Download Spring and Autumn Migration in Arctic Shorebirds: Same Distance

AMER. ZOOL., 35:222-233 (1995)
Spring and Autumn Migration in Arctic Shorebirds:
Same Distance, Different Strategies1
KATHLEEN M. O'REILLY AND JOHN C. WINGFIELD
Department of Zoology, NJ-15, University of Washington, Seattle, Washington 98195.
The Arctic is an extremely inhospitable region for most of
the year, but during the summer months it bursts with life. A major
proportion of avian species nesting in the Arctic are shorebirds (order
Charadriiformes; suborder Charadrii). They migrate thousands of kilometers from their wintering grounds to take advantage of abundant food
resources each summer and display a variety of migratory strategies. In
an attempt to classify this variation, not only between spring and autumn
migration, but within a migration, we present four categories. These relate
to the distance a species generally flies between stopovers: short distance
bout, intermediate distance bout, long distance bout, and combinations.
We then explore further differences between spring and autumn migration.
Spring migrants experience poor weather and decreased food availability
as they fly north. Many cope with huge flocks, which serve as protection
from predators, but may also reduce foraging efficiency and increase
aggression. In contrast, autumn migrants generally encounter favorable
weather and ample food. Flock sizes are usually smaller, thus foraging
efficiency is higher and aggression lower than during spring migration.
Physiologically, spring migrants are preparing for breeding and reproductive hormones are secreted. In the Western Sandpiper {Calidris mauri),
luteinizing hormone levels are higher for spring than autumn migrants.
Late spring migrants have higher testosterone levels than either early
spring migrants or autumn migrants. Corticosterone levels are also higher
in spring vs. autumn migrants. Although spring and autumn migrants
travel similar distances, their strategies differ behaviorally and physiologically.
SYNOPSIS.
INTRODUCTION
der Charadrii) are typical migrants that fly
Migration is energetically expensive, given f r o m t h e i r wintering grounds in tropical and
the metabolic fuel required toflylong dis- temperate latitudes to their breeding grounds
tances, and hazardous, given the potential i n subarctic and arctic regions. After breedfor storms, increased predation etc. asso- m8> t h e y % s o u t h d u n n 8 autumn migration
ciated with crossing inhospitable habitat to escape severe winter weather and to take
such as oceans and/or deserts (Kersten and advantage of feeding opportunities in
Piersma, 1987; Bairlein, 1988; Biebach, warmer climates. Although many shore1990). However, the benefits of breeding in birds travel between fixed breeding areas
localities that are hostile except for a brief, a n d wintering locales (but see Hahn et al,
highly productive time (usually summer), U 9 9 5 1 o n s P a t i al opportunism in some speoutweigh the costs of migration. Note also ?ies)> s P n n 8 a n d autumn migrations differ
that migration involves at least two trips— i n m a n v ways.
Aft
er introducing different types of shoreflight to the breeding site and then the return.
bird
migration and the possible combinaShorebirds (order Charadriiformes: subortions of migratory bout strategies, in this
. „ .
,
, contribution we explore the behavioral,
. _
•From the Symposium Endocrinology ofArctic Birds
and Mammals presented at the Annual Meeting of the
American Society of Zoologists, 27-30 December 1993,
at Los Angeles, California.
prnino;rni
a n f i nhvsinirxnrai difference*
ecological, and physiological dltterences
between spring and autumn migration using
the Western Sandpiper {Calidris Mauri:
222
STRATEGIES OF SHOREBIRD MIGRATION
Scolopacidae) as an example, but drawing
from other North American scolopacid
shorebirds for comparison.
TYPES OF SHOREBIRD MIGRATION
As in other migratory birds, shorebird
migration patterns are determined by geography and food availability. However, unlike
many passerines, most shorebirds have only
limited access to food during migration. For
example, many passerines flying over land
may stop nearly anywhere to refuel where
seeds and insects are available (Ehrlich et
ai, 1988). Shorebirds, on the other hand,
require specialized habitats such as intertidal rocky shores, estuaries, mud flats, and
wetlands on which to feed on invertebrates
(Johnsgard, 1981; Myers et al, 1987; Paulson, 1993). This variation in geography and
food availability leads to variation in length
of flights between refueling stops. In an
attempt to categorize this variation, three
types of migratory strategies are proposed
that could in turn have profound influences
on how birds prepare for, and orchestrate,
migratory processes. European shorebirds
have been categorized into three types of
migration —hop, skip, and jump—by
Piersma (1987). We have modified this classification in light of potential endocrine bases
and as a framework for designing future
experiments. We refer to Piersma's "hop,
skip and jump" categories as different types
of migratory bouts. Each bout consists of
preparations (e.g., hyperphagia and fattening) and the actual single flight taken before
the individual resettles to refuel and initiates the next migratory bout. The migration
itself may consist of varying numbers of
migratory bouts depending upon distance
to be traveled and the potential for refueling
stops along route. Our classification, with
some examples of North American shorebirds is as follows:
Short distance bout
Migrants fly short distances (1-100 km)
between refueling and are similar to some
passerines in that their food is readily available along most of the migratory route. This
classification is equivalent to Piersma's
"hop" strategy. Note that "short distance
bout" does not necessarily mean a short dis-
223
tance migrant. Individuals mayflyonly short
distances in each migratory bout but may
cover long distances of several thousand
kilometers during many short distance
bouts. The Black Turnstone (Arenaria
melanocephala) is a short hop migrant since
it prefers to forage on rocky shores, which
are nearly continuous along its Pacific Coast
migratory route (Johnsgard, 1981; Marsh,
1986; Paulson, 1993). This is illustrated in
Figure 1.
The Red Phalarope (Phalaropusfulicaria)
is another example of a species with short
distance bouts (Fig. 1). Although this species migrates over the ocean, a geographical
barrier for nearly all shorebirds, Red Phalaropes are pelagic, wintering and migrating
far offshore (Johnsgard, 1981; Paulson,
1993). Red Phalaropes eat zooplankton on
the sea's surface, so many stops for food
along the migratory route are possible.
However, phalaropes tend to concentrate
where plankton densities are highest, such
as upwelling zones (Stanford, 1953; Brown
and Gaskin, 1988), so there is potential for
intermediate distance bouts.
Intermediate distance bout
Shorebirds in this category have food available only intermittently along the migratory
route. The distribution of ideal foraging
habitat may not be regular, thus longer distance flights (100-2,000 km) may be interspersed with relatively short distance flights
during migration (equivalent to "skip" in
Piersma, 1987). As with short distance
bouts, this does not necessarily mean that
intermediate distance bouts are all intermediate distance migrants. Several intermediate distance bouts may be made by a
long distance migrant. Western Sandpipers
migrate using flights of intermediate distance (Fig. 1), with the length offlightwithin
a bout probably dependent on factors such
as the distance to the next available habitat,
the condition of the individual, weather, and
food availability.
Western Sandpiper migration between the
Alaskan breeding grounds and the Pacific
Northwest is illustrative of the variability
in flight distance within a bout. During
spring migration, short to intermediate
flights are taken, with stops at coastal mud-
224
K. M. O'REILLY AND J. C. WINGFIELD
WESA
RUTU
FIG. 1. Map of North America showing spring (solid arrows) and autumn (dashed arrows) migration routes of
selected scolopacid shorebirds. Letters denote field sites for Western Sandpiper study: a) Hartney Bay, Alaska;
b) False Bay, Washington; c) Grays Harbor, Washington; d) Bodega Bay, California. Standard abbreviations
denote species (see text for scientific names): BLTU -Black Turnstone; BTCU—Bristle-thighed Curlew; REPH—
Red Phalarope; RUTU—Ruddy Turnstone; SESA-Semipalmated Sandpiper; WESA—Western SandpipeT.
Labeled sites are major stopovers.
STRATEGIES OF SHOREBIRD MIGRATION
flats in Washington, British Columbia, and
Alaska. Greater than ninety percent of the
species' world population have been estimated to stop at the Copper River Delta in
southcentral Alaska before making the final
trip to their breeding grounds in western
Alaska and Siberia (Isleib, 1979; Senner et
al, 1981; Senner and Howe, 1984). However, on the southbound migration fewer
than ten percent of adults stop at the Copper
River Delta (Senner, 1977; Bishop and
Green, 1993), or other coastal Alaskan sites
used during spring migration (Kachemak
Bay, West, 1992; Stikine River Delta, C.
Iverson personal communication). The first
major refueling stop after the migrants leave
the breeding grounds appears to be the Fraser River Delta, British Columbia (Butler et
al, 1987) and Grays Harbor, Washington
(Paulson, 1993). Thus, autumn migrants
take relatively longer intermediate nights
over the same territory they flew over in
short and intermediate bouts during spring
migration.
225
mated Sandpiper (Calidris pusilla) spring
and autumn migrations adopt different categories of bouts. The central Arctic breeding
population migrates north along the Central
Flyway in intermediate distance bouts.
However, during autumn migration, this
population makes a trans-oceanic flight from
the coastal North Atlantic region to South
America, a distance of at least 3,200 km
(McNeil and Cadieux, 1972; Harrington and
Morrison, 1979; Morrison, 1984; Dunn et
al, 1988; Gratto-Trevor, 1992), i.e., long
distance bout (Fig. 1).
As these two examples show, not every
species may be slotted neatly into one category. Some researchers have proposed that
the Western Sandpiper uses combined short,
intermediate and long distance bout strategies during spring migration with specific
combinations varying among individuals
(Butler and Elner, 1994). Similarly, in a
recent examination of these categories in
European shorebirds, it was concluded that
the different migratory strategies may more
appropriately be applied to individuals
Long distance bout
within a species, rather than to a species as
Birds in this category fly extremely long dis- a whole (Zwarts et al, 1990). Some species,
tances (2,000-5,000+ km) non-stop over a and individuals within a species, may vary
major geological barrier, e.g., oceans, deserts their migratory bout strategy depending on
(equivalent to "jump" category in Piersma, season, environmental variables, and phys1987). Bristle-thighed Curlews (Numenius iological condition. Similar evidence for
tahitiensis) traverse a distance of 4,000 km such mixed strategies has also been proor more between their wintering grounds on posed for passerines flying between Europe
islands in the central and south Pacific Ocean and sub-Saharan Africa (Bairlein, 1985,
and breeding grounds in western Alaska (Fig. 1987; Biebach, 1990).
We feel that this classification of migra1; Handel and Dau, 1988; American Ornitory bouts (both within and between migrathologists'Union, 1983).
tory seasons) has profound implications for
Combinations
physiological and hormonal mechanisms
It is possible for populations within a spe- during a bout (i.e., preparation for and durcies to adopt two categories described above. ing a flight bout). For example, are the horTwo populations of the Ruddy Turnstone monal bases of, and preparations for, a short
(Arenaria interpres) fall into different cate- distance bout the same as for a long distance
gories with one a short distance bout migra- bout? Probably not because fat reserves
tion along the rocky shores of the Pacific required are vastly different, and the physCoast (like the Black Turnstone), and iological problems of flight endurance,
another migrating in long distance bouts dehydration and heat dissipation are also
between Pacific Islands and Alaska (like the greatly different. On the other hand, comBristle-thighed Curlew; Fig. 1; Thompson, mon basic mechanisms may still be found,
1974). Both populations are in the same race but the intensity of response to hormone
signals may vary. Clearly, preparations for
(interpres; Paulson, 1993).
Combinations of different bouts may also migration in spring and autumn may be difoccur within populations. In the Semipal- ferent although the distance between breed-
226
K. M. O'REILLY AND J. C. WINGFIELD
ing and wintering grounds remains the same.
It is hoped that the classification above will
have heuristic value in not only the design
of experiments to determine endocrine
mechanisms, but also choice of species (or
population) as a source of subjects.
It is also clear from category 4 above
(combinations of bout types) that spring and
autumn migration seasons may have very
different physiological requirements and
thus hormone mechanisms may also be different. Even more disconcerting is the
potential variation in bout combination
among individuals within a population. This
raises serious problems for the endocrinologist to overcome, because it is possible that
any one experimental cohort could contain
individuals that have adopted different
migratory bout strategies and thus could
respond to experimental procedures in different ways. Thus it will be crucial in future
investigations to determine the bout strategy of individuals and group them accordingly. It is possible that confounding factors
such as this have led to general confusion
and contradiction in the endocrine literature on control of migration (see Wingfield
et al., 1990Z? for review). Only if we are able
to separate migratory bout strategy at the
individual level can we hope to determine
basic hormone mechanisms. Potentially the
comparison of different bout strategies will
be a very powerful way of determining hormonal bases. The classification above draws
attention to this problem and may lead to
more critically designed experiments.
DIFFERENCES BETWEEN SPRING AND
AUTUMN MIGRATION
In addition to the variation in flight
lengths and preparation (bouts) described
above, shorebirds cope with a suite of environmental and biological changes during
migration. Migrants encounter changes in
weather, food, flock size, and activity compared to breeding or wintering. They also
experience differences between spring and
autumn migration. The main factors driving these differences are the destination
(breeding vs. wintering grounds) and the time
of year (spring vs. summer/autumn) of each
migration.
Spring migration
During spring migration, shorebirds
encounter poorer weather as they near their
breeding grounds. Storms and colder temperatures cost the birds energetically as they
maintain body temperature (Wiersma and
Piersma, 1994). Windy conditions may
inhibit their ability to feed efficiently (Pienkowski, 1981). Foraging success may be
reduced since invertebrate prey are often
inactive in cold weather and may burrow
deeper, rendering them inaccessible (GossCustard, 1969; Zwarts and Wanink, 1993).
However, at some stopover sites prey availability is highest during spring migration
(e.g., Delaware Bay Limulus eggs; Myers et
al, 1987).
Social changes also take place during
spring migration. Spring migration is characterized by huge multi-species flocks at
stopover sites {e.g., number of shorebirds
on peak day: 459,000—Copper River Delta,
Alaska, Bishop and Green, 1993; 587,000Grays Harbor, Washington, Paulson, 1993;
>500,000-Bay of Fundy, New Brunswick,
Morrison, 1984). Spring migrants are synchronized because only a narrow window
of time exists on the breeding grounds in
which to lay eggs. Laying begins as soon as
snow melts off the tundra.
High densities of shorebirds at stopover
sites influence social behaviors such as
aggression and flocking. The nature of
aggression varies among species and is difficult to assess (reviewed by Myers, 1984).
Some studies report increased per capita
aggression with increasing densities (Recher
and Recher, 1969; Burger et al, 1979;
Young, 1989) while others report decreased
aggression with increasing densities (Burger
et al, 1979; Stawarczyk, 1984). At very high
densities, aggression appears to be reduced,
presumably because the cost of defense is
too high (Recher and Recher, 1969; Myers
et al, 1980). Note that increased aggression
reduces foraging efficiency (Stinson, 1980;
Fleischer, 1983).
Although shorebirds are known for their
flocking ability nearly year round, it is most
evident during spring migration. Flocking
benefits individuals by reducing the probability of being preyed upon, confusing
STRATEGIES OF SHOREBIRD MIGRATION
predators, and spotting predators quickly
(Goss-Custard, 1970; Hamilton, 1971; Page
and Whitacre, 1975). This protection comes
at the cost of foraging efficiency. Flock mates
may hamper prey detection and deplete prey
(Goss-Custard, 1980; Wilson, 1991). In large
flocks, time spent foraging may be reduced
with increased frequency of escape flights,
whether due to more false alarms or more
predation attempts (Buchanan et al, 1988).
While many environmental and social
factors reduce foraging efficiency during
spring migration, shorebirds are under pressure to accumulate enough fat for migration.
In addition, individuals arrive on the breeding grounds with fat reserves for displaying
(males) and laying eggs (females; Holmes,
1972; Pienkowski et al, 1979; Davidson
and Evans, 1986).
Autumn migration
In contrast to spring migrants, autumn
migrants encounter better weather as they
fly south. Food availability is generally
higher during autumn migration since
invertebrates are more active in warmer
weather and more abundant after reproduction (Goss-Custard, 1969; Beukema,
1974; Wilson, 1989; Wilson, 1991; Zwarts
andWanink, 1993).
Migration in autumn is more protracted
than in spring. Adults usually leave the
breeding grounds before immatures (Recher,
1966; Page etai, 1972; Page, 1974). Species
vary in departure time depending on their
molt schedule (Holmes, 1972; Morrison,
1984). For example, Dunlin (Calidris alpina)
remain on the breeding grounds to molt,
then head south during autumn (Holmes,
1966; Page, 1974). Most other sandpipers
molt largely after autumn migration, thus
heading south during summer (Holmes,
1972; Page et al., 1972; Morrison, 1984;
Paulson, 1993). Due to these differences in
timing of migration, flock sizes during
autumn migration are generally smaller than
spring migration. Smaller flocks may allow
an increased foraging efficiency because of
reduced interference, increased detectability of prey (Goss-Custard, 1980; Wilson,
1991), reduced aggression (Myers, 1984),
and reduced predator disturbance (GossCustard, 1970; Page and Whitacre, 1975),
227
although risk of predation may be higher
(Buchanan et al, 1988).
Autumn migrants have fewer environmental and social barriers to foraging efficiently and subsequent fat deposition than
spring migrants. Unlike spring migrants,
autumn migrants may arrive on their wintering grounds lean without compromising
their reproductive success.
Clearly, spring and autumn migrations are
different phenomena, and the strategies used
by migrants differ at the levels of season,
species, and individuals. Next, we explore
how hormonal mechanisms may modulate
different migratory strategies.
HORMONAL CORRELATES OF
MIGRATION IN WESTERN SANDPIPERS
An important unifying feature of spring
and autumn migration is the need to acquire
enough energy, in the form of fat, to fly
between the wintering and breeding grounds
(reviewed in Berthold, 1975; Blem, 1980;
Zwarts et al, 1990). The deposition of fat
and hyperphagia necessary to obtain energy
are mediated by hormones (reviewed by
Wingfield et al, 19906). Hormones also play
an important role in zugunruhe (migratory
restlessness) and the increase in muscle mass
and metabolic rate necessary for the long
flight (Wingfield et al, 19906). We present
correlational data from the Western Sandpiper on three hormones thought to be
important for migration.
Methods
Between 1989 and 1993, Western Sandpipers were caught during spring migration
at Bodega Bay, California, Grays Harbor,
Washington, and Hartney Bay, Alaska.
Autumn migration samples were obtained
from Bodega Bay, Grays Harbor, and False
Bay, Washington (see Figure 1 for location
of sites). Birds were caught in mist nets set
on mudflats between dawn and dusk. Small
blood samples (10-80 nl) were collected from
a wing vein into heparinized microcapillary
tubes after puncture with a 26 gauge needle.
After tubes were sealed and centrifuged,
plasma was drawn off with a Hamilton
syringe and stored at -20°C. Plasma levels
of hormones were measured by radioimmunoassay using procedures described else-
228
K. M. O'REILLY AND J. C. WINGFIELD
I
4.0-|
O
O
3.0 i
E
J
2.0-1
O)
•§
1.0 i
0.0
early
late
stage of spring migration
2.0-|
0)
8
1.0-
(0
0)
early
late
stage of spring migration
FIG. 2. Plasma levels of luteinizing hormone (upper
panel) and testosterone (lower panel) during spring
migration in adult male Western Sandpipers. Columns
are means and vertical bars are standard errors. Sample
size appears in each column. ** denotes significance at
P = 0.002 (ANOVA, F-test). *** denotes significance
at P = 0.001 (ANOVA, F-test).
where for luteinizing hormone, testosterone
and corticosterone (Wingfield et al, 1991).
Results and Discussion
Results are presented for adult male
Western Sandpipers. For the purposes of
this paper, samples collected in California
(Bodega Bay) and Washington (Grays Harbor and False Bay) are categorized as early
spring migration. Late spring migration
samples were collected in Alaska (Hartney
Bay). Autumn migration samples were collected at the same sites as early spring
migration samples.
Luteinizing
hormone. —Late spring
migrants had significantly higher levels of
luteinizing hormone than early spring
migrants (Fig. 2, P = 0.002, ANOVA,
F-test). Autumn migrants had basal levels
of luteinizing hormone (a gonadotropin
produced by the pituitary gland which promotes steroidogenesis in the gonads). There
is little evidence for a direct role of luteinizing hormone in migratory preparation.
However, as a trigger of testosterone production, luteinizing hormone does have an
indirect role (see below). Conversely, migration may suppress secretion of gonadotropins and gonadal recrudescence. Schwabl et
al. (1984) found a negative correlation
between migratory disposition and plasma
levels of luteinizing hormone and testosterone in male European Blackbirds (Turdus
merula). This may hold gonadal development and associated reproductive behavior
in abeyance until spring migration is completed and the individual is on the nesting
ground. In autumn, reproductive hormones
are extremely low and probably not important in regulating migration.
Testosterone. —Autumn migrants had
basal levels of testosterone which did not
differ significantly from early spring
migrants' testosterone levels. However,
plasma levels of testosterone were significantly higher in late spring migrants than
early spring migrants (Fig. 2, P = 0.001,
ANOVA, F-test). Late spring migrants are
within a week of arrival on the breeding
grounds, thus higher levels of testosterone
are presumably associated with preparation
for breeding. Since testosterone levels are
nearly basal during early migration, there is
no evidence for a major role of testosterone
during migration in Western Sandpipers. In
fact, migration may actually suppress testosterone secretion, as suggested by Schwabl
etal. (1984).
These results are consistent with passerine studies indicating testosterone is not
necessary during migration, but rather well
before migration begins. Male passerines
castrated just prior to migration exhibit
similar levels of fat deposition and zugunruhe as intact males (Brambling Fringilla
montifringilla, Lofts and Marshall, 1961;
Golden-crowned Sparrow Zonotrichia atricapilla, Morton and Mewaldt, 1962). However, studies on White-crowned Sparrows
229
STRATEGIES OF SHOREBIRD MIGRATION
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Zonotrichia leucophrys (Mattocks, 1976) and
White-throated Sparrows Zonotrichia albicollis (Weise, 1967) reveal testosterone has
a role in vernal premigratory fattening and
zugunruhe prior to photostimulation. When
gonadectomized before days lengthen (i.e.,
December), males (and females, Schwabl et
al, 1988) failed to undergo premigratory
fattening and zugunruhe. Preparation for
autumn migration was unaffected in gonadectomized birds (Mattocks, 1976; Schwabl
et al, 1988).
Higher testosterone levels during late
spring migration at Hartney Bay may influence social behaviors. Aggressive acts such
as chasing, vocalizing, and "tail-up" displays were more numerous at Hartney Bay
than at early spring migration sites (K.M.
O'Reilly and J.C. Wingfield, in prep.). High
testosterone levels are associated with
aggression in many avian species in the temperate zone (Wingfield et al, 1990a), but
see Hunt et al. (1995) for an Arctic species.
To what extent such social interactions have
the potential to disrupt preparations for the
final stages of migration is unknown. However, comparisons with autumn migrants in
which these aggressive displays are largely
absent may be useful. This is not to suggest
that agonistic behavior does not occur in
autumn, just that the displays in spring and
autumn are qualitatively and quantitatively
different.
Corticosterone. — We present results for
maximum corticosterone level, defined as
the level present fifteen minutes or more
after capture. Corticosterone levels increase
after capture, so basal levels can only be
obtained within a few minutes of capture
(see Wingfield et al, [1995] for further
details). Maximum levels of corticosterone
may indicate differences in the adrenocortical response to capture stress. After fifteen
minutes, most individuals analyzed plateau
at maximum, or near maximum, levels of
corticosterone (Fig. 3). Late spring migrants
had significantly higher maximum corticosterone levels than either early spring or
autumn migrants (Fig. 3, P < 0.05, ANOVA,
F-test).
Several studies suggest that corticosterone, a glucocorticosteroid produced by the
adrenal gland, has a role in migration. In a
I o
o-
0
15
o
1
1
1
30
45
60
minutes after capture
early spring
late spring
autumn
migration
FIG. 3. Upper panel: plasma corticosterone levels as
a function of minutes after capture in adult male Western Sandpipers during spring and autumn migration.
Corticosterone levels from individuals bled 15 min or
more after capture were defined as maximal. Lower
panel: maximum plasma levels of corticosterone during migration in Western Sandpipers. Columns are
means and vertical bars are standard errors. Sample
size appears in each column. * denotes significance at
P < 0.05 (ANOVA, F-test).
comparative study of migrants and nonmigrants, the former had significantly higher
in vitro production of corticosterone by cultured glands during the spring and autumn
migratory periods than the latter (Peczely,
1976). Schwabl et al. (1991) found a diel
pattern in corticosterone levels during the
autumn migratory period in captive Garden
Warblers (Sylvia borin). Since this pattern
of high nocturnal levels (when birds would
beflying)and low diurnal levels (when birds
would be resting and feeding) disappeared
when the migratory phase was experimentally interrupted, the authors concluded that
corticosterone may have a role in the control of migratory behavior (but see Gwinner
230
K. M. O'REILLY AND J. C. WINGFIELD
et al. (1992) for evidence of low corticosterone levels during nocturnal migratory
flight). No diel pattern in corticosterone levels was observed in Western Sandpipers, but
perhaps this is not surprising since tidal patterns also play a role in the timing of migratory flight in shorebirds. High corticosterone levels were observed in Bar-tailed
Godwits (Limosa lapponica) just after a
migratory bout, decreasing as they rested
and refueled at a stopover site (Ramenofsky
etal, 1995).
Hartney Bay is at the western end of the
Copper-Bering River Delta complex. Previous researchers (i.e., Senner, 1977) have
proposed that sandpipers at Hartney Bay
have obtained most of their fuel at more
eastern sites in the Delta, thus Hartney Bay
(late spring) migrants may be temporally
closer to a migratory bout (as a group) than
early spring or autumn migrants (some may
have just arrived, others may be about to
depart). Higher maximum levels of corticosterone in late spring migrants may be an
indication of their physiological preparation for migration.
Another possible explanation for the high
maximum corticosterone levels in late spring
migrants is that social and ecological
demands, such as increased aggression, huge
flock sizes (the largest encountered during
migration), poorer weather, and food availability, may sensitize the sandpipers to capture stress. In autumn, social interactions
decrease, weather and food availability
improve, perhaps resulting in lower levels
of maximum corticosterone.
CONCLUDING REMARKS
Our data on Western Sandpipers represent an attempt to assess possible hypotheses to be tested regarding common basic
mechanisms of migration at the level of differences between seasons within a single
species. Individual differences have not been
addressed, although some distinction was
made between stages of migration (early vs.
late). It is critical that further research
address individual differences in preparation during short, intermediate, and long
bouts. Radio-telemetry studies are crucial
in determining the flight paths and stopovers of individuals. Western Sandpipers
fitted with radio-transmitters during spring
migration in 1992 revealed a large variation
in flight time with the fastest individual
arriving at the Copper River Delta from San
Francisco Bay in two days and the slowest
in fifteen days (C. Iverson, S. Warnock, N.
Warnock, R. Butler, and M. Bishop unpubl.
data; West, 1992; Bishop and Green, 1993;
Butler and Elner, 1994). Endocrine research
during migration, coupled with knowledge
of bout length and migratory strategy, will
clarify some of the differences between spring
and autumn migration, and within the
migratory period itself. The importance of
studying a species at more than one site during migration is demonstrated by the differences in hormone levels between early
and late spring migrants, and by the ecological differences between spring and
autumn migration. Arctic shorebirds provide a useful model group for endocrine
research during migration since they breed
under similar time constraints in the short
Arctic summer, yet have a great variety of
migratory strategies to compare and contrast. Some questions that generate testable
hypotheses from the field work described
above are as follows:
1) Are preparations for short, intermediate,
and long bouts similar?
2) Do individuals utilize the same foods
during various stages of migration and
do these give the same benefits, both
qualitatively and quantitatively?
3) Do social relationships in flocks change
in spring vs. autumn? If so, what are the
effects on migratory preparation?
4) What are the metabolic consequences of
short, intermediate, and long bouts for
further preparation? Do spring migrants
shorten bouts as they encounter poorer
ecological conditions? Do autumn
migrants lengthen bouts as conditions
improve (i.e., fat is more easily replenished)?
Future work addressing these questions will
greatly improve our understanding of the
ecology and physiology of migration.
ACKNOWLEDGMENTS
Much of the research presented here was
supported by grants DPP-8901228, DPP-
STRATEGIES OF SHOREBIRD MIGRATION
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