Tree Swallow Life History

Habitat

Tree Swallows breed in fields, marshes, shorelines, wooded swamps, and beaver ponds throughout northern North America, preferring to live near bodies of water that produce multitudes of flying insects for food. For nesting they need old trees with existing cavities (typically made by a woodpecker), or human-made nest boxes. Migrating and wintering birds use habitats similar to their breeding habitat, except they may have no need for cavities and are free to live in open areas. Back to top

Food

Tree Swallows live on a diet of insects, though they occasionally capture other small animals and may eat plant foods during bad weather when prey is scarce. They feed from dawn to dusk in sheltered areas full of flying insects, usually foraging no more than 40 feet from the ground. Tree Swallows eat all kinds of flying insects: dragonflies, damselflies, flies, mayflies, caddisflies, true bugs, sawflies, bees, ants, wasps, beetles, stoneflies, butterflies, and moths, as well as spiders, mollusks, and roundworms. Their prey may be smaller than a grain of sand or up to two inches long. They chase prey in the air, with acrobatic twists and turns, and sometimes converge in large numbers in an insect swarm. During the breeding season, Tree Swallows eat high-calcium items like fish bones, crayfish exoskeletons, clamshells, and eggshells of gulls or loons.Back to top

Nesting

Nest Placement

Tree Swallows nest in natural cavities of standing dead trees, old woodpecker cavities, or nest boxes. On occasion they nest in hollow stumps, building eaves, Wood Duck nest boxes, holes in the ground, old Cliff Swallow burrows, or other unconventional sites.

Nest Description

The female does most of the nest building, taking between a few days and two weeks to finish the job. She collects material on the ground near the water’s edge, usually within 100 feet of the nest site. The nest is often made entirely of grass, but may include pine needles, mosses, rootlets, aquatic plants, animal hair, and artificial materials like cellophane or cigarette filters. Within the cavity, the female presses her body against the nest material to shape it into a cup, about 2–3 inches across and 1–2 inches deep, and lines it with many feathers of other bird species. In some populations the male gathers most of the feathers, and in others the male and females split the duty evenly.

Nesting Facts

Behavior

Tree Swallows are highly social, forming large migratory and wintering flocks; and pairs often nest close together, particularly where nest boxes are numerous. Agile fliers, Tree Swallows tend to glide more than any other swallow species. They bathe by flying low over the water and skimming their bodies against the surface, then rising quickly while shaking off droplets. Tree Swallows line their nests with feathers, and they seem to display or even play with these feathers during the early nesting season. A bird flies above the nest with a feather held in its bill; sometimes this leads to chases, and sometimes the bird drops the feather, causing an aerial free-for-all to see which bird retrieves it. Tree Swallows pair up to breed but often mate secretly outside the pair. Occasionally a male attends two mates in separate nest sites. Though an individual swallow may have the same mate several years in a row, it is probably faithful to the site rather than the mate. Nest predators include rat snakes, raccoons, black bears, chipmunks, mink, weasels, deer mice, feral cats, American Kestrels, Common Grackles, American Crows, and Northern Flickers. Outside the nest, adults are hunted by Sharp-shinned Hawks, Merlins, Peregrine Falcons, Great Horned Owls, and Black-billed Magpies. The swallows commonly swarm and dive-bomb predators while giving alarm calls. Back to top

Conservation

Tree Swallows are common but overall populations declined approximately 0.7% per year for a cumulative decline of about 30% between 1966 and 2019, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 20 million and rates them 6 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Tree Swallow numbers are probably most limited by available nest sites, and as people put up more nest boxes their range has been expanding, particularly southward. But nest boxes account for only a small fraction of Tree Swallow nest sites. Natural cavities, where most Tree Swallows build their nests, have been disappearing for the past 200 years as people clear the land, manage woodlands, cut down older trees, and remove dead trees. Tree Swallows eat a high-insect diet, which can expose them to high levels of pesticides and other contaminants such as PCBs and mercury. They also show a sensitive response to climate change. As spring temperatures have warmed since the 1960s, Tree Swallows’ average date for laying their first egg has moved nine days earlier in the year.

Credits

Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2019). The North American Breeding Bird Survey, Results and Analysis 1966–2019. Version 2.07.2019. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

Winkler, David W., Kelly K. Hallinger, Daniel R. Ardia, R. J. Robertson, B. J. Stutchbury and R. R. Cohen. (2011). Tree Swallow (Tachycineta bicolor), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Winkler, D. W. (1993). Use and importance of feathers as nest lining in Tree Swallows (Tachycineta bicolor). Auk 110 (1):29-36.

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