Birds
American Kestrel
Falco sparverius · also called sparrow hawk, killy hawk
The American kestrel is a falcon the size of a jay — a small, sharp, beautifully marked bird that hovers over roadside fields and drops on grasshoppers and voles. It is the most common raptor in North America, and one that has been slipping away for half a century without anyone being quite sure why.
Overview
Falco sparverius ranges from Alaska to Tierra del Fuego, in open country, farmland, deserts, and cities. It is only 22–31 cm long and about 100–120 g, with the pointed wings and fast flight of a true falcon. It eats mostly large insects and small mammals, and caches surplus food.
Colour and false eyes
Unusually for a raptor, the male and female look different — blue-winged male, rusty female. Both have two black false-eye spots on the back of the head that may deter attacks from behind.
Hunting by hover and by UV
It hangs on beating wings over a field with its head held dead still, then drops. It also sees ultraviolet light, and vole urine trails glow in UV, so a bare-looking field is marked up with the routes its prey uses.
Status
Least Concern globally but declining across much of North America since the 1960s — the cause debated, and the subject of large nest-box research networks.
American Kestrel: common questions
Is the American kestrel the same as a sparrow hawk?
The name "sparrow hawk" has been used for the American kestrel in North America and for a completely different bird, the Eurasian sparrowhawk, in Britain, which causes confusion. The American kestrel is a small falcon, a member of the genus Falco, related to the peregrine and the merlin, and it hunts mostly insects and voles by hovering and pouncing. The Eurasian sparrowhawk is an accipiter, a woodland bird-hunting hawk with short rounded wings and a long tail that ambushes small birds in fast dashes through cover. The two are not closely related, and modern usage prefers "American kestrel" to avoid the mix-up.
How does hovering help the kestrel hunt?
Hovering lets the kestrel search a patch of open ground thoroughly from directly above, which a perched bird cannot always do and a bird in normal flight passes over too quickly. By facing into the wind and matching its airspeed to the wind speed, the kestrel holds its body still relative to the ground and, crucially, keeps its head locked in one position so its eyes have a steady image to scan. When it spots movement or a UV-marked vole trail, it stops beating its wings and drops. Hovering is energetically expensive, so kestrels do it in short bouts and switch to perch-hunting when a good vantage point is available.
Why are American kestrels declining?
No single cause has been established, and it is probably several acting together. Suggested factors include the loss of the old trees and wooden buildings that supply nesting cavities, the intensification of farmland reducing the grasshoppers and other large insects the birds rely on, exposure to modern pesticides and rodenticides, mortality from West Nile virus, and increased predation by Cooper's hawks, which have themselves rebounded and moved into the open and suburban habitats kestrels use. The decline is well documented through long-running nest-box monitoring and hawk-migration counts, but stopping it requires knowing which of these matters most, which is why it is a research priority.
Can I put up a nest box for kestrels?
Yes, and it is one of the more effective things an individual can do for the species. American kestrels take readily to a wooden box of the right dimensions, roughly 20 by 20 centimetres inside and 30 to 45 centimetres deep, with a 7.5 centimetre entrance hole, mounted 3 to 6 metres up on a pole, isolated tree, or building edge, facing away from prevailing weather, and overlooking open grassy hunting ground. A few centimetres of wood shavings inside gives the eggs a base. Organisations such as the American Kestrel Partnership provide plans and a way to report occupancy, which feeds directly into the research on the decline.