Tree Swallow
The Tree Swallow (Tachycineta bicolor) is bird of open fields and meadows. Its name is derived based on its nesting habits, relying primarily on tree cavities excavated by woodpeckers or other species, although they willingly nest in constructed nest boxes. Due to the scarcity of suitable nesting sites, Tree Swallows are highly competitive nesters. They winter further north than other swallow species, and are among the earliest arriving migrants to nesting sites. They defend their selected sites with intensity – a casual hiker roaming too close to a Tree Swallow’s nest may be startled by the bird’s dive-bombing behavior.
The Tree Swallow is one of 86 species of swallows in the family Hirundinidae worldwide, and one of eight swallow species that breeds in North America. They breed throughout most of the U.S., except for the Southwest (western parts of Texas, Oklahoma, and Kansas, most of New Mexico and Arizona, and southern parts of Nevada and California), throughout most of Canada and throughout Alaska. They winter along both coasts, from the lower Cheseapeake Bay into Florida, the Gulf Coast, south Texas and Mexico as well as along the Califonia coasts into the Baja Peninsula
What it looks like
The Tree Swallow is a medium-sized swallow, dark above sharply contrasting with white below. Adult males are an iridescent steel blue color on their back contrasting with bright white lower parts extending from the chin and throat to the base of the tail. They have sooty gray wing and tail feathers. Females are similarly patterned, with young females brown above and older females tending to show more green and duller colors than males. An adult male is pictured here.
Breeding Behavior
Males often arrive and defend nest sites before the female’s arrival, with pairing behavior beginning immediately. Most pairs maintain monogamous bonds during the season, although a small proportion (about 8%) of males will mate with two females and protect two nest sites during the season. On rare occasions, two females will even occupy the same nest site. The degree of polygny (the practice of having more than one wife at a time) is thought to be positively related to food abundance. With plenty of food, a single male can support two broods at the same time.
Feeding Behavior
Tree Swallows feed mainly on flying insects, pursuing individual prey in flight. From dawn to dusk, they perform acrobatic twists and turns as they chase a variety of insects. Their diet includes flies, dragonflies, damselflies, mayflies, caddisflies, bees, ants, wasps, beetles, butterflies, and moths. Most nestlings are fed with larger insects, while smaller insects are consumed in flight.
Tree Swallows often are seen swooping over bodies of water, often picking off insects from the surface. They drink in flight by making contact with the surface of the water at normal speed, and also may be seen perched in the rain darting its head about to catch individual raindrops.
Conservation status
Although Tree Swallows are common and widespread, their numbers have declined by about 50% over the last half century. While currently not categorized as a species of concern, that will likely change if the current trajectory continues. Their decline is due to several factors, including forest management practices and lost or degraded wetlands that reduce nest sites and suitable habitat. While the proliferation of nest boxes, such as bluebird boxes, have helped to offset this loss, nest boxes only provide sites for about 2% of the Tree Swallow population.
That’s it for this week. Next week I’ll talk about the Blue Jay.
If you missed last week’s write up on the Sanderling click here.