Avoiding Extreme Environments: Migratory Birds
Scott R. McWilliams, Assistant Professor
Department of Natural Resources Science
Scott McWilliams earned a BS in psychobiology and environmental studies
from Hiram College, an MS in animal ecology from Iowa State University, and
a PhD in ecology from the University of California, Davis. He has been at URI
since 1998. He teaches wildlife ecology and management techniques, and physiological
ecology. His research focuses on the nutrition, physiology, ecology, and behavior
of wild vertebrates, especially amphibians, migratory waterfowl, and songbirds.
Most terrestrial ani mals living in sea sonal
environments like New England survive the winter by either reducing their activity
and exposure during extreme conditions or by simply leaving and thus avoiding
extreme conditions altogether. Familiar examples of the former are mammals such
as ground squirrels and bats that hibernate and bears that suspend much of their
activity during winter but do not truly hibernate. The more common strategy
for birds is to leave and migrate to regions with more favorable winter conditions.
Among the most famous and familiar group of migratory animals are the songbirds.
However, insects, fishes, and mammals (including snowbirds of our own species)
also provide fascinating examples of migration.
Migration is not a particularly simple solution
to the problems presented by extreme conditions in northern, seasonal environments.
Consider a year in the lives of migratory songbirds. They often leave their
breeding areas when food is still abundant. They travel long distances between
breeding and wintering areas and contend with uncertain weather and often unfamiliar
environments as they attempt to rest, avoid predators, and refuel at stopover
sites. On wintering areas (mostly in the southern United States and the tropics
for migrants that breed in New England), migrants must compete with other migratory
species and with residents more familiar with this relatively benign but crowded
environment. Finally, if they survive the fall and winter, migratory birds often
return north to breeding areas before food is abundant. In short, while it allows
birds to avoid cold, food-poor environments during winter, migration clearly
poses other challenges.
Much of what we know about bird migration comes
from banding programs around the globe. At banding stations, birds are passively
captured with fine mesh nets (mistnets), banded, and released Research biologists
at URI, including Peter Paton, myself, and a small flock of graduate and undergraduate
students, maintain a banding station during fall migration in Kingston, Rhode
Island, and intermittently on Block Island. Doug Kraus, a former professor of
chemistry at URI, banded birds each year for more than 35 years at the Kingston
Wildlife Research Station.
Long-term data from banding programs provide
important information about timing and routes of migration and long-term changes
in populations of migratory birds as a consequence of changes in the environment.
Banding programs in eastern North America, including our program in Rhode Island,
have demonstrated that young birds making their first southward migration are
especially common along the coast. It is hypothesized that this "coastal
effect" occurs because young birds stray more than adult birds and because
the Atlantic Ocean provides an obvious boundary that concentrates off-course
young birds. Regardless of its cause, the phenomenon has clear implications
for conservation. Effective management of coastal habitats for migrants during
stopover would enhance many songbird populations by improving survival of first-year
birds.
Long-term data from banding stations, however,
tells us little about the process of bird migration, how the birds actually
do the work of migrating. Much of my research in association with graduate and
undergraduate students at URI is focused on this aspect of bird migration.
Birds prepare for migration by increasing their
body weight, sometimes by as much as 100 percent, while storing fat and protein.
Birds accumulate fat and protein reserves by eating more and by being selective
in what they eat. Migrating birds may increase their feeding rates fourfold.
They are able to eat more food in part because the size of their guts increases.
Without changes in their digestive system, the size of the gut would limit food
intake and slow the accumulation of fat and protein reserves. Interestingly,
this increase in feeding rate and gut size can be induced in captive migratory
birds by decreasing daylight hours in the fall or increasing daylight hours
in the spring. Seasonal changes in daylength initiate hormonal changes that
mediate a host of physiological changes that include eating more, gaining weight,
and flight activity at night. The ability to respond to reliable environmental
cues such as photoperiod is critical for migrating birds because they can anticipate
seasonal changes in environments and then prepare for migration by storing energy
and nutrients.
The astute reader may wonder what happens to
migrating birds as they move from southern New England in the fall where days
are shortening to south of the equator where days are lengthening? If photoperiod
alone synchronized annual cycle events, birds would begin preparing for the
reproductive season prematurely with regrowth of the gonads and an immediate
return to the north. Birds circumvent this problem because the timing of migration,
reproduction, and molt are determined by photoperiod in conjunction with an
endogenous circannual rhythm (a cycle of approximately one year). This endogenous
timing mechanism can be demonstrated by maintaining animals in constant conditions.
Birds maintained in constant photoperiod display a cycle of gonadal growth,
molting, and migratory fattening with a period of about one year. Environmental
stimuli such as photoperiod are important because they synchronize the endogenous
circannual rhythm. As one might expect, birds that breed in more northerly latitudes
are induced to reproduce or migrate with longer day lengths than birds that
breed in more southerly latitudes.
After accumulating the necessary fat and protein
reserves, birds begin their migration from breeding to wintering areas. For
most temperate zone breeding songbirds, migration itself involves many flights
interspersed with stopovers to rebuild energy and nutrient reserves. Thus, during
migration, birds alternate between periods of high feeding to rebuild their
reserves at stopover sites and periods devoid of feeding when they expend their
reserves as they travel between stopover sites. We have shown that short periods
without feeding cause the bird's gut to atrophy, which in turn extends the stopover.
URI graduate student Megan Whitman is studying how birds stopping over on Block
Island rapidly rebuild fat and protein reserves in certain habitats. Her work
will help determine which habitats are most desirable for birds during their
migratory stopover on Block Island.
Selective feeding and high feeding rates allow
birds to satisfy the high energy and nutrient demands of migration. Many insectivorous
songbirds switch to feeding primarily on fruits during migration, which may
conserve energy since harvesting abundant fruits requires less energy than pursuing
insects. Fruit may also be a good source of certain unsaturated fats that are
important during migration. URI graduate student Barbara Wilson and undergraduate
student Wendy Wehunt have shown that certain unsaturated fats preferred by migrating
birds may facilitate rapid fattening and enhance performance during migration.
This recent work is among the first studies to demonstrate that wild birds discriminate
between diets that differ slightly in fat content.
Songbirds provide those of us in seasonal environments
like New England with an obvious harbinger of spring and fall. Their absence
in winter is a testament to the difficulties of survival for small birds living
in places with limited food and cold temperatures. Avoiding winter by migrating
south requires a suite of physiological adjustments and adaptations that we
are just beginning to fully understand. Our immediate challenge is to learn
enough about the migration process so we can conserve and manage the natural
areas that are important for birds during migration.