A Tornado of Birds

Along the Connecticut coast, birds are now gathering in large flocks as they get ready to fly to southern climes. Among them is the Tree Swallow (Tachycineta bicolor). By September, flocks of Tree Swallows will build into the thousands, called staging, before flying south.

This is a special event in nature’s seasonal calendar. At dusk along the lower Connecticut River in New England, Tree Swallows gather into large flocks, often flying in from upriver. I was privileged to witness this firsthand. Birds flew all around me and I watched them skim across the water to bathe and drink. As many as one million birds may funnel down like a tornado to settle for the night among Phragmites grasses on one of the islands in the river, in what legendary artist and ornithologist Roger Tory Peterson called “one of the great ornithological wonders of the world.” At a certain point at twilight you will see and hear thousands of birds spinning and swirling downward and, a few minutes later, all are quiet and still for the night. Local tour boats schedule their cruises to coincide with this amazing event (search “Connecticut River swallow cruise” in your browser).

Tree Swallows are found in open and partially open habitats, meadows, grasslands, and brushy areas, particularly near water. They favor areas that provide lots of flying insects and hunt for them over both water and fields. Swallows have also been known to grab insects off the water’s surface.

As secondary cavity nesters, Tree Swallows often use holes in standing dead trees that were previously excavated by other birds, like woodpeckers. People who have helped bluebird populations by putting up nest boxes and discouraging the aggressive, introduced House Sparrow (Passer domesticus) from using them have also helped the Tree Swallow, which nests in boxes with the same size hole and in the same habitat.

The female Tree Swallow will lay four to seven eggs in the nest cavity and incubate them for about 14 days. Both parents feed nestlings and the young leave the nest about 20 days after hatching. Their diet is mostly insects, including flies, beetles, winged ants, and mosquitoes. But during the colder months, when it is in its wintering range, a Tree Swallow will often switch up to 20% of its diet to fruits such as Southern Bayberry (Morella cerifera), as does the Yellow-rumped Warbler (Setophaga coronata) and Hermit Thrush (Catharus guttatus). A diverse, omnivorous diet allows these birds to stay longer on their breeding grounds, migrate shorter distances, and return to their nests earlier in the season, thereby decreasing competition for nesting sites.

The Tree Swallow is a very vocal species and often “chatters” away with bubbly gurgles, whining sounds, and chips. Both males and females sing both in flight and from perches.

Although overall Tree Swallow populations seem to currently be stable, as insectivores they are vulnerable to the continued use of pesticides. Also, in the last hundred years many dead trees in yards and woodlots have been cut. Try to phase out your pesticide use and consider leaving snags, standing dead tree trunks, in your yard. When I need to take down a tree, I just have the tree service leave about 20 feet (about 6 meters) of the trunk.

You can also add an Eastern Bluebird (Sialis sialis) nest box to your yard, thereby attracting Tree Swallows. Don’t forget to add native, berry-producing shrubs such as Bayberry to your yard. With your help, we’ll be able to see these whirling bird tornados for years to come.