Tree swallow

Quick Take

• Tree swallows switch between two completely opposite social behaviors depending on the season, and the trigger behind that shift is more cutthroat than you would expect. See the seasonal shift →
• The female's egg-laying strategy doubles as a survival tactic, one that forces her to make a harsh choice when food runs scarce. Explore the egg-laying strategy →
• Despite being one of North America's most common songbirds, the tree swallow faces a survival statistic that makes its population numbers hard to believe. Check the survival statistics →
• Scientists single out this one bird species above others as a window into broader animal behavior, and the reason why reveals something unexpected about how research works. See why scientists study them →

Easily identified by the iridescent hue of the plumage, the tree swallow is among the most common songbirds of North America. Their complex sounds cut through the crisp spring air of the breeding season. The popularity of other songbirds like the bluebird and house sparrow has benefited the tree swallow as well; all of them share an affinity for the same artificial nesting holes.

4 Amazing Tree Swallow Facts

• Tree swallows spend most of their lives flying in the air or resting on high perches. They try to spend as little time as possible on the ground, where they are most vulnerable to predators.
• The tree swallow is an important cornerstone of the local food system wherever it’s found. It keeps a check on insect populations and also provides an excellent source of food for other predators.
• Females produce their eggs in a staggered sequence of one per day until all of them have been laid. This allows the mother to prioritize older chicks in times of food scarcity.
• The tree swallow is considered to be a model organism. Scientists study this species to learn new facts about other species.

Where to Find the Tree Swallow

The tree swallow has a truly impressive range across North America, with a breeding range spanning from Alaska and Canada south through the United States, and a wintering range that extends along the Gulf Coast south to Panama. They’re also sometimes found on the Caribbean islands. Tree swallows tend to live near meadows, marshes, shorelines, and wooded swamps.

Nests

The tree swallow takes up residence in small tree hollows, standing dead trees, nest boxes, and even buildings or houses. Their nests are composed of grass, moss, weeds, roots, pine needles, and other plant matter. The nests are then lined with bird feathers (not their own) to provide comfort.

Classification and Scientific Name

The scientific name of the tree swallow is Tachycineta bicolor. Tachycineta, which means “quick-moving” in Greek, is a genus of American swallows. The name bicolor refers to the two-colored pattern of the feathers.

Size, Appearance, and Behavior

The tree swallow is a small bird, measuring up to 5.5 inches in length and weighing around 0.7 ounces (20 grams). It has small feet, a forked tail, and relatively large wings compared to body size. By far the most conspicuous feature is the iridescent green-blue feathers adorning the back and head. This is contrasted with the white breast and the black fringes on the wings and eyes.

One of the more unusual facts is that the tree swallow has two different social arrangements depending on the seasons. For most of the non-breeding months, it congregates into huge flocks of thousands to roost at night. But this behavior suddenly changes in the breeding season, when the competition for nesting sites becomes incredibly fierce. Tree swallows will aggressively defend a territory about 30 to 50 feet around the nest from other members of the same species.

The tree swallow primarily communicates with each other through complex vocalizations. Up to 14 different calls have been identified that correspond with various emotional states and behaviors, including anxiety, physical attraction, submission, distress, and even a desire for food. These calls are generally composed of three parts (or a combination of these parts): a chirp, a whining sound, and a gurgle. While both sexes can make these standard calls, only the males are fully capable of singing. These songs appear to be a form of territorial signaling against potential interlopers.

Migration and Timing

These birds migrate together in large flocks toward the southern United States and Mexico for the winter. This process begins as early as July and peaks sometime in the fall.

Diet

These birds spend most of the day from dawn to dusk foraging for food; they sometimes travel several miles in search of good foraging opportunities. The bird’s main feeding strategies are to capture prey while in flight or skim along the surface of the water with its mouth open. Sometimes, large groups of birds can be seen gathering where insects are particularly abundant.

What does the tree swallow eat?

These birds are omnivores. The bulk of their diet consists of insects like flies, grasshoppers, beetles, and flying ants. The remaining 20% or so consists of plant materials like berries and seeds. This serves as a very good adaptive strategy. While purely carnivorous birds might starve during the winter, this species can survive on only plant material for long periods of time.

Predators, Threats, and Conservation Status

These birds are considered to be a species of least concern by the IUCN Red List. There are currently no significant threats to this species besides natural predators. However, its survival is highly dependent on the availability of good nesting sites, which vary considerably from place to place. There are also some concerns about the long-term effects of pesticides on this species.

What eats the tree swallow?

An adult bird is preyed upon by larger birds such as magpies, falcons, owls, and kestrels. Juveniles and eggs are also vulnerable to raccoons, chipmunks, weasels, snakes, and cats. Groups of swallows will attempt to drive off a predator by mobbing it.

Reproduction, Young, and Molting

These birds begin the annual breeding season in spring by forming temporary breeding pairs. Males will try to court females by showing them their potential nesting sites. While most birds remain monogamous, some will have multiple breeding partners each year.

Mating usually takes place in the summer. After building its nest, the mother lays anywhere between four and seven eggs at a rate of one per day. These eggs take about 14 or 15 days to fully incubate. Both parents take part in feeding the juvenile chicks, but this helpless state does not last for very long. About 15 to 25 days after hatching, the juveniles are ready to leave the nest for the first time; they may still stick around for a short period while learning how to fly and receiving some food from the parents, but they must be fully independent by the time migration begins in late summer or fall. The chicks reach a state of full sexual maturity by the next breeding season.

Unfortunately, this high reproductive rate is a strategic necessity because the survival rate of an adult tree swallow is quite low; Only about 40 to 50% are estimated to survive each year. This makes their average life expectancy just 2.7 years. The oldest ever known tree swallow lived to about 11 years old.

Population

It is estimated that around 19 million of these birds are currently alive. The numbers appear to be decreasing across most of its range.