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MIGRAZIONE : uno straordinario
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Evolution of migration
Several lines of evidence suggest that many birds have an innate capacity to migrate. For example,
several non-migratory, resident species of birds, including Stonechats (Saxicola torquata; Helm and
Gwinner 2006), Silvereyes (Zosterops lateralis; Chan 1994), and White-crowned Sparrows
(Zonotrichia leucophrys; Smith et al. 1969), have been found to exhibit migratory restlessness (or
zugunruhe). In addition, recent comparative studies indicate that migratory behavior has evolved
repeatedly and very rapidly in different avian lineages (Helbig 2003, Outlaw et al. 2003, Joseph
2005, Davis et al. 2006, Outlaw and Voelker 2006). As an example of how rapidly migratory
behavior can develop, House Finches (Carpodacus mexicanus) from a largely sedentary population
in southern California were introduced on Long Island, New York, in about 1940 and, by the early
1960s, many of these eastern House Finches had become migratory (Able and Belthoff 1998).
Based on such observations and studies, some investigators have suggested that migration has been
a common and widespread characteristic of birds for many millions of years and, if so, then many
present-day birds may have inherited the capacity to migrate from their ancestors (Berthold 1999).
In other words, given the proper environmental triggers, this innate migratory program (i.e.,
‘migratory syndrome’) is activated and allows populations or species of birds to rapidly become
migratory. Although there is disagreement about the existence of this migratory syndrome (e.g.,
Piersma et al. 2005), available evidence seems to suggest that most, if not all, birds have the innate
potential to migrate (Salewski and Bruderer 2007). If true, what is the source of this potential?
Unfortunately, as pointed out by Zink (2002), reconstructing the environments where birds evolved
and determining when birds first migrated is not currently possible. However, it is reasonable to
conclude, as also suggested by Steadman (2005), that migratory behavior existed early in avian
history. Thus, for millions of years, natural selection has acted upon some or all of the physiological
and behavioral components important in avian migration, maintaining a diversity of migratory
behaviors, including, often, its suppression (i.e., non-migratory behavior). However, by removal of
the suppression of those genes controlling the various components of migration, migratory behavior
can quickly re-appear in a population or species. Among living birds, then, the expression of
migratory activity is subject to selective pressures, with those pressures determining if birds are
sedentary, short-distance migrants, or long-distance migrants. It is always the case that additional
study can alter current ideas or hypotheses. However, available evidence does seem to support the
migratory syndrome hypothesis and that means that, when examining present-day birds, the focus
must shift from explaining the actual ‘evolution’ of migration (because migratory behavior likely
does not evolve de novo) to trying to determine what factors or selective pressures currently acting
on birds have contributed to their current migratory, or non-migratory, behavior.
Differential and partial migration
Among some species of migratory birds, all individuals in a population or species may share the
same general breeding and wintering areas, with no spatial separation of adults and juveniles or
males and females. However, for other species, particularly among short- and medium-distance
migrants, wintering areas may vary with sex, age, or both. Some individuals may migrate whereas
others do not (partial migration) or some individuals migrate greater distances than others
(differential migration; Figure 14).
Among partial and differential migrants, a number of factors can
potentially influence either a bird’s decision to migrate or not or how
far to migrate, including age, sex, physical condition, size, and
dominance status. In addition, the migratory behavior of partial migrants can either be
obligate, i.e., genetically (innately) fixed at the individual level (Lundberg 1988), or facultative,
with migratory decisions based on conditions that can change over time (Ketterson and Nolan
1983).
Several hypotheses have been proposed to explain partial and differential migration. The Arrival
Time hypothesis proposes that the individuals that establish breeding territories are less likely to
migrate or, if they migrate, to migrate shorter distances because remaining in or near breeding areas
makes it more likely that they will be able to acquire (or reacquire) high-quality territories (King et
al. 1965). The Dominance hypothesis suggests that migratory decisions are based on dominance
status, with subordinate individuals in a population more likely to migrate because, if they stay or
migrate shorter distances, dominant individuals are likely to out-compete them for access to needed
resources (Gauthreaux 1982). The Body Size hypothesis posits that larger individuals with smaller
surface area-to-volume ratios are less likely to migrate or to migrate long distances because they are
better able to withstand colder temperatures and food shortages (Ketterson and Nolan 1976). These
hypotheses are all based on the results of studies conducted in north temperate areas where there
can be extreme seasonal differences in environmental conditions (e.g. temperature and day length)
and food availability. In addition, all three are based on the assumption that staying further north
can be costly due to adverse weather conditions, but can also be beneficial because of shorter
migration distances. Testing these hypotheses is often difficult because, in many species of birds,
males are larger, dominant, and establish breeding territories. In such species, all three hypotheses
lead to the same prediction: larger, dominant males should winter further north. These hypotheses
are also not mutually exclusive; multiple factors can contribute to the evolution of partial migration.
As an example of the difficulty in differentiating among these hypotheses, White-throated Sparrows
(Zonotrichia albicollis) breed across most of eastern Canada and the northeastern United States and,
during the non-breeding season, migrate as far south as the Gulf of Mexico. These sparrows exhibit
differential migration, with males tending to winter further north than females (Figure 12). Male
White-throated Sparrows are larger than and dominant to females (Piper and Wiley 1989), and
arrive in breeding areas one to two weeks earlier than females to establish territories (Falls and
Kopachena 2010). Thus, any or all of the proposed hypotheses (Arrival Time, Dominance, or Body
Size hypotheses) could explain differential migration by White-throated Sparrows.
Some species, however, have characteristics that make them suitable for testing these hypotheses,
and studies have revealed interspecific differences in the factors that have led to the evolution of
partial migration. For example, House Finches (Carpodacus mexicanus) in the eastern United States
exhibit differential migration, with males tending to winter further north than females. This
difference appears to be best explained by the Body Size hypothesis (Belthoff and Gauthreaux
1991) because male House Finches do not defend territories (and so, based on the Arrival Time
hypothesis, they have no need to winter closer to breeding areas) and females are typically
dominant to males (so, based on the Dominance hypothesis, females should winter further north).
However, male House Finches are larger than females and, as predicted by the Body Size
hypothesis, should winter further north because they can better cope with colder temperatures and
reduced food availability.
Partial migration by Lesser Black-backed Gulls (Larus fuscus) appears to be best explained by the
Arrival Time hypothesis (Marques et al. 2010). Older Black-backed Gulls tend to winter further
north, closer to breeding areas, than younger gulls. These gulls exhibit minimal variation in body
size so the Body Size hypothesis cannot explain the age-related difference in migration distance.
The Dominance hypothesis predicts that dominant gulls (those 4 or more years old) should winter
closest to the breeding grounds. However, three-year old gulls that will be breeding for the first time
winter as close or even closer to breeding areas than many older, more dominant gulls, suggesting
that begin close and arriving early in breeding areas best explains the winter distribution of Blackbacked Gulls (Marques et al. 2010).
Few studies have provided support for the Dominance hypothesis. However, Kjellén (1994) found
that, in several species of raptors, juveniles were more likely to migrate than adults and, in addition,
females were less likely to migrate than males. Adult raptors are dominant to juveniles and exhibit
reversed sexual dimorphism, with females larger than and dominant over males. These results,
therefore, support the Dominance hypothesis, with dominant adults and larger, more dominant
females tending to winter further north.
Few investigators have examined partial migration in the tropics where wet-dry cycles predominate.
However, a recent study of Tropical Kingbirds (Tyrannus melancholicus) provided support for the
Food Limitation hypothesis (Jahn et al. 2010). This hypothesis predicts that, among insectivorous
species, larger individuals with greater energetic needs are more likely to migrate to wetter areas to
find sufficient food. In contrast to the other three hypotheses where larger individuals, generally
males, are predicted to be less likely to migrate, Jahn et al. (2010) found that the largest male
Tropical Kingbirds that were typically older and dominant over younger individuals were most
likely to migrate from breeding areas. Because Tropical Kingbirds feed on flying insects and never
forage in flocks, dominance status has less effect on their ability to access resources (compared to
many granivores and omnivores that feed in flocks). What is more important is the abundance of
flying insects. When insect availability drops during the dry season (coinciding with the nonbreeding period), larger males may be unable to meet their energetic needs and must migrate to
wetter areas with more insects. In contrast, smaller individuals require less energy and fewer insects
and need not migrate.
Stopover sites
Migrating birds rely on stored energy and nutrients to fuel their flights, and many birds, especially
small landbirds, cannot store enough energy to fly nonstop between breeding and wintering areas.
So, for most birds, migration is divided into alternating periods of flight and stopover, with time at
stopover sites spent foraging to deposit fuel for the subsequent flight(s) (Figure 18). The time spent
at stopover sites is influenced by a bird’s condition when arriving at a site and by conditions, such
as food availability and weather, at the site. The overall speed of migration is greatly influenced by
the time spent at stopover sites, and this speed can be of critical importance because it determines
when migrants arrive at breeding and wintering sites.
Experiments suggest that stopover duration is short if foraging success is poor and fuel deposition
rates are low or negative (Biebach 1985, Yong and Moore 1993). More generally, Schaub et al.
(2008) found that birds that accumulated fuel stores at medium rates remained at stopover sites
longer than those that either lost fuel stores during their stopover or were able to increase their fuel
stores quickly. However, the decision about when to leave a stopover site also appears to be
influenced by the location of a site, with birds at sites located just before a large ecological barrier
(e.g., a desert) generally stayed long enough to deposit sufficient fuel to cross the barrier (Schaub et
al. 2008).
Yet another factor that can influence stopover duration is weather. A number of studies have
demonstrated that birds tend leave stopover sites when winds are favorable (tailwinds; Richardson
1990, Liechti and Bruderer 1998). However, precipitation can be a complicating factor; rain, for
example, can saturate a bird’s plumage, increase wing loading, and increase rates of heat loss
(Newton 2007a) so birds typically do not leave stopover sites during periods of precipitation.
However, because early arrival times at breeding and wintering sites can be critically important,
extended p eriods of harsh weather conditions (e.g., precipitation) can force birds to depart from a
site even when weather conditions are not optimal, e.g., when there are head-winds (Erni et al.
2002, Jenni and Schaub 2003)
Diurnal vs. nocturnal migration
Many species of birds migrate at night, but some migrate during and day and still others are flexible
and can migrate at any time of day. Most waterfowl, including ducks and geese, are flexible and
migrate at various times of day. However, most long-distance migratory songbirds and shorebirds
migrate only at night, and some songbirds, including finches, swallows, and corvids, as well as
pigeons and doves migrate only during the day (Alerstam 1990).
An important advantage of nocturnal migration is that, for birds that forage during the day, more
time is available for foraging. Nocturnal migrants can initiate migration after a day spent foraging
and storing energy, whereas diurnal migrants must balance flight time and foraging during the day
and spend the night roosting and sleeping. As a result, nocturnal migrants can spend more time
flying, resulting in faster migration (Alerstam 2009). The potential importance of maximizing
foraging time is perhaps best illustrated by shorebirds. Lank (1989) found that shorebirds typically
initiate migratory flights at dusk, after light levels made foraging difficult, but also noted that
shorebirds sometimes initiated migratory flights during the day if foraging at a particular location is
prevented (e.g., due to rising tides).
Beyond increased foraging and flight time, nocturnal migration may also be advantageous because
flying conditions are typically better, with less atmospheric turbulence, less wind, and reduced
evaporative water loss (because temperatures are cooler). Flying at night may also take less energy
because, with cooler temperatures and higher humidity increasing air density, generating lift is
easier (Kerlinger and Moore 1989, Alerstam 2009). In addition, with few or no aerial predators,
predation risk is lower for birds migrating at night (although, in some areas, birds risk predation by
bats; see below). Finally, many birds use navigational cues that are only available at dusk or at night
(e.g., stars).
This animation created by Cornell University researchers illustrates the use of a network of
surveillance weather radar
to record nocturnal migrating birds, bats, and insects in the continental U.S. from sunset to sunrise
on 1 October 2008. The
blocky green, yellow, and red patterns, especially visible on the east coast, represent precipitation,
but, within an hour
after sunset, radar picks up biological activity, as seen in the widening blue and green circles
spreading from the east
across the country. Birds take off, fly past, and get sampled by the radar beam. Note, the black areas
on the map do not
represent places without birds, necessarily, but rather places where radar does not sample
Bird migration and climate change
For birds that breed at mid- to high latitudes, the time of arrival on their breeding areas has been
selected over time so they largely miss the adverse weather conditions of late winter and early
spring and arrive when food availability is increasing. Over the past several decades, however,
global temperatures have increased due to anthropogenic climate change, with corresponding
changes in the timing of seasonal climate conditions. These changes in temperature have not been
uniform across the globe; temperatures in the Northern Hemisphere, particularly at higher latitudes,
have increased more than those in the Southern Hemisphere, and temperatures in Europe and
northern Asia have increased more than those in most of North America (excluding parts of Alaska;
Figure 24)
Figure 24. Increases in temperatures during the period from 2000 to 2009 compared to average
temperatures recorded between 1951 and 1980.
The most extreme warming (shown in red) was in the Arctic. Few areas had cooler temperatures
(shown in blue). Gray areas over parts of the
Southern Ocean are places where temperatures were not recorded. Note that temperatures have
increased more in Europe and Asia
than in North America. (Source: http://en.wikipedia.org/wiki/File:GISS_temperature_200009_lrg.png).
Not surprisingly, increasing temperatures have altered the timing or phenology of biological
processes in many areas, but particularly at higher latitudes in the Northern Hemisphere. For
example, in the Arctic, where annual average temperatures have increased at almost twice the rate
of the rest of the world (Callaghan et al. 2005; Figure above), warmer winters and earlier springs
have advanced the date of peak abundance of arthropods by about 7 days (Tulp and Schekkerman
2008). Climate change has also altered the phenology of migration for many species of birds. For
example, Végvári et al. (2010) determined first-arrival dates (the day on which the first individuals
of a migratory species is observed in the spring) of migrants in eastern Hungary (47°N latitude) and
found that, during the period from 1969 to 2007, the time of arrival was significantly earlier for 45
of 177 species (25.4%; Figure 25). First-arrival dates of short-distance migrants (species wintering
north of the Sahara Desert) were advanced more than those of long-distance migrants (species
wintering south of the Sahara in tropical Africa) (Figure 25a below). The shorter the migration
distance, the greater the tendency for earlier arrival dates (Figure 25b below). Similarly, Murphy-
Klassen et al. (2005) examined a 63-year dataset (1939 to 2001) of first spring sightings for 96
species of birds in Manitoba (50° N latitude) and found that 25 species (26%) had significant trends
for increasingly early arrival. As also found in Hungary, more short-distance migrants (33.2% of
species) exhibited significant tendencies for earlier arrival dates than long-distance migrants (18.8%
of species).
Available evidence indicates that a mismatching of food demand and availability is having a
negative impact on populations of some species of migratory birds, but relationships between global
warming, food availability, and timing of bird migration are complex. For example, species of birds
breeding in the same habitat may experience different mismatches or be able to avoid such
mismatches due to subtle differences in food habits (Jones and Cresswell 2010). In addition, climate
change may cause shifts in or expansion of breeding and wintering ranges, affecting the degree to
which food demand and availability might or might not mismatch. Clearly, additional study is
needed to better understand how global warming is impacting the behavior, breeding success, and
population status of migratory birds. Although increasing global temperatures have clearly altered
the migratory behavior of many species of birds, the potential future impacts of global warming on
birds are likely to be even more pronounced. If, as predicted by some, global mean temperatures
increase by up to 8 degrees C by the year 2100 (Figure 27), the resulting changes in climate and sea
levels will undoubtedly have tragic consequences for many species of birds.
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