Tree Swallow Tachycineta bicolor
Like its Barn Swallow cousin, the Tree Swallow is in the scientific family Hirundinidae, but in a different genus, Tachycineta, a word derived from an ancient Greek word meaning “quickly moving.” The bicolor part of the scientific name is obvious from the species’ contrasting hues of white on the underparts, and metallic-blue-green on the upperparts of adults. Wing and tail feathers are dark gray. Tree swallows are very capable fliers and do have forked tails, though they lack the lateral streamers of Barn Swallow tails.
Tree Swallows breed widely across North America, wintering in southern states as well in Mexico and parts of Central America. Here in Maine, they typically arrive in April and depart in August. Tree Swallows may nest in tree cavities, especially in holes made by woodpeckers, but are more commonly observed setting up housekeeping in nest boxes, for which they may aggressively compete, sometimes to the death, with other cavity-nesting species such as Eastern Bluebirds, House Sparrows, and House Wrens, as well as with conspecifics. Although Tree Swallows aggressively defend their nest cavities during breeding season, they are highly social otherwise, and may roost together in large numbers by late in the summer.
Like Barn Swallows, Tree Swallows drink and bathe in flights close to a water surface, though they also take advantage of rainfall for both purposes. They forage over open areas like fields, meadows, and marshes, eating mostly flying insects along with a small percentage of plant foods, primarily bayberry, especially in cold weather. Prey items are pursued in the air and only rarely gleaned from surfaces. Various kinds of flies seem to be the preferred foods.
Once the nest cavity has been appropriated, females do most of the construction. Nests are often built entirely of grass but may also contain other plant materials like pine needles. When the floor of the cavity has been well covered, the top of the nest is layered with body feathers of other species, sometimes quite profusely. Eggs are white, most commonly numbering 4-7. Females do almost all of the incubating, which lasts roughly two weeks.
Hatchlings have only sparse natal down and are brooded by the female, while being fed by both parents. Nestlings are often fairly active, fluttering their wings and moving around the cavity, sometimes coming to the cavity entrance to be fed. The period from hatching to fledging is usually about 3 weeks. During that time, young birds are often parasitized by blowfly larvae, and may succumb to starvation in cold, rainy weather. Nestlings may be orphaned if the parents go missing, if their nest box is damaged or if they are evicted by intruders who want the cavity for themselves.
However, if all has gone well during this time period, young birds are able to fly when they leave the cavity for the first time, some taking only short flights with others taking quite long ones. Parents may continue to feed fledglings for several days, and the youngsters may or may not return to the cavity after their first flight. Soon enough, juveniles and adults will mix in foraging and roosting groups that often include other swallow species. Tree Swallows almost never double clutch, but may renest if the first attempt fails.
Recent decades have seen some decline in Tree Swallow populations in northern parts of their breeding range, probably attributable not only to decreases in aerial insects but also to loss of natural nesting sites, like dead snags. Nest boxes provide sites for only a small portion of the breeding population; they are insufficiently available to compensate for the lack of usable tree cavities. Like Barn Swallows, Tree Swallows are a Species of Special Concern in Maine.
More information about their natural history may be found at https://www.allaboutbirds.org/guide/Tree_Swallow