One of the amazing spectacles of fall on Nantucket is the annual “swallow tornado” as migrating Tree Swallows mass in the air and pour down into their temporary roost for the night. We start to see them during the day around the island: a few in Tom Nevers, or over the Milestone Cranberry Bog, in Bartlett’s Farm fields, or Head of the Plains.
They are seldom single birds: a group of a dozen, perhaps, or a hundred, or several hundred, or even a thousand or more. They perch on bushes or snags, sometimes giving dead trees the appearance of a brief flowering of blue leaves. They swoop and dart over fresh-water ponds, they cruise in the wind over salt marshes, back and forth and up and down, making aerial sweeps for insects.
Although fall is the peak time to see them en masse on Nantucket, some have been around all summer. We see them starting in late March or early April arriving from the south, a sign of spring. Many will keep moving as their migration carries them further north. A few will find nest boxes here attractive and stay to breed, spending the summer raising a family.
They are cavity-nesters, meaning that in the wild they depend on finding hollows in trees or old woodpecker excavations. In fact this is what probably gave them the name “tree” swallows, despite the fact that they are most often seen out in the open over ponds, marshes or fields. But they are not really very fussy, and will readily use bird houses. They will also use boxes put up for other birds such as mallards or wood ducks.
Past experience shows they can also eke out spaces under the eaves of buildings, in a fire hydrant, even a hole in the ground. At least once a pair built a nest in the railing of an active ferry, where they traveled back and forth completely unconcerned at the human presence or changes of location.
They build a nest of grasses lined with feathers, white by preference. The female does most of the actual nest-building, although the males also collect feathers. This is said to have something to do with claiming a breeding territory, as the males arrive first to defend a homestead. But nests are not always built right away when birds arrive. They wait until conditions are favorable for peak availability of food. The females lay from one to nine small conical white eggs, at the rate of one per day. Four to seven is average.
Once the clutch is complete they incubate the eggs for about two weeks, although the timing can vary considerably depending on weather conditions. When the young begin to hatch the parents remove the eggshells and drop them at a distance, or in water, presumably to discourage predators from locating the nests. They usually raise four to six young and can breed every year, but sometimes individuals will skip a season for reasons still unclear. They sometimes renest, although second broods are not as common as in some other species.
Primarily insect-eaters during the breeding season, they are faster than a speeding mosquito, which they can catch on the wing, and consume by the mouthful. Caterpillars, small insects and insect larvae form the mainstay of their summer diet. This has made them a valuable and beloved sight over pond and marsh.
But part of their hardiness also comes from their ability to eat fruits such as bayberries. In an April snowstorm or a cold spell in fall, they can subsist on a vegetarian diet. In fall the bayberry crop provides the fat to sustain them on their long journeys south. Another important ecological “service” they provide is hidden in this fact. They also gather on sand dunes, where they load up on grit to fill their crops. This abrasive helps them digest the tough waxy berries. But it also provides the scarification without which the seeds do not germinate. By carrying the seeds in their digestive system and excreting them on the dunes they seed otherwise barren soils with bayberries, an important plant in the succession of dune to upland. There are nitrogen-fixing nodules on the roots of bayberries, thus also nourishing other plants. This stabilizes the dunes as they retreat inland.
Nantucket is not the tree swallows’ only summer residence. Scattered across the northern half of North America they nest almost to the Arctic Circle. They winter along coasts from Baja California in the west and North Carolina in the east south to Panama. Banding data has shown that they can live to 12 years of age.
In the fall the best time to see them is about half an hour before sunset. Flocks of Tree Swallows skim the hills, gliding right overhead, almost close enough to touch. Darting back and forth, the birds get in their last mosquito snacks before bedtime. Unlike many other songbirds, Tree Swallows migrate by day, roosting overnight in wetland reeds. Although families separate during the breeding season, on migration they collect into huge, uncountable flocks.
Like a spill of pepper in the sky, each bird hardly more than a dark dot, they form a blanket, a trail of smoke, a cloud. Are there 10,000? 50,000? More? Flock after flock, they gather up, spin together, break apart and fly higher and higher. Sometimes a group will break off and look for another spot. Finally coalescing into a swarm, they stream down, a slender ribbon, a waterfall, a double helix of birds, finding perches in the reeds for the night.
Sometimes a late Merlin or Cooper’s Hawk in search of a bedtime snack chases the swallows, driving them this way and that, testing their strength and agility. If the raptor finds a bird weak or injured enough it may catch one. This is the key to migrating in such big numbers. The odds of being eaten are much less when you are one of 50,000. If you are one of 10, your chances are not nearly as good.
Despite big numbers in migrating swarms, breedingbird surveys show the overall population has declined by nearly 50 percent in the last 50 years. But as beautiful and useful birds they are worthy of active protection and appreciation.
Virginia Andrews writes “Island Bird Sightings” for The Inquirer and Mirror, Nantucket’s weekly newspaper.
