One of the most impressive feats in nature is the ability of
birds to travel hundreds or even thousands of miles to distant
lands where they breed and spend the winter. We tend to think
of migration as the seasonal movement of birds each fall and
spring; in fact, these are just the peak times. Pick any month
of the year, and you'll find some percentage of the western
hemisphere's five billion birds in the midst of migration.
Their journeys can take weeks or months to complete, depending
on the species and difficulties encountered along the way.
Migration Basics
Migratory birds include Neotropical songbirds, hawks,
shorebirds, and waterfowl, among others. The most familiar
migrants, at least to urban dwellers, are probably Canada geese
and Neotropical migratory birds. Canada geese are typically
thought to migrate along one of four identified
flyways—Pacific, Central, Mississippi, and Atlantic—although
these bands of movement are general in nature. Neotropical
songbirds breed in Canada and the United States in the summer
and spend their winter months in Mexico, Central America, South
America, and the Caribbean islands.
Impending cold weather is one of a number of factors that
encourage birds to migrate. The main trigger is food depletion,
such as when insects go into hibernation or when snow cover
makes seeds inaccessible. Diminishing daylight means that there
is less time to feed, and birds begin to have difficulty
maintaining sufficient energy stores to cope with lower
temperatures. This cause-and-effect relationship creates
another signal that it's time to head south. Once the instinct
to migrate is triggered, activities such as feeding, resting,
and aggression are often suppressed, allowing the bird to focus
on little else but reaching his or her destination.
One reason many of us don't notice migration on a large
scale is that much of it occurs at night when the air is cool
and calm and there are few predators. Warblers, vireos,
thrushes, and tanagers are all nighttime flyers, as are
shorebirds. Daylight migrants include ducks, geese, cranes,
loons, swallows, and swifts. Soaring birds, such as hawks and
vultures, migrate during the day to take advantage of the warm
updrafts created by heat from the sun.
Tools of Navigation
Birds use mountain ranges, narrow peninsulas, and coastlines
as landmarks to help them navigate. We also know they use the
setting sun and star patterns for migration, and some bird
species appear to rely on the magnetic field of the Earth.
Overcast or foggy nights can hamper the progress of birds who
rely on the stars for navigational cues. Birds have help,
however, from seasonal wind patterns that blow in the general
direction they need to travel. Whatever the means of
navigation, it is extraordinary that many species are capable
of returning not only to their home ranges, but to their exact
nesting or winter feeding sites.
Aerial Stunts
Among the many species that migrate, there are a few whose
feats are, in a word, astonishing. When blackpoll warblers
leave New England for South America in the fall, they begin a
nonstop flight that takes a minimum of 72 hours and covers
2,000 miles. The tiny ruby-throated hummingbird uses less than
one gram of fat from its four-ounce body to complete a 500-mile
flight across the Gulf of Mexico. That's energy efficiency!
Perhaps the champion of them all, though, is the Arctic tern,
who breeds in the most northerly tip of North America and
winters in the Antarctic, some 10,000 miles away.
Threats to Migrating Birds
Migrating flocks have declined by nearly 50 percent since
the 1960s. Changes that humans have made to landscapes in the
United States, such as clearing and fragmenting forests and
draining wetlands—and such as clearing rain forests for
sun-grown coffee in wintering locations—have greatly reduced
migratory bird populations. Other harmful factors include
pesticides, severe weather, predators, food scarcity, and
collisions with communications towers and brightly lit office
buildings.
In addition to the disappearance of habitat in seasonal
ranges, migratory birds are losing their "rest stops." About
half of all birds depend on wetland habitats to some degree,
and Neotropical migrants look for remnants of forested areas.
Snow geese, for example, rely on marshy areas for aquatic
plants and invertebrates. They may fly day and night before
finding their next stopover point.
What We Can Do
We need to protect wetlands, forests, and other valuable
habitats that birds need during their journeys. We also can
help migratory birds by adding water, natural food sources, and
cover in our backyard sanctuaries and community green spaces,
so that they can serve as stopover points. The natural
challenges of migration are many. If we can eliminate or
minimize the hardships that we've added, birds will be able to
safely continue their spectacular journeys between distant
lands.
