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.
Article by FCFCDB
Nature note for 4/26/20