By Lee Franks
The American kestrel, Falco sparverius, is a colorful jay-sized falcon commonly seen perched on the PG&E power towers at the west end of the Park, watching for unwary grasshoppers, birds and rodents. When not perched, they can often be seen hovering, the single most strenuous flight maneuver a bird can execute, above potential prey. All falcons are skilled hunters, and they have a unique, tooth-like projection on their hooked bills that can quickly crush the neck of small prey. The American Kestrel’s species name, sparverius, is Latin for “pertaining to sparrows,” an occasional prey item. It was formerly known as the Sparrow Hawk.
A small (10.5″) falcon easily identified by its bright rusty back and tail, and striking blue-gray wings. Their cheeks are white with two vertical black stripes. They are whitish below with a rusty wash. When seen in flight from below, they show a distinctive row of translucent spots on the trailing edge of their wings. They show the characteristic falcon in-flight silhouette: long, pointed wings, deep wing-beats, and long tail.
Primarily a “sit-and-wait” hunter, locating prey visually from exposed hunting perches. Most capture attempts occur within 50m of their perch. Hover-hunting usually occurs when suitable perches are absent. They hover by facing into the wind with wings in the glide posture. Wing and body posture are adjusted continuously to wind turbulence while the head remains remarkably fixed.
Most prey are captured on the ground, although some individuals become adept at capturing flying insects or small birds. They make direct flight to prey, which is seized with one or both feet. Small prey are consumed on the ground, and larger prey are typically carried back to the hunting perch. They will cache uneaten remains of prey and surplus kills, to be used during periods of unfavorable weather or to meet the needs of a growing brood. Time spent on the ground is typically only a few seconds, but occasionally they will spend several minutes on the ground when faced with difficult prey
Despite their size, kestrels successfully defend their territories against larger raptors, such as Red-tailed Hawks and Cooper’s Hawks. Also, they are often successful in evicting or out-competing nest competitors such as woodpeckers and squirrels.
American Kestrels have a fairly limited repertoire consisting of variations on 3 main vocalizations; klee (a rapid series of 3 to 6 notes which carry far), whine (may last as long as 1 to 2 minutes, and is used by both sexes during courtship), chitter (most frequently used call by both sexes in their interactions). Kestrel chicks are able to give weak, but consistent peep calls at 1 day old, to beg for food. This food-begging call increases in intensity as they grow. Nestlings can whine, chitter, and klee at about 2 weeks of age.
Males locate and inspect potential nest cavities within their established territories, then escort the female to them. The female makes a selection from those shown to her. Kestrels prefer tree cavities surrounded by large open patches covered with short ground vegetation with adequate hunting perches nearby.
No nesting material is brought to the nest cavity. The female simply hollows out a shallow depression in any loose material on the cavity floor. One egg is laid every other day until a clutch of 4 to 5 is achieved. Both sexes incubate the eggs for approximately 30 days. A clutch of 5 eggs takes 2 to 3 days to hatch, each egg taking 48 to 52 hours from pip. The female helps the young out of the shell, which is either trampled into the nest or eaten by the female. Brooding by the female begins immediately at hatching and lasts 8 to 10 days.
Young depart the nest at 28 to 31 days old, but are dependent on their parents for food for about 12 to 14 days after fledging. Flight agility increases noticeably during the first week after nest departure. Perch-hunting success occurs 3 to 5 weeks after nest departure.
The Birds of North America, No 602, 2002; John A. Smallwood and David M . Bird.