents suffer only if their particular area is
destroyed, but migrants could suffer if any one of
several areas important to them is lost. In this sense
they have, on average, more chances than residents of
being affected – adversely or otherwise – by human
action. They live in multiple jeopardy.
Another much neglected aspect concerns the
interactions that occur between different populations
or different sectors of the same population. Thus, if
individuals from two or more breeding populations
occur together in the same staging or wintering areas
(like many shorebirds) where their numbers are
limited, and feed on the same prey, the dynamics of
the separate breeding populations can be interlinked,
as the survival rate of individuals from one population is likely to depend on the overall size of both
populations (Dolman & Sutherland 1995). Further
complexities arise where individuals from a single
population winter in different habitats or in different
geographical regions, as in partial migration or in
differential migration where birds of different sexes
or ages migrate to different regions. In such cases,
adversity in one region could have complex effects on
populations, leading to population decline through
poor recruitment, unequal sex ratios or other types
of demographic disruption, as yet largely unexplored.
With so many travel lanes in bird migration,
populations from different breeding or wintering
areas may often use the same stopover sites. This gives
great potential for competition: if one population
passes first, it may deplete the food stocks for later
ones, and if different populations are present at the
same time, the individuals in both may suffer from
depletion and interference. In other words, although
living apart for most of their lives, the annual few
weeks of contact on stopover sites could ultimately
influence the size of one or both of any two competing
populations. This is an aspect of stopover biology that
has so far received little attention, but it could have
enormous repercussions for competing populations.
I am grateful to Andy Gosler, in his capacity as editor, for
encouraging me to write this article, and to Will Cresswell
and Bill Sutherland, in their capacity as referees, for helpful comments on the manuscript.
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Received 17 November 2003;
revision accepted 19 February 2004.