Whirlwinds of feathered bodies, iridescent beetle-blue on top
and snowy below, are touching down all along the eastern
Flocks move in a loose collection of tumbles and dives, sweeping
across fields and swamps. They pepper the sky, often collecting
over bodies of water to skim for insects and catch a drink. As the
sun sets, the scattered birds pull together, gathering like a
At the peak of migration, flocks of tree swallows can contain
hundreds of thousands of birds. Doppler weather radar - yes,
weather radar - has revealed that staging points are relatively
evenly spaced, almost always 62 to 93 miles apart. Migration flows
down the eastern seaboard in a multi-month game of hopscotch as the
birds make (comparatively) leisurely stopovers one roost after
In the northern parts of their range, swallows begin to move
south in July. By early September, most have left their breeding
grounds, which extend across most of the top third of the United
States and well into Canada. Stragglers, however, are common.
Unlike many songbirds, tree swallows migrate during the day and
fly low, foraging as they go. This makes their migration much more
visible than, say, a warbler's nocturnal high-altitude odyssey. In
terms of distance covered, tree swallows fail to impress: most
settle down for the winter in relatively nearby Florida, or in
Central America and the Caribbean. (Compare this to a barn swallow
that might winter deep in South America.)
Tree swallows are one of the earlier songbirds to return in
spring, arriving in parts of Vermont and New Hampshire as early as
March. Their early arrival is related to the fact that they're
obligate cavity nesters, meaning that they don't construct their
own nests. Instead, they depend on woodpeckers and their ilk to
provide tree cavities for nesting. Often, tree swallows are found
in beaver habitat, moving in after woodpeckers excavate holes in
dead or dying flooded trees. Tree swallows will also take readily
to nest boxes. In some areas, the proliferation of nest box
"bluebird trails" has been a boon to their populations.
Competition over holes to call home may spur their early spring
arrival. First come, first served, and when you cannot create holes
yourself, your options are limited to what is available any given
A second defining characteristic: unlike all other swallows,
which dine exclusively on insects, tree swallows are able to digest
fruit and seeds, including the fruits of bayberry and viburnum.
Tree swallows that find themselves in a cold snap with reduced
insect activity may instead descend upon a berry bush and make a
feast of it.
It's this ability to do a cedar waxwing impression that lets the
birds move north months ahead of other swallows. They don't need to
wait for the spring buffet of insects to appear before they get to
the business of nesting; in the meantime, they can supplement their
diet with last year's leftover berries.
After an intense period of jostling for position on breeding
grounds, tree swallows breed for a few months in late spring and
early summer. As soon as the young are fledged, they begin to
migrate south again.
Although the tree swallow is one of the best-studied birds in
all of North America, its migration ecology is still poorly
understood. During the winter, they seem to continue their
migratory habit of moving from roost to roost, following food
supplies. There are no good records of when birds begin to leave
their wintering grounds in spring; all we know of their return trip
is when they begin to arrive back in their breeding range. As such,
our picture of their migration, stunning though it is to witness,
remains rather impressionistic.
That impression is one of a roving species that only seems to
settle in one place as long as it needs to breed or wait out the
worst of the temperate winter. Their fluid, maneuverable migration
is a delight to behold on a September day.
Kenrick Vezina works for the Genetic Literacy Project and is a
freelance writer, naturalist, and raconteur based in the Greater
Boston area. Illustration was drawn by Adelaide Tyrol. The
Outside Story is assigned and edited by Northern Woodlands magazine
and sponsored by the Wellborn Ecology Fund
of NH Charitable Foundation.