Sunday, May 26, 2024

A juvenile male kestrel flies over Medford. (Photo: Brian Rusnica)

Known previously as a sparrow hawk, the American kestrel (Falco sparverius) was renamed by the American Ornithologists’ Union in 1983 because the bird looks and acts like a European kestrel. It’s the smallest falcon in North America – about the same size as a blue jay and weighing from 3 to 6 ounces (about the same weight as a cup of blueberries).

DNA analysis has since revealed that this bird is more closely related to American falcons than to European kestrels, even if this raptor has evolved to fill the same small-prey niche as European kestrels. 

A female American kestrel scans for prey. (Photo: Richard George)

Since kestrels depend on insects to supplement their diet, most of them do not stay in New England during the winter. They migrate south, spending the colder months in the mid- to southern United States, Mexico or the Caribbean. This is a good time to observe returning kestrels because they migrate north in April.

According to Brian Rusnica, the president of Eastern Mass Hawk Watch, the best kestrel migration site in Massachusetts is at Plum Island, where the organization runs a daily count throughout April at Lot 1 at the Parker River National Wildlife Refuge. Visitors are encouraged.“The best conditions are dry days with moderate northwest winds. Last year we counted 926 migrant kestrels total, with 402 of them coming on a single massive day – April 15, 2022,” Rusnica said. “The single-day record for American kestrels in Massachusetts was 550 on April 20, 1995, also at Plum Island. Under the right conditions, kestrels will migrate up the coastline and can be seen zipping over sand dunes, sometimes in bursts of up to 10 at a time.”

An adult female kestrel with its prey in Medford. (Photo: Brian Rusnica)

Kestrels have not always been so common here. Birders first spotted a kestrel nest in Cambridge on 1877, and 11 years later, another. By the 1920s, kestrels lived in Boston’s Public Garden and other urban areas, but the birds prefer grasslands and farmlands where they can find small prey. Today, as farmlands disappear, the number of birds is dropping. Kestrel populations in the Eastern United States are only a fraction of what they were at their peak. On the Cape, they are almost completely gone.

Kestrels are one of the few birds that can hover in a headwind, as this female demonstrates. (Photo: Richard George)

To hunt, the American kestrel often perches in a tree or on power lines and from there scans the ground for prey. It plunges down and pounces, seizing the prey with one or both talons. It captures grasshoppers, beetles, moths and other insects, mice, voles, frogs and small birds. It sometimes preys successfully upon birds its own size, carrying larger creatures up to a perch to eat. Because they are ambush predators that perch and wait, they are less muscular than other falcons that spend large amounts of time flying and chasing what they eat. American kestrels require less food per day than their more muscled cousins.

In 1922, farmer Floyd Bralliar described how these “sparrow hawks” attacked his chicks:

The hawk watches until he feels sure of his prey, then swoops downward straight as an arrow, strikes the bird in the back with his talons, and with his powerful beak tears the top of the head off. The point of the beak is sunk into the base of the skull, and the skull is torn off with a swift forward motion. I succeeded in getting a number of chickens immediately after the hawk struck them, and everyone had the whole upper part of the skull torn off, the brain exposed, and the medulla mangled with the point of the hawk’s beak.

Each attack occurred quickly: “A long straight swoop, a flash of wings, and the hawk is off with its prey so quick bright things come to confusion.”

An adult male kestrel surveys the scene from its perch in Somerville. (Photo: Brian Rusnica)

American kestrels can see ultraviolet light. Since rodents leave urine trails that reflect it, kestrels are able to track a potential meal back to its home. Then the bird has only to perch and wait for the rodent to emerge.

Kestrels have an amazing ability to hover. When there’s a good headwind, they can flap their wings and maneuver their tail to hover in one spot while searching the ground below for prey.

A kestrel with a meal over Somerville in June 2020. (Photo: Brian Rusnica)

During courtship, the male kestrel may bring the female gifts, such as headless mice or dead birds. Once paired, the kestrels don’t built nests but instead lay their eggs in cavities in trees, buildings, cliffs or other structures. The female lays about five eggs, and the male and female incubate the eggs, although the female does the vast majority of this work. In about a month the hungry chicks hatch. A kestrel chick eats twice as much as an adult kestrel, or about two or three mice per day. 

In a month, the chicks are full sized, their wings are developed, and they are ready to take their first flight. Once the chicks are flying, the kestrel family often hunts together. This helps the young birds learn and improve their hunting techniques before they must survive on their own.

A male American kestrel surveys the landscape. (Photo: Richard George)

The American kestrel in some places is called a grasshopper hawk (because of its diet) or a killy hawk (because of its klee, klee, klee call). Kestrels live about five years in the wild. In one 2002 study, humans caused 43 percent of kestrel deaths, and predation by other animals accounted for less than 3 percent of deaths. Data shows that the American kestrel population in the Northeast is in a sharp decline. Lack of nesting cavities and declining farmlands may be one cause, but the biggest cause is likely insecticides and pesticides that kill the insects these birds rely on for survival.

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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.