A yellow-rumped warbler is spotted with its fall plumage. Credit: Courtesy of Bob Duchesne

Probably not much is going to happen in your backyard this week. But it might. Migration is in full swing, and unusual birds can pop up anywhere. It’s happening in my yard right this minute.

Birds fly south for the winter and return in the spring, but it’s not as simple as it sounds. They choose when to fly and when to lie low. No bird wants to fly hundreds of miles into a headwind, nor does any bird want to get eaten during the flight. Thus, most of the songbirds fly at night, avoiding potential predators, enjoying a helpful tailwind. Some nights there is a lot of movement; some nights there is none.

Consider further that there is a lot of variability on where and when birds start and stop their flights. The common yellowthroat has a nesting range from the southern tip of Florida to the northern tip of Newfoundland. Migrants along this vast expanse take wing only when local conditions are right. That will change by the day, and even the hour. A passing front might inhibit departure in the early evening, only to encourage it later. Off they go, a little behind schedule.

Likewise, birds will typically fly until sunrise, but settle down early if conditions deteriorate. There’s no planning ahead, no destination in mind. They just plop down wherever seems suitable. Apparently, my yard is suitable, because a host of songbirds dropped in overnight, and now they’re all foraging through the treetops, refueling for the next favorable breeze.

I suspect most people wouldn’t notice. The twittering from all the treetop migrants is hushed, but persistent. It comes and goes, as they work their way through my yard, and then on to the neighbor’s. Eventually they circle back through, again and again. It’s a foraging flock, a behavior typical of late summer.

As birds are getting ready to migrate, they often form into mixed-species flocks, foraging together. There are more eyes looking for food and watching for danger. Without the need to stay put and feed young, the birds gain an advantage by moving around together. It also happens during migration when a bunch of different species travel together on the same favorable wind, and then fall out in the same place at dawn.

There’s an eclectic band of birds feeding over my garage at the moment. Warblers include black-throated green, black-and-white, yellow-rumped, magnolia, northern parula, blackpoll and a couple of bay-breasted. Both golden-crowned and ruby-crowned kinglets are up there, along with a rather noisy blue-headed vireo. A Swainson’s thrush just perched on the porch railing. There’s a furtive female scarlet tanager sneaking around in the thicker leaves, showing me little. She gave me fits over proper identification.

Foraging flocks often cluster around chickadees and nuthatches, since they are bold, alert and quick to sound the alarm. Each time today’s flock works its way back into my yard, I can hear the chickadees leading the way.

The occasional foraging flock offers both opportunity and challenge. It’s an opportunity to see a lot of different species at once, without much effort, and not just at home. Whenever I walk anywhere this time of year, I’m alert for a sudden burst of activity, and start sorting through the birds. I hit several flocks on a hike through Katahdin Woods & Waters National Monument two weeks ago.

The challenge comes in sorting them out. Many are in their “confusing fall plumage.” Most warblers don’t change very much, if at all. Female and young black-throated green warblers don’t have black throats. Otherwise, they’re the same. Black-and-white warblers and northern parulas have the same plumage year-round. Yellow-rumped warblers go drab, but otherwise the pattern is the same.

Alas, some warblers change a lot. Bay-breasted warblers don’t nest in my yard, but there’s a male and a female in today’s flock, and I had to look three times at each to be sure of the identification. They are one of four warbler species that are wicked troublesome in autumn: bay-breasted, blackburnian, Cape May and blackpoll. They shed their bright colors, and mostly go a bland yellowish-green. Every year, I have to crack open a field guide to remind myself what field marks I’m supposed to be looking for. Let’s see, which one has the yellow legs? (Blackpoll.)

It’ll all be over soon. The foraging flocks will be gone. Until then, it’s best to carry binoculars when you go out to get the mail. You never know.

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Bob Duchesne, Good Birding

Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at duchesne@midmaine.com.