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American Kestrel
Falco sparverius
– Family
Authors: Smallwood, John A., and David M. Bird

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Male American Kestrel, Bosque de Apache NWR, NM, November.
Figure 1. Distribution of the American Kestrel.
Female American Kestrel, Davis, CA, January.

This species account is dedicated in honor of Elizabeth Rawlings, member of the Cornell Lab of Ornithology's Administrative Board.

Formerly known as the Sparrow Hawk, the American Kestrel is the smallest, most numerous, and most widespread North American falcon. Of the 13 kestrel species that occur throughout the world, it is the only one found in the Western Hemisphere, where as many as 17 subspecies are recognized from Alaska and Canada to Tierra del Fuego, Argentina. This colorful falcon is sexually dichromatic: the male has blue-gray wings and a rufous tail with a single broad subterminal black band, while the wings and tail of the female are rufous with black bars across their lengths. There is considerable individual variation in plumage. The species also exhibits sexual size dimorphism, with the female about 10% heavier than the male.

The American Kestrel inhabits open areas covered by short ground vegetation where it hunts mostly from perches, frequently from utility wires along roadside berms, but also by hovering, especially when suitable perches are lacking. A hovering bird faces into the wind, with head apparently fixed in space, while the wings alternately flap and glide and the tail constantly adjusts to each eddy in the breeze. It feeds on arthropods and small vertebrates, usually capturing these on the ground, although some individuals become proficient at capturing insects and small birds in flight. The kestrel is attracted to human-modified habitats, such as pastures and parkland, and often is found near areas of human activity, including some heavily developed urban areas.

This falcon is a secondary cavity nester, using woodpecker-excavated or natural cavities in large trees, crevices in rocks, and nooks in buildings and other structures. The availability of suitable cavities appears to limit its populations in many parts of the breeding range. The species readily uses artificial nest boxes, and there is increasing public interest in participating in nest-box programs.

Many aspects of the life history of this species, including foraging and nesting behavior, have been well studied. Notable among these works is a monograph on the behavior and ecology of American Kestrels breeding in natural cavities in northern California (Balgooyen 1976). Other researchers maintain hundreds of nest boxes that support study populations. These investigations have provided information on reproductive strategies (for example, parental investment [Wiebe and Bortolotti 1995], adaptive shifts in offspring sex ratios [Smallwood and Smallwood 1998], postfledging behavior [Varland et al. 1991], and geographic variation in plumage characteristics [Smallwood et al. 1999]). Such long-term studies of marked individuals are particularly valuable as they likely will yield important data on demography and on lifetime reproductive success (that is, a direct measure of fitness).

The American Kestrel is an excellent laboratory animal as well. It readily breeds in captivity (Willoughby and Cade 1964) and served as the primary model for bioaccumulation of organochlorine pesticides in birds of prey (Porter and Wiemeyer 1969, Lincer 1975). It was the first falcon to be produced by artificial insemination (Bird et al. 1976) and the first produced from frozen semen (Brock and Bird 1991).