Tree Swallow

Until frost arrives, the skies are full of them

By Susan Campbell

Perhaps you have seen them: huge flocks of small birds, wheeling over open water or marsh, twisting and turning in deliberate arcs. They are most likely migrant swallows. And at this time of year, most will be tree swallows. Sometimes numbering in the thousands, swallows are feasting on the plethora of late-season flying insects. The skies will be filled with these birds until frost moves them farther south to warmer climes, where food is still plentiful.

This handsome species is shiny green above and white below in adult plumage. Young of the tree swallows are a dusky gray-brown with a pale grayish breast band and pale lower parts. But both age classes have a black mask. Their call is a characteristic high-pitched liquid chirping, which becomes a harsh chatter when alarmed.

Tree swallows are mainly insectivorous, but they will feed on berries (such as those of wax myrtle) during cold or wet periods in the fall. It is an odd sight: plants covered with fluttering birds, all tugging at tiny fruits. Most commonly they consume insects on the wing, swallowing anything small enough to enter their throat. Although they may pick insects out of the air, they will simply consume whatever may enter their mouth when flying through clouds of small flies, midges and the like. Furthermore, they may feed very close to the ground or water surface, especially in cool or windy weather.

In late afternoon, large numbers of tree swallows will descend into thick vegetation to spend the night. In areas devoid of trees or shrubs, they will use marsh grasses as cover. Wind and rain as well as large owls such as great horned or short-eareds on the hunt are threats in the darkness.

This species is a cavity nester, historically using holes in dead trees or adjacent to water. But with fewer dead trees on the landscape, the birds can be found using nest boxes or other man-made crevices. In fact, where the right sized bird boxes have been erected in wet habitats, tree swallows have moved right in. Only very large bodies of water attract these birds in the warmer months, since they need a steady stream of large flying insects while raising their families. Even so, there are more locations boasting breeding pairs in the Piedmont and coastal sections of the state each spring. Females can occasionally be found using wood duck or purple martin houses when smaller boxes are unavailable.

Farther north, where more tree swallows breed, they may be found on small farm ponds as well as the shores of large lakes. They are loosely colonial, with a few to several pairs found in close proximity. Brood size is dependent on food availability, with four to six young produced depending on insect population numbers. In years of especially abundant insects, males may even bond with multiple females (behavior referred to as polygyny), further increasing local tree swallow productivity.

Tree swallows form huge, often combined flocks during fall migration. Barn, cliff and northern rough-winged swallows may accompany them. We now find a cave swallow or two mixed in with them in late fall or early winter. If you are scanning a big group of swallows with binoculars, you may get lucky and spot one of these birds: a stockier individual with a pale rump patch and throat. Although typically a southwestern species, we now know that some caves do wander north and east, turning up along the Atlantic coastline late each year.

But know that swallows will be with us along the southern coastline for some weeks yet. Even though the nights are getting chillier, as long as the insects are still active around our beaches, marshes and sounds, so too, will be the graceful tree swallows.

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