Ecology

Decline of the American kestrel

Rusty Ryan
Frederick County Forestry Conservancy

(9/2022) A once common sight around Frederick County was the kestrel, or sparrow hawk, hovering above an open field or perched on utility lines searching for a meal. Spotting kestrels is becoming harder to do these days as their populations decline. The American kestrel (Falco sparverius) is the smallest and most widespread of the falcons in North America.

Sparrow hawks are about the size of a mourning dove, and they are found throughout the United States, Mexico, and Canada. The kestrel prefers open grasslands and desert, typically nesting in hollow trees near these open areas. Kestrels lay five to seven eggs in the spring which hatch in about 30 days; the young mature quickly, usually leaving the nest in a month or so.

Unlike other falcons and birds of prey, the kestrel does not use speed or brawn to capture prey. Kestrels, instead, use their natural hovering ability and good eyesight to locate and capture prey. This small raptor has a very diverse diet that consists of insects, lizards, mice, snakes, and smaller birds. The kestrel has two spots on the back of its head that look much like eyes, which disorient larger hawks that could potentially prey upon them. Given the Kestrel’s high population potential, general diet, and widespread distribution, why are the populations declining over much of its range?

Long term population surveys have determined that the kestrel has been declining over much of its range since 1984, at an average rate of 3% per year. Maryland’s peak kestrel population was in 1989, and from there it has seen a steady decline. There have been a number of theories for this decline, including the presence of West Nile Virus (first detected in the US during 1999.) The increase in Cooper’s Hawks population, a main predator of the kestrel, may also be to blame.

The destruction of grassland habitats and decline of suitable hollow trees for perching and nesting maybe contributing factors, as well. The increased use of pesticides does not help, and the kestrel’s general inability to cope with increasing human populations might be a contributing factor, based on the fact that kestrel declines are highest in the more densely populated sections of the country.

A number of studies have focused on the effects of these various factors, and so far no definitive cause has been identified. As an example, Cooper’s Hawks seem to have developed more successful methods of capturing kestrels, but the hawk does not inhabit much of Canada where population declines are also being noted. In addition, Kestrel populations began their decline in 1984 – much sooner than the advent of West Nile Virus in 1999.

Most researchers theorize that the decline is the result of a combination of factors, and more research is needed. Many organizations are trying to preserve suitable grassland habitats and retain hollow, den trees and perches around the perimeter of these fields.

In addition, kestrels readily utilize suitable nest boxes, so many programs focus on erecting nesting structures around meadows. Hopefully, these conservation practices, combined with increasing knowledge on the causes of decline will enable us restore this beautiful bird to our landscapes.

Hawks of Frederick County

Hawks are medium-sized birds of prey that are widely distributed throughout the globe. Hawks have sharp bills, large talons, strong legs, and very keen eyesight, including photo receptors that pick up ultra violet light and magnetic fields.

Hawks are believed to be some of the more intelligent species of bird. Like most birds, many hawks migrate in the fall and spring. Some hawks, such as the Swainson’s hawk, migrate great distances—about 12,000 miles round trip. Some migrate in large flocks of 1,000 or more birds, like the Broad Wing hawk.

In the hawk family, the female tends to be significantly larger than the male. There are two main groupings of hawks— the Accipiter and Buteo, based upon the bird’s general appearance and how it flies and hunts for prey. Accipiters tend to have short wings and long tails. They fly low to the ground, and flap their wings a lot. They tend to inhabit forested areas, and they will sit on a perch waiting for unsuspecting prey swooping down for the kill.

The accipiter tends to feed on other birds. Examples of accepters include the Cooper’s hawk, Sharp shinned Hawk, Sparrow Hawk, and Goshawk. Buteo hawks tend to glide about at higher elevations, and swoop down to catch their prey. Buteo hawks have large broad wings, short tails, and very keen eyesight. They tend to be larger in size than accipiter’s.

Hawks tend to nest in trees; some hawks, like the goshawk, will vigorously defend their nest against intruders. Most species of hawks are very territorial unless they are in migration mode.

Red Tail Hawk

The red tail hawk is considered to be the most prevalent hawk found in North America. This hawk gets its name from its very conspicuous red tail. The red tail is a member of the Buteo family of hawks. They have broad wings and a relatively short tail, adaptations that it uses while soaring high in the sky searching for food. Like most Buteos, red tails tend to feed on small mammals like rabbits, squirrels, rodents, and snakes, but they are opportunists and will take an occasional bird or fish.

Red tail Hawks are generalists—they can be found in all kinds of settings such as open land, dessert, forests, swampy areas, and suburban settings. These hawks are often spotted on utility lines, scanning the ground for food. They tend to be year-round residents in most areas, except for the far north where the bird will migrate to warmer climates in the fall. A common occurrence of hunting red-tails involves the hawk being mobbed by birds, usually crows. When a solitary red tail is attacked by numerous birds, grievous harm or death for the hawk can occur. Red tails are the most popular bird used in falconry in the United States.

Cooper’s Hawk

The Cooper’s hawk is a medium-sized hawk that has a fairly wide distribution across the United States and Mexico, and is quite common in Maryland. The Cooper’s Hawk is an Accipiter, meaning that it tends to perch in trees and utility lines, and swoop down to catch its prey. The bird was named after William Cooper who was a famous Naturalist in the 1800’s. Cooper’s hawks are very quick and agile, with strong legs and large talons, traits that serve them well when hunting for birds and small mammals.

Cooper’s Hawks target game birds like pheasant, quail, and grouse, along with other small and medium-sized birds. It was the bird’s propensity to target game birds that led to large scale bounties being placed on the hawk in the 1800’s early 1900’s that drastically reduced their numbers. The populations rebounded quickly after protective measures were put on this bird and other raptors, so today its population is fairly robust.

While the Cooper’s Hawk feeds on some birds that are declining such as prairie chicken, cerulean warbler, and golden winged warbler, the declines that these birds are witnessing is due more to loss of suitable habitat rather than predation. One reason for the smaller sparrow hawk’s population decline is due to competition by the Cooper’s hawk. The Cooper’s hawk is agile, fast, and adaptable, traits that serve this bird well in the natural world.