Like cooler weather, the screeching of thousands of birds swarming downtown city trees and above thoroughfares such as East Main Street is a harbinger of September.

Some are relatively harmless, such as European starlings. Some are violent, winged assassins — take the red-tailed and red-shouldered hawks, which also are migrating this time of year. Red-tailed hawks, which typically weigh two and a half pounds with wingspans of 49 inches, are known to swoop down and snatch up squirrels, mice and small birds. They've also reportedly attacked small dogs or cats, but such attacks are rare, says Sergio Harding, a nongame bird conservation biologist for the Virginia Department of Game and Inland Fisheries.

With the proliferation of urban chickens, however, there could be some cause for concern. Hawks eat chickens.

“Occasionally, somebody has guinea fowls or has domestic chickens, there are predation events, where their chickens are being taken,” Harding says. But it's more likely that a resident hawk is the culprit in a chicken attack, he says, as they're more apt to know the terrain and chicken hot spots. Migrating hawks are less familiar.

Starlings roost every evening around dusk, says Michael Wilson, a research biologist at the Center for Conservation Biology at the College of William and Mary and Virginia Commonwealth University. Now that breeding season is over the birds are feeling less territorial and more apt to congregate, he says. Brought to the United States in the 19th century, European starlings thrive as the aviary equivalent of squirrels, able to pick food from city refuse and nest in buildings.

Another species of noisy birds, purple martins, fled Richmond in mid-September. Native to tropical climes such as Central and South America, purple martins fly north to mate in the spring. After local conservation efforts by bird enthusiasts, the city's purple martin population swelled to upwards of 10,000 this year, Wilson says.

They alight on a line of Bradford pear trees near the 17th Street Farmers' Market from early July to early September in what Wilson calls a “premigratory roost” in swooshes of 500 to 1,000 birds. Then they catch up, or squabble.

“Sometimes there's fights over perch space,” Wilson notes.

With purple martins gone, now roaming are European starlings, owls and red-tailed hawks — who snacked this year on purple martins, Wilson says: “Red-tails were coming down and plucking [them].”