Each autumn, millions of tree swallows (Tachycineta bicolor) accompanied by a few rough-winged (Stelgidopteryx serripennis) and barn (Hirundo rustica) swallows, leave northern climates on both sides of the Atlantic for their southward migration. These small birds disperse widely during the day, sometimes congregating at high quality food sources, as they fly along their migratory routes. In late afternoon, the birds aggregate in increasingly dense masses over reed beds to sleep. These evening flocks move rapidly across the landscape, sometimes flying low, sometimes high, often splitting into smaller groups and fusing back into a single unit, always increasing in numbers as new birds fly in from surrounding areas.
As the evening deepens toward dark, the flocks coalesce into a dense, tornado-like cone of swirling bodies. The noise of their wings becomes intense and sounds like a wind-driven storm. This cone of a half million or more birds abruptly descends into the reeds and disappears. The noise and tumult cease and the reeds turn dark from the blue-black backs of the birds.
Why the Swallows Do This
Possibly the most important reason for the swallows congregating is that there is safety in numbers: a bird in a small group is more likely to be eaten than is a bird in a large flock. Although the conspicuous large mass of wheeling birds might attract predators, once the flock exceeds several hundred birds, the effect of a predator becomes negligible.
Prior to migration, local populations of swallows gather into smaller flocks which join the migratory hordes as they initiate their southward journeys. As small flocks gather together before settling down for the night, stray birds that might not know the locations of the roosts join the masses of birds funneling into the roost and enjoy the safety that communal roosting provides.
In a mass of half a million densely packed birds overnight temperatures of the roost are several degrees above ambient temperatures. Increased temperature means the birds do not expend as much energy to stay warm during inclement weather, or a bird may pull through the night if it did not find quite enough food the previous day to survive the overnight chill.
The locations of roosts remains constant, changing only after catastrophic alterations of the environment. Young of the year learn these locations as they make their first trip southward with their parents, then teach the roost locations to their own young.
For most of the day, the roost has very few birds in its vicinity. As birds are only found near the roost as they enter and leave it, most foraging occurs thirty or more miles away during the day. Thus, there is little competition for food at the roosts and the birds form compact groups without displaying aggression.
Where the Roosts Are
Roosting sites are found in reed beds located about sixty to one hundred miles apart along the migration route – usually near the shore. A particular site may be the focus of a half million or more birds that visit nightly for six to eight weeks from late August until October. Individual birds move from one roost to the next in a single day then stay put for some time before moving along the migration routes. Thus, the composition of a flock fluctuates daily as birds leave to continue their journey and others arrive from more northerly roosts.
Connecticut's only roost is on Goose Island, in the Connecticut River, just north of the I-95 bridge. I have visited other northern roosts near Ithaca, New York and Cape May, New Jersey in areas where they can find plentiful insects and berries nearby. While European swallows fly to Africa to roost by the tens of millions; North American swallows winter in Central and South America, near Vacherie, LA, and I have seen them in the Florida Everglades, North Carolina and Georgia, the pine woods of Greater Abaco Island in the Bahamas in flocks of several hundred thousand to a million or so. Few tree swallows winter in Central and South America, but most individuals of the other species of swallows migrate that distance.
What the Birds Feed On
Being primarily insectivorous, the swallows feed on the myriads of insects flying above nearby salt marshes, ponds, fields, woods, and waterways. Scientists estimate that each migrating swallow eats about 1000 insects every day, and summer birds will remove several times that amount to feed their young. Thus, these birds perform a huge service to humans as they eat mosquitoes along their way.
During peak migration in the north, the bayberry (Myrica pennsylvanica) bushes on Atlantic coastal beaches and other berries at other locations often feed hundreds of thousands of tree swallows daily. I often see flocks of thirty or forty thousand swallows swooping into and out of these bushes to eat the oil-rich bayberries. The huge numbers of birds rapidly strip all berries from many bushes while still overlooking a few bushes that provide food for winter residents. The oils allow the swallows to rapidly boost their fat reserves before making their next migratory flights.
Bayberries also form a significant portion of the diets of wintering tree swallows in Florida. Flocks there may alternately descend into bayberry bushes to eat the berries, then fan out over nearby fields and waterways gleaning insects. This berry-eating behavior is not displayed in the Bahamas where most fruit is carbohydrate loaded and not oil-rich.
As the swallows of autumn migrate, birdwatchers in the northeastern United States wishing to locate these flocks would do well to visit the bayberry laden beaches of Cape Cod, Long Island, Staten Island, New Jersey, and the Delmarva Peninsula during September and October. In December and January, large flocks can be found in the Everglades. Springtime migrations are more rapid as the birds rush northward swept by the southwesterly winds and spend less time foraging on the beaches.
More information on swallow migration can be obtained from: Robert W. Butler, 1988. Population Dynamics and Migration Routes of Tree Swallows, Tachycineta bicolor, in North America. J. Field Ornithol. 59(4):395-402.
Albert Burchsted - Ph.D. in animal behavior, field biologist, and photographer. Al leads nature study walks and is an environmental consult in SE ...
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