In the bird world, it’s always a time of transition. Whether it’s mid-December or mid-August, some bird somewhere is on the move, or molting from one plumage to another, or changing their behavior in response to cues from the environment, like day length.
I’ve talked a lot about shorebird and seabird migrations in recent weeks, but you don’t necessarily have to hit the beaches to see evidence of the change of seasons among the local birds.
In fact, a few songbirds are already starting their migrations. Like clockwork every July, yellow warblers and northern waterthrushes finish nesting and start to move around, turning up in places they don’t breed. If you listen closely, you’ll notice they start giving their migratory flight calls more frequently around now. These high-pitched calls are used primarily by songbirds when they are migrating at night, presumably as contact calls to orient them to one another in the darkness. On a night with northerly winds from now through the fall, it’s possible to literally hear bird migration by listening to the night sky. While warbler flight calls tend to all sound the same, birds like Bobolinks and thrushes have distinctive flight calls that can be discerned by ear.
The increase in flight calls happening now is part of a suite of behaviors known as migratory restlessness. Luckily, the researchers who first described this phenomenon had the presence of mind to give it one of those cool, big German names, which in this case is “zugunruhe”. Zugunruhe, or migratory restlessness if you hate awesome German words for some reason, was formally described by researchers using what’s called an Emlen funnel. A bird, often a White-crowned Sparrow, is placed in this funnel with an ink pad at the bottom, so the direction they are trying to go is betrayed by their inky little footprints. Researchers can then manipulate a series of variables, like images of the starry night sky, temperature, day length, and barometric pressure, to see how the birds respond. It turns out that all of these variables can affect the birds’ behavior at different times of year, depending on their migratory state. For example, when the birds are presented with a night sky during fall, all of their restless movements are toward the southern part of that fake sky, regardless of where true south is, which is how we know that birds can use the stars to navigate.
The Gay Head cliffs of Aquinnah on Martha’s Vineyard is one of the classic spots to witness bird migration in the northeast, especially in September and October, but a few migrants were already noted passing the cliffs this week. Two Cliff Swallows and two Purple Martins, neither of which breed on the island, zipped past on Monday, along with a few dozen Cedar Waxwings and almost 300 Red-winged Blackbirds, all clearly migrating. These west-facing cliffs concentrate the migrants who are orienting west toward the mainland to avoid being stuck out over the ocean. They linger at the cliffs until they get up the nerve to hurl themselves out over the water again to continue their migration, giving birders a chance to see sometimes spectacular numbers of migrants milling about.
Don’t worry if you’re too busy with your freeloading summer guests to get out and enjoy the birds right now; the migration will be going on through at least October. And by that point, your own Zugunruhe may have kicked in, sending you migrating down to that time share in Naples.
