The American Kestrel is the smallest falcon in North America. In addition to requiring open space for hunting, they seem to need perches for hunting from, cavities for nesting, and a sufficient food supply.
It is the only North American falcon to habitually hover with rapid wing beats, keeping its head motionless while scanning the ground for prey. The kestrel commonly perches along fences and powerlines. It glides with flat wings and wingtips curved upward. It occasionally soars in circles with its tail spread and its wings flat.
This falcon species is not long-lived. The oldest banded wild bird was 11 years and seven months old while a captive lived 17 years. A mortality rate average of 57 percent was found. First year mortality rates have declined since 1945 with a decrease in shooting. Major causes of death include collision with traffic, illegal shooting, and predation.
The American Kestrel is a common prey item of other raptors, including Red-tailed Hawks
In summer, kestrels feed largely on grasshoppers, dragonflies, lizards, mice, and voles. They will also eat other small birds. Wintering birds feed primarily on rodents and birds. However, due to its diminutive size even in open cerrado habitat, mixed-species feeding flocks will hardly consider it a threat. The birds characteristically hunt along roadsides from telephone wires, fence posts, trees or other convenient perches when not flying in search of food. When they are flying and looking for food they frequently hover with rapid wingbeats.
Because it feeds on both insects and vertebrates, the American Kestrel maintains fairly high population densities. It has a small breeding home range, from 1.75 square miles to 2 square miles. Territory size has been estimated at 269 acres much larger wintering home ranges.
Several hunting techniques are used by the American Kestrel. It will hover over one spot and when prey is sighted the kestrel will partly fold its wings and drop lower once or several times before striking. When the prey disappears the falcon will glide in a semicircle before turning back into the wind to hover again. It will also soar in circles, or figure eights, using the same stooping tactics as when hovering.
The kestrel commonly hunts from elevated perch sites, waiting for prey to move on the ground. The kestrel bobs its head and pumps its tail just before attacking.
Other prey capture techniques include direct pursuit, landing and flushing prey from the ground (especially for grasshoppers) and then taking them in flight, capturing flying insects from an elevated perch, and nest robbing including the burrows of Bank Swallows and the nests of Cliff Swallows. It is also an occasional bat catcher, taking bats from their tree roosts, or striking bats in flight from above or as the bats leave or enter caves. The kestrel will kill and cache food items.
American Kestrels will use holes in trees, rock cavities and crevices in cliffs, artificial nest boxes, or small spaces in buildings. The number of suitable breeding cavities limits this species’ breeding density. The American Kestrel has adapted well to nest boxes. In one program, nest boxes were fixed to the backs of signs along a freeway thus allowing kestrels to breed in areas formerly devoid of nest sites. Pairs nesting in boxes on poles have much higher nesting success than pairs using boxes on trees. No nest is built inside. In nest boxes sawdust and wood shavings may be a suitable substrate for the eggs. Males and females defend the nest against intruders, with the male maintaining a small core territory and the female defending the nest cavity directly rather the surroundings.
Both sexes take turns incubating their eggs, a very rare situation among North American birds of prey where the female usually incubates exclusively. Correspondingly, both sexes develop bare oval patches on each side of their breasts where the warm bare skin can contact the eggs for warming.
As this bird occurs over a wide range and is not generally rare, the IUCN classifies it as a Species of Least Concern. Local populations may fluctuate according to resource availability, and birds may become locally extinct if habitat deteriorates.
The American Kestrel’s North American population has been estimated at 1.2 million pairs, with the Central and South American populations being as large. It is possible that the clearing of parts of North America for agriculture in the last two hundred years has caused the American kestrel population to increase. The southeastern race, Falco sparverius paulus, is in serious decline (an 82 percent decrease since the early 1940s in north central Florida) possibly due to habitat loss and loss of nest sites, and has been listed in Florida as “threatened”. Threats to the species as a whole include loss of nest sites, pesticide poisoning (dieldrin and DDT, among others), and death through collisions with vehicles as well as shooting.
Hunting Kestrels are also at risk of predation by cats, dogs, and other raptors, in particular Cooper’s Hawk (Accipiter cooperii).