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Breeding Birds of the Bull Run Mountain

Tree Swallow (Tachycineta bicolor)

The tree swallow breeds over most of North America and winters as far south as Central America, although many spend the winter in the southeastern United States. The sexes are similar in appearance with the adult female only slightly paler. First year females and juveniles are brownish on the back with white underparts. The adult male is especially striking with snowwhite underparts and metallic blue and iridescent green back. To appreciate its beauty, it should be observed in sunlight as it courses over marshes and meadows searching for insects.

Unlike the barn swallow, the tree swallow nests in tree cavities. With the loss of habitat however, the tree swallow has readily adapted to bluebird nest boxes. A pair of tree swallows used a bluebird box in the field by the Mountain House (see article in this newsletter).

For breeding, tree swallows like open areas such as fields and marshes, usually near water. The nest is usually an open cup made mostly of dry grasses and is always lined with many large feathers, usually from waterfowl, but also from chickens or other birds. While the female does most of the nest building, the male may fly great distances to find the large feathers used in constructing the nest. The feathers keep the nestlings, which are born blind, warm as well as reducing the number of ectoparasites.

The female incubates the 4 to 6 white eggs. They hatch in about 13 to 16 days. The female is fearless as she remains on the nest after the nest box is opened (and will not leave) and can be stroked while remaining on the nest. Once the eggs hatch, the young will fledge in anywhere from 16 to 24 days. Tree swallows, like other swallows, don't have much of a song, but make short twittering notes and whistles.

All swallows, including tree swallows are flying insect eaters, however tree swallows have adapted to eat plant foods such as bayberries. This is why some can be found in the winter months in the mid-Atlantic region and further north. Not only is it the most northern wintering swallow, but also the first to return to its nesting sites in the spring.

When most of our tree swallows leave in the fall, they gather in huge flocks numbering in the hundreds of thousands. These flocks are usually seen along our coasts at places such as Bombay Hook NWR in Delaware and Chincoteague NWR in Virginia. They wheel around over the marshes or low trees and, as darkness approaches, begin to drop down in large numbers until all are settled down for the night.

Tree swallows may live from 6 to 9 years in the wild as recovered bands have shown. Of an estimated 1/2 million birds banded, only abut 4,000 bands have been recovered. If you find a wild bird with a leg band, report the number to the Bird Banding Lab at 1-800-327-BAND (2263).