- 4.7–5.9 in
- 11.8–13.8 in
- 0.6–0.9 oz
- Smaller than a Purple Martin; slightly larger than a Bank Swallow.
- Hirondelle bicolore (French)
- Golondrina invernal (Spanish)
- Migrating and wintering Tree Swallows can form enormous flocks numbering in the hundreds of thousands. They gather about an hour before sunset and form a dense cloud above a roost site (such as a cattail marsh or grove of small trees), swirling around like a living tornado. With each pass, more birds drop down until they are all settled on the roost.
- Tree Swallows winter farther north than any other American swallows and return to their nesting grounds long before other swallows come back. They can eat plant foods as well as their normal insect prey, which helps them survive the cold snaps and wintry weather of early spring.
- The Tree Swallow—which is most often seen in open, treeless areas—gets its name from its habit of nesting in tree cavities. They also take readily to nest boxes.
- Tree Swallows have helped researchers make major advances in several branches of ecology, and they are among the best-studied bird species in North America. Still, we know little about their lives during migration and winter.
- The oldest Tree Swallow on record was at least 12 years, 1 month old when it was captured and released by an Ontario bird bander in 1998.
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.
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.
- Clutch Size
- 4–7 eggs
- Number of Broods
- 1-2 broods
- Egg Length
- 0.7–0.8 in
- Egg Width
- 0.5–0.6 in
- Incubation Period
- 11–20 days
- Nestling Period
- 15–25 days
- Egg Description
- Pale pink, turning to pure white within 4 days.
- Condition at Hatching
- Helpless, with closed eyes and pink skin sparsely covered with down.
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.
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.
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.
Tree Swallows are common but their populations declined by 1 percent per year between 1966 and 2010, resulting in a cumulative decline of 36 percent, according to the North American Breeding Bird Survey. Partners in Flight estimates the global breeding population at 17 million with 46 percent spending some part of the year in the U.S., 65 percent in Mexico, and 54 percent breeding in Canada. They rate an 8 out of 20 on the Continental Concern Score and are not on the 2012 Watch List. 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 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 through bioaccumulation 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 of laying their first egg has moved nine days earlier in the year.
- Robertson, R. J., B. J. Stutchbury, and R. R. Cohen. Tree Swallow (Tachycineta bicolor). In The Birds of North America, No. 11 (A. Poole, P. Stettenheim, and F. Gill, eds.). The Birds of North America Online, Ithaca, New York.
- Partners in Flight. 2012. Species assessment database.
- Sauer, J.R., J.E. Hines, J.E. Fallon, K.L. Pardieck, D.J. Ziolkowski Jr., and W.A. Link. 2011. The North American Breeding Bird Survey, Results and Analysis 1966–2010. Version 12.07.2011. USGS Patuxent Wildlife Research Center, Laurel, MD.
- USGS Patuxent Wildlife Research Center. 2011. Longevity Records of North American Birds.
- USGS Patuxent Wildlife Research Center. 2012. North American Breeding Bird Survey 1966–2010 analysis.
- Winkler, D. W. 1993. Use and importance of feathers as nest lining in Tree Swallows (Tachycineta bicolor). Auk 110:29-36.
Long-distance migrant. Tree Swallows begin migrating south in July and August, flying during the day and roosting in large flocks at night. Eastern populations probably migrate along the Atlantic coast to winter in Florida and Central America. Except for the individuals that winter in Cuba, most Tree Swallows probably migrate along the coast to their wintering grounds rather than going over the Gulf. Populations from the Midwest and the Canadian prairies may follow the Mississippi River southward; Western populations probably travel along the eastern Rockies and along the Pacific coast.
If you live in their breeding range, there’s a good chance you can attract Tree Swallows to your yard by putting up a nest box (find nest box plans at our NestWatch project). Tree Swallows may supplement their insect diet with berries, such as fruit from bayberry and myrica shrubs. During the breeding season, when they need extra calcium to produce eggs, the swallows may search through backyard compost piles for pieces of eggshells to eat.
Find This Bird
Tree Swallows are easy to find in much of North America from spring through fall. Head to open fields or marshes adjacent to bodies of freshwater. Scan the air for flying birds, look along utility wires and shrubs for perched birds. Also check any nest boxes you happen to see; in summer Tree Swallows spend lots of time sitting on or flying around them. Tree Swallows are vocal; listen for their sweet, chirping calls as they wheel around overhead in pursuit of insects. In the winter you can find Tree Swallows in the extreme southeastern and southwestern United States as well as south of the border.
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Tree Swallow Farmer: a Cornell researcher's 22-year fascination with swallows. Story and photos in Living Bird magazine.