I looked up on the last day of July at the sound of purple martins, and instead of a swirling flock of feeding birds, I saw a line of the giant swallows flying south toward their winter home, perhaps in Brazil. Fall bird migration is well underway, despite the fact that your yard sounds the same as it has all summer.
Some birds never migrate, for example our Carolina wrens and northern cardinals and white-breasted nuthatches. Young of these species will disperse from their place of birth to an available territory, perhaps next-door, perhaps hundreds of miles away, but such a one-way journey is not a migration. Migration is a regular round-trip.
Other species are complete migrants in which every member of the population leaves the breeding ground before food runs out in autumn and spends the winter far away. Flycatchers are an example. Before the end of September all of our breeding eastern kingbirds; eastern wood-pewees; and acadian, least, alder, willow and great-crested flycatchers will have departed on solo nocturnal journeys to the tropics.
The migration of one flycatcher, the eastern phoebe, was the first subject of modern ornithological research into migration, when John James Audubon attached silver wire to the legs of nestling phoebes on his property in Pennsylvania to determine whether they would return the following year. Yet we still don’t fully understand their migration. Eastern phoebes will be present in small numbers through our increasingly mild winters, although the vast majority still migrate south to the Gulf Coast of the U.S. or Mexico. Phoebes may be evolving into partial migrants, in which some members of the population migrate and others do not, based on genetic differences. This would be advantageous to the non-migratory portion of the population in years when food is available throughout the winter, but their genetic contribution to future populations may be severely reduced with a single ice storm.
Is that phoebe searching for small fruits and larval insects in your December garden the same one that nested under the porch awning in May? This would be evidence of partial migration in the population — most leave but there is a resident population. It could be that breeding phoebes are still complete migrants, and the ones we see in winter have migrated here from the northern part of their range. The miniaturization of tracking devices will soon reach the point that we can answer that question directly, finally resolving the research question that Audubon initiated in 1804.
But does it really matter if we know the provenance of our wintering eastern phoebes, or American robins, eastern bluebirds, brown thrashers, great blue herons, red-tailed hawks or Canada geese? No, in the sense that we should maintain great bird habitat with native plants and insects on our properties year-round, for residents and migratory visitors alike.
However, it does matter a lot whether we understand migration patterns or we want to protect bird populations. Whether we like it or not, humans have so drastically modified the habitats and climate of the planet that we are now responsible for managing bird species. Their fates are in our hands. If we don’t know when and where birds need food, peace and water during their complex and evolving migratory journeys, we will not be able to save them from extinction.
Dan Cristol teaches in the biology department at William & Mary and can be contacted at [email protected]. To discover local birding opportunities, visit williamsburgbirdclub.org.
