Updated, Thursday, Sept. 8, 8:30 am
Poking around the FARM Institute’s fields at Katama Farm last Saturday, I saw something I haven’t seen in decades: three American kestrels in the air at the same time.
Kestrels are not exactly rare on Martha’s Vineyard. They occur regularly, if sparingly, during spring migration, and equally regularly but in somewhat larger numbers in the fall. A few linger into winter most years. The birds at Katama were surely migrants, because this small falcon, once a common breeder on the Vineyard, no longer nests here.
As a young birder in the Boston suburbs back in the 1960s, I grew almost tired of kestrels. They were common “wire birds,” perching on power cables along roadsides. But the numbers of kestrels breeding in the state began to plummet around that time. According to data presented by the Massachusetts Division of Fisheries and Wildlife, breeding kestrels in southern New England declined at a rate of about 10 percent annually during the last four decades of the 20th century. While not yet listed under the state’s endangered species act, this elegant bird is listed as a top conservation priority in the Bay State’s Wildlife Action Plan.
Elsewhere across its vast breeding range (most of North America, and portions of Central and South America as well), this species still flourishes: On the dry grasslands of the Southwest, for example, you can find scores of kestrels in a day. But numbers on the Vineyard have echoed the regional trend. Formerly a common breeding bird here, kestrels long since stopped nesting on the Island, and numbers detected during migration and in the winter have likewise plunged.
As a small falcon, the kestrel is most likely to be confused with the merlin, once a rarity here, now appreciably more common than the kestrel during migration. Merlins are on average a bit larger, though the two species overlap in size. Seen well, a kestrel is much more colorful: The larger females are rusty birds above and pale below, with the males sporting gray wings that contrast with a reddish back. Merlins, in contrast, are plain gray (males) or brown (females and young) above, with streaked underparts.
But even at long range, the manner of flying separates these two falcons. Merlins fly strongly and directly, seemingly with an infinite reserve of power at their disposal for climbing or accelerating. Kestrels are slower, more inclined to meander and to glide rather than flap, with a looser, floppier wing beat. Merlins are notoriously cranky, rarely passing up a chance to harass another hawk; kestrels typically mind their own business. While both species can hover in flight, this is rare behavior in a merlin but a characteristic maneuver in kestrels.
The hovering relates to the preferred prey of these birds. Merlins are bird killers, typically patrolling in flight or scanning from a perch before launching an explosive attack on a flying songbird. The kestrel, in contrast, has evolved to focus mainly on insects. They take large beetles, grasshoppers, and, especially, dragonflies, which they often consume in flight, holding the prey in one foot and eating it like a Popsicle. Perhaps not coincidentally, the timing and path of kestrel migration in New England closely approaches the migration of green darner dragonflies.
Typically for this species, the birds at Katama last weekend were alternately patrolling and hovering over a mowed section of pasture, occasionally swooping down to seize grasshoppers. The species is capable of taking small mammals such as voles or mice, as well as small birds. But why bother, when insects abound?
Migrating somewhat earlier than our other falcons, perhaps because they rely so heavily on cold-blooded prey, kestrels probably peak in number here sometime around the middle of September. Open and agricultural land of any kind may attract them; also, kestrels often turn up at the corners of the Island — Wasque and Gay Head — where geography tends to concentrate both prey species and moving hawks.
The regional plunge in kestrel numbers loosely tracks another process evident in the New England landscape: the reclamation of former pasture and farmland by forests, or the conversion of farmland to housing or industrial uses. These are open-country birds, shunning woodland and preferring grassland, shorelines, marshes, and other open areas. Their numbers in the first half of the 20th century were probably inflated by the legacy of land clearing and farming in New England; during the span of my lifetime, ecological succession and human needs have rendered much kestrel habitat unsuitable for this small hawk.
As a species, the American kestrel is secure; it has a huge breeding range, and across a large part of that range it remains a common bird. But its dramatic decline in the Northeast vividly illustrates how changes in human social and economic patterns play out as unintended consequences in the natural world.
Due to a production error, an earlier version of this story used a photograph that showed a kestral not native to America. This photograph is the correct species.