For those of us who live in the Northeast, it may seem obvious that many birds migrate to avoid cold winters. But that’s not the case.
“It’s not because of the cold,” says Christopher W. Leahy, the Gerard A. Bertrand chair of natural history and field ornithology at Mass Audubon. “Birds are among the best-insulated creatures around. It’s all about food.”
Even if we understand that it’s plentiful bugs and other provisions rather than sunny beaches that inspire birds to travel south, the process doesn’t seem to make much sense. Why do migrating birds spend so much time and energy on long, risky journeys to and from our region when they could just stay in warmer climates full of their favorite foods year-round?
There are two current theories of migration, Leahy says in his encyclopedic book, The Birdwatcher’s Companion to North American Birdlife. The first is that when glaciers covered much of North America, birds from the Northeast were forced into what are now tropical regions but as the glaciers retreated gradually reoccupied their former ranges. The second theory is that some present-day North American migrants originated in the tropics and expanded northward. For example, tanagers evolved in the tropical region of our hemisphere, where the greatest number of tanager species still occurs. Then at some point, a few species, such as scarlet and summer tanagers, dispersed northward. When colder weather arrived and they couldn’t find food, they “were forced to retreat to their ancestral homeland,” Leahy says. Now the behavior has become “hard-wired.”
These aren’t mutually exclusive theories. Some birds may have followed the “north-to-south” path and others the “south-to-north” path. The explanations work for the great majority of migrating birds, especially insect-eating songbirds, Leahy says.
One reason birds don’t stay in the tropical areas may be competition there. The wintering ranges are often much smaller than the ranges that North American migrant birds enjoy for the rest of the year. Birds also are very specific about what they eat during the spring, returning to the same breeding grounds year after year to forage. This dependence on narrow ecological niches may contribute to their need to migrate.
We may never know all the reasons, Leahy says. “Some of the answers don’t partake of logic. In some cases, it seems, birds are hard-wired to migrate by evolution.” Yet they change their behavior based on conditions. For example, yellow-rumped warblers, which have learned to feed on fruit as well as insects, will winter near the coast of New England if the crop of bayberries is strong, migrating only a short distance, if at all. But if the crop is bad, they will migrate as far south as Guatemala.
“Birds aren’t automatons,” Leahy says. “They can make choices. It enhances survival.”