Driving home from a visit to Hirundo Wildlife Refuge last week, I was treated to the sight of a large flock of common nighthawks foraging near the Penobscot River in Old Town.

I was barely able to count the birds because their flight was so swift and erratic; there were at least 30 – and perhaps as many as 50 – of the birds hunting the evening sky for flying insects.

Nighthawks are slightly smaller than blue jays in length, with a larger wingspan of two feet – quite conspicuous even at a distance. Easy field marks are the white bars at the base of their primary feathers – during flight, these appear as comma-shaped marks toward the end of the wings. In addition, males have bright white throats and a white stripe edged by a black band at the end of their tails.

The name “nighthawk” is misleading, for they are neither hawks nor active at night. In fact, they are members of a family that includes the whip-poor-will; and they are primarily active either side of dawn and dusk – sometimes during full daylight as well. Bright lights such as street lamps attract their winged prey, which may induce them to remain active further into the night.

Their Latin name hints at an old myth surrounding these birds – they were once thought to enter barns at night to suck the milk of goats-hence the moniker “goatsucker.” They have also been nicknamed “bullbats,” because of their peculiar, but graceful bat-like flight. Actually, their airy, floating flight reminds me more of a moth’s.

Researchers say there has been a decline in this bird’s numbers throughout the United States and Canada, possibly as a result of “increased predation, indiscriminate use of pesticides, and habitat loss.” Unfortunately, this has become the mantra for almost any wild bird population.

Nighthawks are ground nesters that rely on camouflage to protect themselves and their eggs. Both are so cryptically colored they are hard to spot, even when one is very close and looking directly at them. Their favored nesting areas are ones that enable them to blend in and disappear: pebbly ground, sandy areas, clearcuts, even flat, gravel rooftops. They will also roost on the ground, and it is their habit of doing this at night, on gravel roads, that has caused them to be flattened by passing vehicles.

Because they rely completely upon flying insects for food, nighthawks are very susceptible to unseasonably cold weather. They attempt to avoid this by being one of the latest spring and earliest fall migrants (shorebirds start their fall migration even earlier – in July). In fact, the group I saw may have been a migratory flock, for they usually begin traveling south by late August on one of the longest migratory trips undertaken by any North American bird – one that brings as far as the tip of South America.

I wondered if the birds I had seen were locals or travelers from farther north; they might have been both. Sometimes migratory flocks will join up with residents for brief insect-catching forays, at times bringing hundreds of birds together in preparation for their big migratory push.

It was a thrill to see them, and again I marveled at the spectacle of migration.

Chris Corio, a volunteer at Fields Pond Nature Center in Holden, can be reached at fieldspond@juno.com