It exhibits sexual dimorphism in size (females being moderately larger) and plumage, although both sexes have a rufous back with noticeable barring.

[4] The American kestrel was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the current binomial name Falco sparverius.

[5] Linnaeus based his account on the "little hawk" that had been described and illustrated by the English naturalist Mark Catesby in his book The Natural History of Carolina, Florida and the Bahama Islands that was published between 1729 and 1732.

This could change in the future if continued genetic research more precisely determines the evolutionary history of the American kestrel within the genus Falco.

[16] The pectoral flight muscles of the American kestrel make up only about 12% of its body weight, as compared to about 20% for the strongest flying falcons such as the peregrine.

Their less muscular body type is adapted to energy-conserving ambush hunting, rather than spending large amounts of energy-consuming time on the wing and getting into long tail-chases of bird prey.

Their lean build and energy-conserving strategy allow a lower daily food intake than if they were more strongly muscled, yet with enough strength to commonly take bird prey as large as themselves, and occasionally larger.

The flight of the American kestrel is not so dramatic and swift as more muscular falcons such as merlins and peregrines, but their efficient adaptation to a broader diet of more available smaller prey, and need for less food per day, has resulted in there being many more of them.

[20] The function of these spots is debated, but the most commonly accepted theory is that they act as "false eyes", and help to protect the bird from potential attackers.

[24] American kestrels are found in a wide variety of habitats, including grasslands, meadows, deserts and other open to semi-open regions.

[25] The American kestrel is able to live in very diverse conditions, ranging from above the Arctic Circle,[26] to the tropics of Central America, to elevations of over 4,500 m (14,800 ft) in the Andes Mountains.

Before striking, the kestrel characteristically bobs its head and tail, then makes a direct flight toward the prey to grab it in its talons.

Much like the red-tailed hawk, American kestrels conserve energy in a hunt and pick their attacks with care as to position and odds of success.

One study found that an American kestrel pair "foraged in ways that minimized the costs of energy acquisition in its particular situation".

They generally prefer natural cavities (such as in trees) with closed tops and tight-fitting entrances that provide for maximum protection of the eggs and young.

[citation needed] In ecological terms, the reproductive pattern of the American kestrel leans towards a small bird "r selection" strategy.

[citation needed] This said, older American kestrel pairs generally have larger brood sizes and produce more viable offspring than younger conspecifics.

[58] American kestrels are often useful in scientific studies on animal physiology, and are typically captured using the bal-chatri method or raised in nest boxes for experiments.

[60][61] American kestrels' response to environmental stress is measured as blood concentration of corticosterone (CORT), a hormone produced by the hypothalamic-pituitary-adrenal (HPA) axis that releases stored energy for essential body functions.

[59] Thus, high levels of traffic disturbance and human development surrounding American kestrel nests are found to increase stress hormones leading to reproductive failure.

[59] Since American kestrels are carnivores, toxic chemical runoff ingested by their prey can concentrate at high levels in their blood.

Wild kestrels are subject to immunomodulation, or an altered immune response, to polybrominated diphenyl ethers (PBDEs), a group of industrial flame retardants that may leach from factories into the environment.

[67] While it is unknown what role eye color plays in visual acuity, this may be of greater concern to birds like kestrels who rely heavily on vision for hunting.

Such behaviors include frequent copulation, food transfers, male posturing, nest box inspection, and specific mating calls (7).

Kestrels were also found to be 20 to 30 times more sensitive to secondary poisoning from diphacinone than other birds like Northern bobwhite and mallard ducks.

[1] The Peregrine Fund, a leading non-profit organization advancing research and conservation of birds of prey worldwide, launched the American Kestrel Partnership in 2012.

Each breeding season, the American Kestrel Partnership features a live-streaming video feed[78] from the nest box located at The Peregrine Fund's campus in Boise, Idaho.

It is often considered a beginner's bird, though the careful weight control needed to maintain the kestrel's desire to aggressively hunt takes skill.

Falconers experienced in extracting the best performance the species is capable of, report they are highly reliable on the normal game of sparrows and starlings.

The advantage the American kestrel offers the experienced falconer is its suitability to simple and urban falconry not requiring large tracts of land or the use of hunting dogs.