Male Tree Swallows can be identified by their
glossy, iridescent, dark blue or blue-green
plumage. Their cheeks and entire underparts are
Adult females resemble the males but tend to be a
little more greenish. It takes two years for
females to attain their adult blue-green plumage;
thus, females in their first breeding season (in
other words, about 10 months after they fledged)
are light brown on the back with darker bits of
green scattered through the upperparts,
especially on the head and wings.
Juveniles are a dull gray brown. The tail and
wings are slate brown with a slight greenish
tint. The belly and breast are white, and often
the breast has a faint gray band.
Distribution and Breeding Habitat
Tree Swallows are found all across the
continental United States and Canada, and in
parts of Alaska, as far north as cavities are
available, avoiding only the southeastern corner
of the U.S. In winter, they migrate to Mexico,
Florida, and Central America, but they
occasionally winter as far north as the
mid-Atlantic coast and the central valley of
California. Tree Swallows prefer open habitats,
such as the edges of woods, and areas near water,
including marshes, shorelines, and swamps. Tree
Swallows are quite tolerant of humans.
|Tree Swallow Range
The Tree Swallow
breeds from western and central Alaska
and central Yukon to northern Quebec and
central Labrador south along the Pacific
Coast to southern California and
south-central New Mexico, generally
sporadic or irregular as a breeder east
of the Rocky Mountain States and south of
the upper Mississippi and Ohio Valleys,
or along the Atlantic Coast south of
Massachusetts. Winters from southern
California, southwestern Arizona, Texas,
the Gulf Coast, and the Atlantic Coast
from New York south to Central America.
The Tree Swallow prefers open woodlands
near ponds, small lakes, or marshes.
Occurs around farmlands, river
bottomlands, beaver ponds, wooded swamps
and marshes where dead standing trees are
in or near water.
Tree Swallows are aerial feeders. They forage
over land and water, swerving and dipping to feed
on flying insects such as beetles, midges, crane
flies, horseflies, ants, moths, grasshoppers,
dragonflies, and mayflies. In winter, they
sometimes feed on berries, especially those of
the wax myrtle. Tree Swallows and Myrtle Warblers
are the only birds that can digest these berries,
and this might help explain why Tree Swallows
winter farther north than any other North
American swallow. Tree Swallows forage mainly
near their nest sites, either alone or in loose
Pair Formation and Territoriality
Outside of the breeding season, Tree Swallows are
highly social birds. During the breeding season,
however, these swallows are territorial and
aggressively defend their nests against other
Tree Swallows (called conspecifics) and against
other species. They are solitary nesters, but
they may nest in loose aggregations with nests 10
to 15 meters from one another. They frequently
compete for nest sites with House Wrens and House
Arrival at the breeding site is age- and
sex-dependent. Generally, older birds arrive at
the breeding grounds before younger birds, and
males arrive before females. Females arrive up to
a week later, and pair bonds form shortly
thereafter. Pairs begin to defend their sites
Both sexes defend their territory and their nest.
The birds are most territorial before the eggs
are laid, and they defend a large area. Although
they initially defend more than one nest site on
their territory, only one nest site is actually
used. They become less territorial and defend a
smaller area as the nesting cycle progresses.
Although Tree Swallows are generally monogamous,
some males may have more than one mate
simultaneously. Pair bonds last for the duration
of the breeding season. Often, the same pairs
will mate in consecutive breeding seasons. This
appears to be the result of both sexes returning
to the same nest site year after year, rather
than the result of a long-term pair bond.
Nest Building: After returning
to the breeding site, Tree Swallows wait before
they begin nesting. In Upstate New York, nest
building tends to begin in April, but nests are
seldom finished before early May. The timing of
nest building and all subsequent reproductive
activities depends upon latitude and weather. If
pairs nest too early in the season, they risk
nest failure because of poor weather. Cold, wet
weather reduces insect abundance and may cause
nesting pairs to abandon their nests in search of
Nesting is quite synchronous in Tree Swallows;
that is, females tend to begin nesting within a
week or 10 days of their neighbors. Tree Swallows
nest in natural tree cavities, woodpecker holes,
and nest boxes. The female builds the nest. This
process is governed by the weather. In typical
springs, the nest can take up to three weeks to
complete, but if conditions are very good and the
birds are late, the nest can be completed in a
few days. Nests are typically 1 to 10 meters off
the ground, usually composed of dry grasses or
pine needles. The nest cup is lined with feathers
collected by the male and female. Feathers
continue to be added during egg laying and
Egg Laying: The onset of
egg-laying is highly variable. It is influenced
by weather and age of the female, rather than how
soon pairs return to the breeding site or
complete their nest. In some populations, eggs
are laid beginning in late April; in others,
egg-laying begins in late May. Throughout most of
the Northeast, the bulk of egg laying usually
takes place in mid-May.
Females lay one egg per day, but there are
occasionally large gaps between the laying of
consecutive eggs. Eggs are laid in the morning,
usually within two hours of sunrise. The eggs are
pure white and become slightly glossy after a few
days of incubation.
The average clutch contains four to seven eggs.
Nevertheless, clutch size is affected by the
scarcity of food and lateness in the breeding
season. Also, older females tend to have larger
clutches than younger females.
Incubation: Incubation begins
the day the penultimate, or next-to-last, egg is
laid. But in periods of cold weather, a female
may postpone the onset of incubation for up to
two weeks. This delay does not affect the
viability of the eggs or the success of the nest.
The incubation period is 14 to 15 days. Females
incubate the eggs, but males will guard the nests
when females are absent.
Nestling Care: Eggs hatch within
one or two days of each other, but bad weather
can lengthen the hatching period. Females brood
their nestlings for three days, after which they
only brood at night and during uncommonly cold
weather. The young are fed soon after they hatch,
and both parents share this duty equally. In
situations where one adult has disappeared, a
single parent can raise the young alone.
Nevertheless, retarded nestling growth, brood
reduction, and starvation are typical outcomes.
Fecal sacs are removed from the nest until the
young are 14 days old. Nestlings fledge after 16
to 30 days, and many are virtually independent
immediately upon their departure.
Tree Swallows have one brood per breeding season.
Pairs will nest again if the first
attempt fails, but true second broods are
extremely rare. The same nest sites are used for
more than one breeding season, and pairs will
often nest on top of old nests.
Winter Movement and Dispersal
In fall and winter, Tree Swallows form migratory
flocks that can contain anywhere from a few
thousand to a million individuals. They head to
the southern coast of the United States, Mexico,
and Central America.
In early spring, a small percentage of the young
return to their birthplaces. They do not breed on
the natal territory itself but rather settle some
distance away. This distance varies, depending on
Site fidelity in adults is highly variable.
Anywhere between 13 and 60 percent of the birds
return to their former breeding territories, the
percentage depending greatly upon winter
mortality and previous breeding success. Females
show less site fidelity than males, especially
following an unsuccessful breeding season.