Tree Swallows are streamlined small songbirds with long, pointed wings and a short, squared or slightly notched tail. Their bills are very short and flat. Adult males are blue-green above and white below with blackish flight feathers and a thin black eye mask. Females are duller with more brown in their upperparts, and juveniles are completely brown above. Juveniles and some females can show a weak, blurry gray-brown breast band.
Tree Swallows are about the size of a sparrow. They have a length of 4.7-5.9 inches (12-15 centimeters), a weight of 0.6-0.9 ounes (16-25 grams), and a wingspan of 11.8-13.8 inches (30-35 centimeters).
Tree Swallows occur in most of North America to Central America. They are long distant migrants. They begin migrating south in July and August, flying during the day and roosting in large flocks at night. 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, or human-made nest boxes. Migrating and wintering birds use habitats similar to their breeding habitat, except 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 also eat 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.
Tree Swallows nest in natural cavities of standing dead trees, old woodpecker cavities, or nest boxes. On occasion they nest in other unconventional sites. 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. 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 have one to two broods with four to seven pale pink eggs turning to pure white within 4 days in each clutch per year. The eggs are 0.7-0.8 inches (1.7-2 centimeters) in length and 0.5-0.6 inches (1.3-1.4 centimeters) in width. Incubation period is 11-20 days and nesting period is 15-25 days. At hatching, the hatchlings are helpless, with closed eyes and pink skin sparsely covered in down.
Tree Swallows are highly social, forming large migratory and wintering flocks. 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.
They commonly swarm and dive-bomb predators while giving alarm calls.
Tree Swallow songs are high-pitched and liquid, composed of three sounds—a chirp, a whine, and a gurgle—which they mix and match in a variety of patterns. Both males and females sing.
Mates call to each other with the same chirps and gurgles that also appear in their songs. In addition, Tree Swallows give aggressive chatters, shrieking alarm calls, harsh distress calls, and amorous ticking sounds.
Tree Swallows may also snap their bills defensively while perched, to keep other individuals from coming to close.
Tree Swallows are common but they have suffered a fairly large decline recently. During this same period, though the species decreased in the east and west, populations in central North America increased. This bird's 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. 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.
- 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, 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.
- The Tree Swallow—which is most often seen in open, treeless areas—gets its name from its habit of nesting in tree cavities.
- 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 and 1 month old when it was recaptured and released during banding operations in Ontario in 1998.
- Tree Swallows are also called Golondrina Bicolor (in Spanish) and Hirondelle bicolore (in French).