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Revision as of 18:38, 6 October 2010

American Kestrel
A male American Kestrel
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Aves
Order:
Falconiformes
Family:
Falconidae
Genus:
Falco
Species:
F. Sparverius
Binomial name
Falco Sparverius
Linnaeus, 1758
American Kestrel range
  Year Round
  Summer (breeding)
  Winter (nonbreeding)
Male upperparts pattern
Adult female located in Manitoba, Canada

The American Kestrel (Falco sparverius), sometimes colloquially known as the Sparrow Hawk, is a small falcon found in the Western Hemisphere. It is the most common falcon in North America, as it can live in a wide variety of habitats. At 19–21 centimetres (7–8 in) long, it is also the smallest falcon in North America. It exhibits sexual dimorphism in size and plumage, although both genders have a rufous back with noticeable barring. Juveniles are similar to the adults.

The American Kestrel hunts by hovering in the air with rapid wing beats or perching and scanning the ground for prey. Its diet typically consists of grasshoppers, lizards, mice, and other small birds. It nests in cavities in trees, cliffs, buildings, and other structures. The female lays 3–7 eggs, which both sexes help to incubate. It is a common bird to be used in falconry, especially by beginners.

Their breeding range extends from central and western Alaska across northern Canada to Nova Scotia, and south throughout North America, into central Mexico and the Caribbean. They are local breeders in Central America and are widely distributed throughout South America. Most birds breeding in Canada and the northern United States migrate south in the winter. It is the only kestrel found in the Western Hemisphere, though is a very rare vagrant to western Europe.

Description

The American Kestrel is the smallest falcon in North America–about the size of a large thrush, such as the American Robin (Turdus migratorius).[2] The American Kestrel is sexually dimorphic, although there is some overlap in plumage coloration within the species. The bird can be 19–21 centimetres (7–8 in) long and have a wingspan of 50–60 centimetres (20–24 in), with the female typically larger than the male. The male can weigh 103–120 grams (3.6–4.2 oz), and the female 126–166 grams (4.4–5.9 oz).[3] However, these subtle differences are often difficult to discern in the field.

Plumage has more variation between the sexes. Males have blue-grey wings with black spots, while the undersides are white with black barring. The back is rufous, with barring on the lower half. The belly and flanks are white with black spotting. The tail is also rufous, with a white or rufous tip and a black subterminal band.[4]

The back and wings of the female American Kestrel are rufous with dark brown barring. The undersides of the females are creamy to buff with heavy brown streaking. The tail is noticeably different from the male's, being rufous in color with numerous narrow dark black bars. Juveniles exhibit coloration patterns similar to the adults.[4] In both sexes, the head is white with a bluish-grey top. There are also two narrow, vertical black facial markings on each side of the head, while other falcons have one.[5] Two black spots (ocelli) can be found on each side of the white or orangish nape.[6] 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.[7] The wings are moderately long, fairly narrow, and taper to a point. While perched, the wingtips are noticeably shorter than the tail tip.

Vocalizations

The American Kestrel has three basic vocalizations – the "klee" or "killy", the "whine", and the "chitter."[8] The "klee" is usually delivered as a rapid series – klee, klee, klee, klee when the kestrel is upset or excited. This call used in a wide variety of situations and is heard from both sexes, but the larger females typically have lower-pitched voices than the males. The "whine" call is primarily associated with feeding, but is also uttered during copulation. The "chitter" is used in activities which involve interaction between male and female birds, including courtship feeding, copulation, and the feeding of nestlings.[9] Nestlings can produce calls similar to those of adults at 16 days old.[10]

Systematics

Until the sixth edition of the AOU Checklist of North American Birds was published by the American Ornithologists' Union in 1983, the most commonly used name for the American Kestrel was the Sparrow Hawk or Sparrowhawk. This was due to a mistaken connection with the Eurasian Sparrowhawk in the genus accipiter. The sixth edition of the AOU Checklist corrected this, officially renaming the bird American Kestrel. Several other local names for the kestrel are also in use, including Grasshopper Hawk, due to its diet, and Killy Hawk, due to its distinct call.[11]

The American Kestrel's scientific name was given by Carolus Linnaeus in his 18th Century work Systema Naturae under the name Falco sparverius. The genus refers to the falcate, or hooked, shape of the beak, and the species name means "pertaining to a sparrow", referring to the bird's small size and occasional hunting of sparrows.[11]

17 subspecies of the American Kestrel are recognized, generally based upon plumage, size, and vocalizations:[12]

  • F. s. sparverius (Linnaeus, 1758) is the nominate subspecies. It is found in most of the United States, Canada, and Mexico.
  • F. s. paulus (Howe and King, 1902) is found in the Southeast United States, from Louisiana to Florida.
  • F. s. peninsularis (Mearns, 1892) is found in southern Baja California.
  • F. s. tropicalis (Griscom, 1930) is found from southern Mexico to northern Honduras.
  • F. s. nicaraguensis (Howell, 1965) is found in Honduras and Nicaragua.
  • F. s. sparveroides (Vigors, 1827) is found in Cuba and the Isle of Youth, and southern to central Bahamas.
  • F. s. dominicensis (Gmelin, 1788) is found in Puerto Rico through the Lesser Antilles to Grenada.
  • F. s. caribaearum (Gmelin, 1788) is found in Hispaniola.
  • F. s. brevipennis (Berlepsch, 1892) is found in the Netherlands Antilles.
  • F. s. isabellinus (Swainson, 1837) is found from Venezuela to northern Brazil.
  • F. s. ochraceus (Cory, 1915) is found in eastern Colombia and northwest Venezuela.
  • F. s. caucae (Chapman, 1915) is found in western Colombia.
  • F. s. aequatorialis (Mearns, 1892) is found in northern Ecuador.
  • F. s. peruvianus (Cory, 1915) is found in southwest Ecuador, Peru, and northern Chile.
  • F. s. fernandensis (Chapman, 1915) is found on the Juan Fernández Islands off Chile.
  • F. s. cinnamominu (Swainson, 1837) is found in Peru, Chile, and Argentina.
  • F. s. cearae (Cory, 1915) is found from northeast Brazil south to eastern Bolivia.

Ecology and behavior

American Kestrels are found in a wide variety of habitats, including grasslands, meadows, deserts, and other open to semiopen regions. They can also be found in both urban and suburban areas. Requirements for a kestrel's habitat include open space for hunting, perches, and cavities for nesting (whether they be natural or man-made).[13] The American Kestrel is also able to live in very diverse conditions, including at an elevation of over 4,500 metres (14,764 ft)* in elevation in the Andes of South America.[14] Its distribution ranges from northern Canada and Alaska, down to the southernmost tip of South America, Tierra del Fuego.

American Kestrels in Canada and the northern United States typically migrate south in the winter, sometimes going as far as Central America and the Caribbean. Birds that breed south of about 35 degrees north latitude are usually year-round residents. Migration also depends on local weather conditions.[15] There is often is a sex difference in wintering kestrels' selection of habitat. Females are usually found in open areas more often than males during the non-breeding season. Explanations for this behavior include the two sexes utilizing their habitats differently, or the larger females arriving to the preferred habitat first and excluding males from their territory.[16]

The American Kestrel is not long-lived, having an average lifespan of 5 years and 2 months. The oldest banded wild bird was 11 years and seven months old,[17] and the oldest captive kestrel was a 14 year old male house pet.[18] The kestrel has an average mortality rate of 57 percent.[18] In a study, humans accounted for 43.2% of 1,355 reported deaths, which included direct killing and roadkills, while predation accounted for 2.8%.[19] This statistic is likely biased, however, as reported deaths are usually found near or in areas populated by humans.

Female about to pounce on a snake near Morro Bay, California

Feeding

American Kestrels feed largely on small creatures such as grasshoppers, dragonflies, lizards, mice, and voles. They will also eat other small birds. The Kestrel has also been reported to have killed larger animals such as snakes, bats, and squirrels.[20] Part of the reason the Kestrel maintains high population densities is because it can eat a wide variety of both insects and vertebrates. The American Kestrel's primary mode of hunting is by perching and waiting for prey to come near. The bird is characteristically seen along roadsides or fields perching from objects such as trees, telephone wires, or fence posts. It also hunts by hovering in the air with rapid wing beats and scanning the ground for prey. Other hunting techniques include low flight over fields, or chasing insects in the air.[21]

Prey is almost always always caught on the ground. Before striking, the kestrel characteristically bobs it's head and tail, then makes a direct flight toward the prey to grab it in its talons. During the breeding season, the bird will carry large prey back to his or her mate or young. One study found that an American Kestrel pair "foraged in ways that minimized the costs of energy acquisition in its particular situation", according to one study. For example, if a particular area falls below the average success rate for capturing prey, the bird will move to a different area.[22]

Reproduction

Male (right) presenting grasshopper to female

American Kestrels are sexually mature during their first spring.[23] In migratory populations the males arrive at the breeding ground before females, then the female selects a mate. Pair bonds are strong, often permanent. Pairs also usually return to previous nesting sites in consecutive years, since it would give birds an advantage over younger or invading individuals to already be familiar with the hunting grounds, neighbors, predators, and other features of the site.[24] Males perform elaborate dive displays to advertise their territory and attract a mate. These displays consist of several climbs and dives, with 3-4 'klee' calls at their peak. Females are promiscuous for about 1-2 weeks after their arrival at the nesting site. This is thought to stimulate ovulation.[25] Food transfers from the male to the female occur from about 4-5 weeks prior to egg laying to 1-2 weeks after.[26]

American Kestrels are cavity nesters, but they are able to adapt to a wide variety of nesting situations. They generally prefer natural cavities (such as in trees) with closed tops and tight fitting entrances, as to provide for maximum protection of the eggs and young.[27] Kestrels occasionally use holes created by large woodpeckers,[28] or use the abandoned nests of other birds, such as Red-tailed Hawks, Merlins, and crows.[29] They have been recorded nesting on cliff ledges and building tops, as well as in abandoned cavities in cactuses.[30] American Kestrels also commonly utilize nesting boxes.[31]

A young American Kestrel enjoying the sun

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. There are from three to seven eggs per clutch, but four to six are average. They hatch 29 to 31 days after being laid. The eggs are typically elliptical in shape, and are white or pinkish-white with an even covering of fine spots and flecks of brown shades, occasionally concentrating as a ring or a cap.

The young grow very quickly, becoming noisy between day 11 and 14 and assuming adult weight in about 2.5 weeks. The young fledge in 30 to 31 days. Early fledgling behavior varies. Broods typically stay together for a week or two. Some broods remain close to the nest area for a week or two while others travel throughout the parents' home range. Young do not disperse (permanently moving more than 0.6 miles (one km) away from the nest area) until 23 to 26 days after fledging. Young disperse as hunting skills develop. Occasionally groups of older juveniles from various broods join together into flocks.

The American Kestrel can raise two broods per year, notably in areas with abundant small mammals e.g. in the southern United States. Replacement clutches can be laid.

Status and conservation

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.

Use in falconry

Male at the Canadian Raptor Conservancy, Canada

American Kestrels (along with the Red-tailed Hawk) are one of two raptors almost universally used by new (apprentice) falconers in the United States. This is largely due to federal laws restricting apprentices to possession of the following raptors

"which must be taken from the wild: an American kestrel (Falco sparverius), a red-tailed hawk(Buteo jamaicensis), a red-shouldered hawk (Buteo lineatus), or, in Alaska only, a goshawk (Accipiter gentilis)."

The American Kestrel is generally considered to be a harder bird to care for due to its small size, quick metabolism, and fragile nature (in comparison with the much larger Red-tail and other raptors used in falconry) and requires extensive weight management, often within a couple tenths of a gram.

Falconers typically use American Kestrels to hunt non-game species: insects, sparrows (typically the invasive European variety), starlings, and grackles. Male American Kestrels, due to their smaller size, are best suited for sparrows and starlings—though particularly skilled and competent males may occasionally tackle grackles. The larger females are slightly less agile than the males but can take larger prey. Females generally are very slightly less successful in taking sparrows in comparison to males, but are more capable with taking starlings, grackles, and a variety of other small birds. Females have even been known to take doves, though this is a very rare occurrence.

Male American Kestrel with handler, San Diego Zoo

Hunting Kestrels are at risk of predation by cats, dogs, and other raptors, in particular Cooper's Hawk (Accipiter cooperii).

Behaviorally, kestrels generally tame down relatively quickly (1–3 weeks) and to a great degree—especially in comparison to other raptors. This tameness helps considerably in day-to-day care, as Kestrels used in falconry should be kept indoors in order to better control weight and protect the bird from predation. The downside of this tameness is the American Kestrel's penchant to begin "screaming" (food begging) – incessant, piercing, loud calls—whenever the falconer appears.

The mortality rate for American Kestrels kept by falconers is estimated to be 7.4% per year, whereas mortality rates for wild kestrels can be well over 65% per year. Most falconers eventually release their kestrels back into the wild once they are of breeding age. There are so few falconers in the United States (estimated at less than 4,000) that contribution to the estimated millions of wild American Kestrels in the United States would be statistically insignificant.

References

  1. ^ Template:IUCN2009.1
  2. ^ Wauer (2005), pp. 6–7
  3. ^ McCollough. "American Kestrel Falco sparverius". University of Michigan Museum of Geology. Retrieved September 13, 2010.
  4. ^ a b "American Kestrel, Falco sparverius". Cornell Lab of Ornithology. Retrieved September 13, 2010.
  5. ^ Tveten & Tveten (2004), p. 210
  6. ^ Clark & Wheeler (2001), p. 252
  7. ^ Negro, Juan José; Bortolotti, Gary R.; Sarasola, José Hernán (2007). "Deceptive plumage signals in birds: manipulation of predators or prey?". Biological Journal of the Linnean Society. 90 (3). Linnean Society of London: 467–477. doi:10.1111/j.1095-8312.2007.00735.x/abstract. {{cite journal}}: Unknown parameter |doi_brokendate= ignored (|doi-broken-date= suggested) (help)
  8. ^ Mueller, Helmut C. (1971). "Displays and Vocalizations of the Sparrow Hawk" (PDF). The Wilson Bulletin. 83 (3). Wilson Ornithological Society: 249–254.
  9. ^ Wauer (2005), pp. 11–12
  10. ^ Smallwoood, John A.; Dudajek, Valerie (2003). "Vocal Development in American Kestrel (Falco sparverius) Nestlings" (PDF). Journal of Raptor Research. 37 (1). Raptor Research Foundation: 37–43.
  11. ^ a b Wauer (2005), p. 4
  12. ^ Smallwood, John A.; Bird, David M. (2002). "American Kestrel: Systematics". Cornell Lab of Ornithology. Retrieved 4 September 2010.
  13. ^ "American Kestrel, Life History". All About Birds. Cornell Lab of Ornithology. Retrieved 3 September 2010.
  14. ^ Fjeldså & Krabbe (1990), p. 112
  15. ^ Wauer (2005), pp. 23–24
  16. ^ Ardia, Daniel R.; Bildstein, Keith L. (1997). "Sex-related differences in habitat selection in wintering American kestrels,Falco sparverius". Animal Behavior. 53 (6). The Association for the Study of Animal Behaviour: 1305–1311. doi:10.1006/anbe.1996.0364.
  17. ^ Clapp, Roger B.; Klimkiewicz, M. Kathleen; Kennard, John H. (1982). "Longevity Records of North American Birds: Gaviidae through Alcidae". Journal of Field Ornithology. 53 (2). Association of Field Ornithologists: 107.
  18. ^ a b Roest, A. I.; Kathleen Klimkiewicz, M; Kennard, John H (1957). "Notes on the American Sparrow Hawk". The Auk. 74 (1). American Ornithologists' Union: 16.
  19. ^ Smallwood, John A.; Bird, David M. (2002). "American Kestrel: Demography and Populations". Cornell Lab of Ornithology. Retrieved 5 September 2010.
  20. ^ Sherrod, Steve K. (1978). "Diets of North American Falconiformes" (PDF). Journal of Raptor Research. 12 (2). Raptor Research Foundation: 103–106.
  21. ^ Collopy, Michael W.; Koplin, James R. (1983). "Diet, Capture Success, and Mode of Hunting by Female American Kestrels in Winter". The Condor. 85 (3). Cooper Ornithological Society: 69–371.
  22. ^ Rudolph, Seri G. (1982). "Foraging Strategies of American Kestrels During Breeding". Ecology. 63 (5). Ecological Society of America: 1268–1276. doi:10.2307/1938854.
  23. ^ "American kestrel (Falco sparverius)". Avian Science and Conservation Centre. 2003. Retrieved 6 September 2010.
  24. ^ Wauer (2005), p. 52
  25. ^ Wauer (2005), p. 54
  26. ^ Smallwood, John A.; Bird, David M. (2002). "American Kestrel: Behavior". Cornell Lab of Ornithology. Retrieved 25 September 2010.
  27. ^ Wauer (2005), p. 55
  28. ^ Gault, Kathleen E.; Walters, Jeffrey R.; Tomcho, Joseph, Jr.; Phillips, Louis F., Jr.; Butler, Andrew (2004). "Nest Success of Southeastern American Kestrels Associated with Red-Cockaded Woodpeckers in Old-Growth Longleaf Pine Habitat in Northwest Florida". Southeastern Naturalist. 3 (2). Humboldt Field Research Institute: 191–204.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  29. ^ Wauer (2005), pp. 55-56
  30. ^ Smith, Dwight G.; Wilson, Charles R.; Frost, Herbert H. (1972). "The Biology of the American Kestrel in Central Utah". The Southwestern Naturalist. 17 (1). Southwestern Association of Naturalists: 73–83.
  31. ^ Rohrbaugh, Ronald W., Jr.; Yahner, Richard H. (1997). "Effects of Macrohabitat and Microhabitat on Nest-Box Use and Nesting Success of American Kestrels". The Wilson Bulliten. 109 (3). Wilson Ornithological Society: 410–423.{{cite journal}}: CS1 maint: multiple names: authors list (link)

Cited books

  • Clark, William S.; Wheeler, Brian K. (2001). A field guide to hawks of North America. New York, NY: Houghton Mifflin Harcourt. ISBN 0395670675.
  • Fjeldså, Jon; Krabbe, Niels (1990). Birds of the High Andes: A Manual to the Birds of the Temperate Zone of the Andes and Patagonia, South America. Svendborg, Denmark: Apollo Books. ISBN 8788757161.
  • L. Tveten, John; Tveten, Gloria (2004). "Our Smallest Falcon—American Kestrel: 198/1996". Our life with birds: a nature trails book. College Station, TX: Texas A&M University Press. pp. 210–212. ISBN 1585443808.
  • Wauer, Roland H. (2005). The American kestrel: falcon of many names. Boulder, CO: Johnson Books. ISBN 1555663532.
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