Every year in the life of a bird is dominated by three big events: migration, molt and breeding.
These punctuation marks in a bird’s annual cycle each require massive investment of energy and an increased risk of death, so birds have evolved a life history that ensures none of the Big Three occurs at the same time. If a bird were to attempt to molt while breeding or migrating, it would surely perish.
Instead, birds generally time their lives to breed first, molt next and migrate after. Evolution has produced lots of variation in the annual cycle of different species, but the typical pattern is common among our songbirds.
After migrating north to breeding grounds, males set up territories, attract later-arriving females, and together they build a nest. Females typically do incubation while males defend the territory against other males with aggressive singing, ensuring a food supply for the babies.
Once the babies fledge, males typically take over feeding the young while females start another nest. When the second nest is done, both male and female molt all of their feathers, often dropping their first feathers the very day the young leave the nest.
Like raising baby birds, feather molt is a costly affair, as growing thousands of feathers requires lots of protein. In addition, birds missing many feathers find flight difficult and are vulnerable to predators. No wonder they try to complete the molt as quickly as possible, enabling them to migrate with a sleek new aerodynamic plumage.
Some species make special migrations with their worn out feathers to remote, predator-free areas and molt even more quickly, rendering them completely flightless for several weeks. This is why wildlife control officers always remove Canada Geese from golf courses in mid summer – they have molted all of their wing feathers and are flightless.
Some species molt very gradually, so that they don’t face the full cost of replacing feathers all at once – an installment plan, of sorts. These can be recognized as they soar overhead by the gaps in the middle of their wings, and include vultures, hawks and gulls.
Once molt is complete, the southward migration begins, either as a series of shorter all-night flights and frantic refueling days, or days and nights spent flying over the ocean powered only by stored fat. Once the wintering ground is reached, the cycle starts all over, with feathers slowly wearing down in the sun and under assault by lice and mites, until they are ready to be replaced again after the next migration and breeding season.
Seabirds, which must be in top-flight condition at all times because they never stop flying, molt just a few feathers at a time all year round. A few species migrate halfway, stop for a month to molt, and continue on their way. And then there are species that live in scratchy marsh grass and must molt twice a year to avoid having feathers worked down to nubs.
Evolution has shaped the migration timing, reproductive habits and molt schedule of each species to adapt to the special demands of its environment.
Cristol teaches in the Biology Department at the College of William and Mary and can be contacted at dacris@wm.edu. To discover local birding opportunities visit williamsburgbirdclub.org.
