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Over the course of a year—or even a single month—we see a notable variety of bird species pass through the Northeast. In just September and October, you could potentially see hundreds of different species in Massachusetts alone; from shorebirds big and small in mid-September, to warblers and thrushes in early October, to sparrows later in the fall. What causes all of these different birds to move at different times in a single migration season? What are these birds doing? What do they eat? How do they do it?? The answer to these questions, to put it simply, comes down to biology.
Birds have a dazzling array of adaptations that allow them to live almost everywhere. Take, for example, the Chimney Swift. As aerial insectivores, swifts rely solely on flying insects as their food source. To accommodate this special diet, they must migrate to different geographic locations as insects become seasonally available. In addition to a limited diet, Chimney Swifts also have specific habitat needs; they use only hollow cavities, like you may find in an old tree, for shelter. As human populations increased, so did our urban settlement, and forested areas that once provided habitat for these birds were destroyed to make way for cities and buildings. Despite facing dramatic population declines due to major changes in habitat availability, the aptly-named Chimney Swift has adapted to using chimneys and other similarly-shaped manmade structures for shelter. Their unusually short legs, small feet and strong claws are perfectly suited to cling to stony surfaces. Now, you’ll often find large flocks of these birds flying into old chimneys at dusk (during late spring, when insects are most abundant here in Massachusetts) where they cling to the structure’s interior by the hundreds—sometimes thousands—to roost (or, rest) and nest.
Or, for a more seasonally-appropriate example, take a look at the Blackpoll Warbler. Blackpolls are a species of warbler named for the males’ black caps (or, “polls” if you’re old school) seen in their breeding plumage. Here at Manomet’s banding lab, we typically catch and band 200-300 of these birds every fall, usually from late-September to mid- to late-October. For Blackpolls—and several other migrant species—the timing of their travel is critical.