September 16, 2007 - Swallow Flocks Welcome to the Nature Notebook. They gather in fives and tens, twenties and thirties this time of year. You can see them sitting on utility lines or flying to nightly roosts before sunset. This post-breeding flocking behavior of Tree Swallows begins once young leave their nests. They vacate their breeding area and join other new fledglings and adults. The birds gather at the best available food source over fields or lakes. These family groups join other swallow families and their numbers grow. These flocks feed, preen, bathe, roost, and fly together throughout migration. Tree swallows migrate later than most birds. Although swallows are insect eaters and linger until the weather turns, they can switch to eating berries to get through periods of cold weather. Sometimes other species of swallows join these large flocks of Tree Swallows as they finally move farther south for the winter. This is PPL's naturalist, Jon Beam, with the Nature Notebook for WVIA.