American kestrel

American kestrel
Male
Female
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)

Adult female in Winnipeg, Canada

The American kestrel (Falco sparverius) is a small falcon, and the only kestrel found in the Americas. It is the most common falcon in North America, and is found in a wide variety of habitats. At 19–21 cm (7–8 in) long, it is also the smallest falcon in North America. It exhibits sexual dimorphism in size and plumage, although both sexes have a rufous back with noticeable barring. Juveniles are similar in plumage to 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 small birds (e.g. sparrows). It nests in cavities in trees, cliffs, buildings, and other structures. The female lays three to seven eggs, which both sexes help to incubate. It is a common bird to be used in falconry, especially by beginners. Though not as aggressive a hunter as many other larger falcons, proper training and weight control by the falconer allows many kestrels to become effective hunters of birds in the size range of sparrows and starlings, with occasional success against birds up to approximately twice their own weight.[2]

Its 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. It is a local breeder in Central America and is widely distributed throughout South America. Most birds breeding in Canada and the northern United States migrate south in the winter. It is an occasional vagrant to western Europe.

Description

The American kestrel is the smallest falcon in North America and, under traditional classification, is the smallest raptor in America.[3] The American kestrel is sexually dimorphic, although there is some overlap in plumage coloration between the sexes. The bird ranges from 12 to 27 cm (4.7 to 10.6 in) in length with a wingspan of 50–61 cm (20–24 in). The female kestrel is larger than the male. The male weighs 80–105 g (2.8–3.7 oz), as opposed to the female, which weighs 100–120 g (3.5–4.2 oz). In standard measurements, the wing bone is 16–21 cm (6.3–8.3 in) long, the tail is 11–15 cm (4.3–5.9 in) and the tarsus is 3.2–4 cm (1.3–1.6 in).[4][5][6]

In contrast to many other raptor species, the sexes differ more in plumage than in size. Males have blue-grey wings with black spots and white undersides 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.[7] 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'.[7] 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.[8] Two black spots (ocelli) can be found on each side of the white or orangish nape.[9] 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.[10] The wings are moderately long, fairly narrow, and taper to a point.

Vocalizations

The American kestrel has three basic vocalizations – the "klee" or "killy", the "whine", and the "chitter."[11] The "klee" is usually delivered as a rapid series – klee, klee, klee, klee when the kestrel is upset or excited. This call is 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 that involve interaction between male and female birds, including courtship feeding, copulation, and the feeding of nestlings.[12] Nestlings can produce calls similar to those of adults at 16 days old.[13]

Taxonomy

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 colloquial names for the kestrel are also in use, including grasshopper hawk, due to its diet, and killy hawk, due to its distinct call.[14]

The American kestrel's scientific name, Falco sparverius, was given by Carl Linnaeus in his 18th century work Systema Naturae.[15] The genus refers to the falcate, or hooked, shape of the beak, and the specific name means "pertaining to a sparrow", referring to the bird's small size and occasional hunting of sparrows.[14]

Seventeen subspecies of the American kestrel are recognized, generally based upon plumage, size, and vocalizations:[16]

Ecology and behavior

Female about to pounce

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. A kestrel's habitat must include perches, open space for hunting, and cavities for nesting (whether natural or man-made).[17] The American kestrel is able to live in very diverse conditions, ranging from above the Arctic Circle,[18] to the tropics of Central America, to elevations of over 4,500 m (14,800 ft) in the Andes Mountains.[19] The bird is distributed from northern Canada and Alaska to the southernmost tip of South America, Tierra del Fuego. It is the only kestrel found in the Americas.[20] It has occurred as a vagrant in the UK, Denmark, Malta and the Azores.[21]

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° north latitude are usually year-round residents. Migration also depends on local weather conditions.[22] Wintering kestrels' choice of habitat varies by sex. Females are found in open areas more often than males during the non-breeding season. A common explanation for this behavior is that the larger females arrive at the preferred habitat first and exclude males from their territory.[23]

The American kestrel is not long-lived, with a lifespan of <5 years for wild birds.[24] The oldest banded wild bird was 11 years and 7 months,[25] while captive kestrels can live up to 14–17 years.[24] In a study, humans accounted for 43.2% of 1,355 reported deaths, which included direct killing and roadkills, while predation (including by larger birds of prey) accounted for 2.8%. This statistic is likely biased, however, as reported deaths are usually found near or in areas populated by humans.[24]

Feeding

American kestrels feed largely on small animals such as grasshoppers, dragonflies, lizards, mice, and voles. They will occasionally eat small birds. The kestrel has also been reported to have killed snakes, bats,[26] and squirrels.[27] The kestrel is able to maintain high population densities, at least in part because of the broad scope of its diet. 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 perched on objects such as trees, overhead power lines, or fence posts. It also hunts by kiting, 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 and birds in the air.[28]

Prey is almost always caught on the ground. Before striking, the kestrel characteristically bobs its 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 its 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". For example, if the success rate for catching prey decreases significantly in a particular area, the bird will move to a different area.[29]

Reproduction

A young bird

American kestrels are sexually mature by their first spring.[30] 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 usually use previous nesting sites in consecutive years. This gives birds an advantage over younger or invading individuals, as they would already be familiar with the hunting grounds, neighbors, predators, and other features of the site.[31] Males perform elaborate dive displays to advertise their territory and attract a mate. These displays consist of several climbs and dives, with three or four "klee" calls at their peaks. Females are promiscuous for about one to two weeks after their arrival at the nesting site. This is thought to stimulate ovulation.[32] Food transfers from the male to the female occur from about four to five weeks prior to egg laying to one to two weeks after.[33]

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.[34] Kestrels occasionally nest in holes created by large woodpeckers,[35] or use the abandoned nests of other birds, such as red-tailed hawks, merlins, and crows.[36] They have been recorded nesting on cliff ledges and building tops, as well as in abandoned cavities in cactuses.[37] American kestrels also commonly utilize nesting boxes.[38]

Three to seven eggs (typically four or five) are laid approximately 24–72 hours apart. The average egg size is 32 mm × 29 mm (1.3 in × 1.1 in), 10% larger than average for birds of its body size. The eggs are white to cream in color with brown or grey splotching. Incubation usually lasts 30 days and is mainly the responsibility of the female, although the male incubates 15–20% of the time. Eggs that are lost are typically replaced in 11–12 days. Hatching takes place over three to four days. Hatchlings are altricial, and are only able to sit up after five days. They grow very quickly, reaching an adult weight after 16–17 days. After 28–31 days, their wings develop and they are able to leave the nest.[39] The young adult kestrels may breed from a year old, and the species has a ten year life expectancy.[40]

Stress physiology

Weather

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[41] Kestrel metabolic rate has been found to increase in response to rainfall, and at ambient temperatures below about 25⁰ C. Kestrel metabolic responses to weather and temperature do not vary, however, with sex.[42] Kestrels will increase their oxygen consumption, and therefore their metabolic rate in cold and wet conditions to counteract heat loss.[42][43]

Environmental disturbance

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. Extended periods of elevated blood CORT levels may direct metabolic energy away from growth and reproduction.[41] Thus, high levels of traffic disturbance and human development surrounding American kestrel nests are found to increase stress hormones leading to reproductive failure.[44] Among successful nests, however, nestlings do not typically experience a higher stress response to environmental human disturbance, suggesting that they can tolerate a considerable degree of human activity near the nest.[41]

Environmental contaminants

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. When PBDEs accumulate in body tissues of kestrels, the T-cell mediated immune response decreases in efficiency. As a result, kestrels that ingest PBDEs may not respond sufficiently to viruses or other invading microorganisms. In addition, certain PBDEs may suppress growth and development of the spleen and bursa in American kestrels[45]

Status and conservation

The American kestrel is likely the most abundant falcon in North America, although its total population is difficult to quantify, as local populations can change quickly due to resource availability. Count data from the USGS Breeding Bird Survey (BBS) indicate that the North American breeding population is experiencing long-term and gradual but sustained declines, with some regions, such as New England and coastal California, exhibiting more rapid declines.[46][47] Count data from raptor migration corridors also indicate regional population declines and largely corroborate BBS data.[48] The North American population has been estimated at 1.2 million pairs, with the Central and South American populations being as large. A smaller estimate is 236,000 birds wintering in North America. A population increase occurred in the 18th and 19th centuries, probably due to deforestation for agriculture. The resulting pastures provided an ideal habitat for kestrels.[24]

Male with handler, San Diego Zoo

The southeastern U.S. subspecies (Falco sparverius paulus) has declined 82% since 1940 due to a decrease in nest site availability. This decline is a result of longleaf pines being cleared from agricultural fields.[49] Despite this, the American kestrel is classed as Least Concern on the IUCN Red List.[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.[50] The American Kestrel Partnership developed and maintains a web-based network for citizen and professional scientists to enter, manage, and consolidate data from kestrel nestbox monitoring programs in the Western Hemisphere. The database is being used by researchers to model and understand relationships between kestrel nesting parameters (e.g., phenology, occupancy, survival, productivity, and nestling weight and exposure to environmental toxins) and environmental factors, such as land use, landscape composition and configuration, climate conditions (e.g., drought), and point sources of environmental toxins. The American Kestrel Partnership's website, with support from the Cornell Laboratory of Ornithology, features two live, streaming video feeds from a kestrel nestbox and breeding pair on The Peregrine Fund's campus in Boise, Idaho.[50]

Relationship with humans

One important use of American kestrels is in falconry. 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, particularly in ambushing this prey by surprise when released out of a vehicle window.[51] More aggressive individuals are sometimes capable of capturing prey up to approximately twice their own body weight, allowing the occasional capture of true game birds such as quail and dove. However, most falconers interested in the reliable taking of such game do prefer larger falcons or hawks. 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. Migratory raptors native to the United States are protected by the Migratory Bird Treaty Act of 1918, so American kestrels are illegal to possess without a permit in the United States, Canada, and Mexico.[52]

References

  1. 1 2 BirdLife International (2012). "Falco sparverius". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
  2. ↑ Mullenix, Matthew (2002). American Kestrels in Modern Falconry. Western Sporting Publications. ISBN 1-888357-05-3.
  3. ↑ Wauer, Roland H.; Clark, William S. (2005). The American Kestrel: Falcon of Many Names. Johnson Books. ISBN 1-55566-353-2.
  4. ↑ McCollough, Kathryn (2001). "American Kestrel Falco sparverius". Animal Diversity Web. University of Michigan Museum of Geology. Archived from the original on 13 September 2010. Retrieved 13 September 2010.
  5. ↑ Ferguson-Lees, James; Christie, David A. (2001). Raptors of the World. Houghton Mifflin. ISBN 0-618-12762-3.
  6. ↑ American Kestrel, Life History, All About Birds – Cornell Lab of Ornithology. Allaboutbirds.org. Retrieved on 2013-02-25.
  7. 1 2 "American Kestrel, Falco sparverius". Cornell Lab of Ornithology. Retrieved 13 September 2010.
  8. ↑ Tveten & Tveten (2004), p. 210
  9. ↑ Clark & Wheeler (2001), p. 252
  10. ↑ 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 (Linnean Society of London) 90 (3): 467–477. doi:10.1111/j.1095-8312.2007.00735.x.
  11. ↑ Mueller, Helmut C. (1971). "Displays and Vocalizations of the Sparrow Hawk" (PDF). The Wilson Bulletin (Wilson Ornithological Society) 83 (3): 249–254.
  12. ↑ Wauer (2005), pp. 11–12
  13. ↑ Smallwoood, John A.; Dudajek, Valerie (2003). "Vocal Development in American Kestrel (Falco sparverius) Nestlings" (PDF). Journal of Raptor Research (Raptor Research Foundation) 37 (1): 37–43.
  14. 1 2 Wauer (2005), p. 4
  15. ↑ Linnaeus, Carolus (1758). Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata (in Latin). Holmiae: Laurentii Salvii. p. 152.
  16. ↑ Smallwood, John A.; Bird, David M. (2002). "American Kestrel: Systematics". Cornell Lab of Ornithology. Retrieved 4 September 2010.
  17. ↑ "American Kestrel, Life History". All About Birds. Cornell Lab of Ornithology. Retrieved 3 September 2010.
  18. ↑ Wauer (2005), p. 15
  19. ↑ Fjeldså & Krabbe (1990), p. 112
  20. ↑ Smallwood, John A.; Bird, David M. (2002). "American Kestrel: Introduction". Cornell Lab of Ornithology. Archived from the original on 26 September 2010. Retrieved 2 November 2010.
  21. ↑ Snow, David; Perrins, Christopher M (editors) (1998). The Birds of the Western Palearctic concise edition (2 volumes). Oxford: Oxford University Press. p. 152. ISBN 0-19-850188-9.
  22. ↑ Wauer (2005), pp. 23–24
  23. ↑ Ardia, Daniel R.; Bildstein, Keith L. (1997). "Sex-related differences in habitat selection in wintering American kestrels,Falco sparverius". Animal Behavior (The Association for the Study of Animal Behaviour) 53 (6): 1305–1311. doi:10.1006/anbe.1996.0364.
  24. 1 2 3 4 Smallwood, John A.; Bird, David M. (2002). "American Kestrel: Demography and Populations". Cornell Lab of Ornithology. Retrieved 27 May 2012.
  25. ↑ Clapp, Roger B.; Klimkiewicz, M. Kathleen; Kennard, John H. (1982). "Longevity Records of North American Birds: Gaviidae through Alcidae". Journal of Field Ornithology (Association of Field Ornithologists) 53 (2): 81–124. JSTOR 4512701.
  26. ↑ Mikula, P.; Morelli, F.; Lučan, R. K.; Jones, D. N.; Tryjanowski, P. (2016). "Bats as prey of diurnal birds: a global perspective". Mammal Review. doi:10.1111/mam.12060.
  27. ↑ Sherrod, Steve K. (1978). "Diets of North American Falconiformes" (PDF). Journal of Raptor Research (Raptor Research Foundation) 12 (2): 103–106.
  28. ↑ Collopy, Michael W.; Koplin, James R. (1983). "Diet, Capture Success, and Mode of Hunting by Female American Kestrels in Winter". The Condor (Cooper Ornithological Society) 85 (3): 69–371. doi:10.2307/1367081. JSTOR 136708.
  29. ↑ Rudolph, Seri G. (1982). "Foraging Strategies of American Kestrels During Breeding". Ecology (Ecological Society of America) 63 (5): 1268–1276. doi:10.2307/1938854. JSTOR 1938854.
  30. ↑ Duncan, James, R.; Bird, David M. (1989). "The influence of relatedness and display effort on the mate choice of captive female American kestrels" (PDF). Animal Behavior (The Association for the Study of Animal Behaviour) 37: 112–117. doi:10.1016/0003-3472(89)90011-0.
  31. ↑ Wauer (2005), p. 52
  32. ↑ Wauer (2005), p. 54
  33. ↑ Smallwood, John A.; Bird, David M. (2002). "American Kestrel: Behavior". Cornell Lab of Ornithology. Retrieved 25 September 2010.
  34. ↑ Wauer (2005), p. 55
  35. ↑ 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 (Humboldt Field Research Institute) 3 (2): 191–204. doi:10.1656/1528-7092(2004)003[0191:NSOSAK]2.0.CO;2. ISSN 1528-7092. JSTOR 3878098.
  36. ↑ Wauer (2005), pp. 55–56
  37. ↑ Smith, Dwight G.; Wilson, Charles R.; Frost, Herbert H. (1972). "The Biology of the American Kestrel in Central Utah". The Southwestern Naturalist (Southwestern Association of Naturalists) 17 (1): 73–83. doi:10.2307/3669841. JSTOR 3669841.
  38. ↑ 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 Bulletin (Wilson Ornithological Society) 109 (3): 410–423. JSTOR 4163837.
  39. ↑ Wauer (2005), pp. 59–63
  40. ↑ "American kestrel (Falco sparverius)". ARKive. Retrieved 30 November 2013.
  41. 1 2 3 Strasser, Erin H.; Heath, Julie A. "Effects of developmental conditions on nestling American Kestrel (Falco sparverius) corticosterone concentrations". General and Comparative Endocrinology 173 (1): 164–170. doi:10.1016/j.ygcen.2011.05.010.
  42. 1 2 Wilson, Glenn R.; Cooper, Sheldon J.; Gessaman, James A. (2004-11-01). "The effects of temperature and artificial rain on the metabolism of American kestrels (Falco sparverius)". Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 139 (3): 389–394. doi:10.1016/j.cbpb.2004.10.009.
  43. ↑ Willmer, Pat; Stone, Graham; Johnston, Ian (2009). Environmental Physiology of Animals (2 ed.). Wiley. p. 126. ISBN 9781444309225.
  44. ↑ Strasser, Erin H.; Heath, Julie A. (2013-08-01). "Reproductive failure of a human-tolerant species, the American kestrel, is associated with stress and human disturbance". Journal of Applied Ecology 50 (4): 912–919. doi:10.1111/1365-2664.12103. ISSN 1365-2664.
  45. ↑ Fernie, Kim J.; Mayne, Greg; Shutt, J. Laird; Pekarik, Cynthia; Grasman, Keith A.; Letcher, Robert J.; Drouillard, Ken (2005-12-01). "Evidence of immunomodulation in nestling American kestrels (Falco sparverius) exposed to environmentally relevant PBDEs". Environmental Pollution 138 (3): 485–493. doi:10.1016/j.envpol.2005.04.008.
  46. ↑ Sauer, J.R.; Hines, J.E.; Fallon, J.E.; Pardieck, J.L.; Ziolkowski,Jr., D.J.; Link, W.A. (2011). "The North American Breeding Bird Survey, Results and Analysis 1966 – 2010. Version 12.07.2011". USGS Patuxent Wildlife Research Center. Retrieved 27 May 2012.
  47. ↑ "American Kestrel Partnership: population declines". The Peregrine Fund. 2012. Retrieved 27 May 2012.
  48. ↑ "Raptor Population Index, Regional Population Trend Summaries 2011". Raptor Population Index. 2012. Retrieved 27 May 2012.
  49. ↑ Hoffman, Mark L.; Collopy, Michael W. (1988). "Historical Status of the American Kestrel (Falco sparverius paulus) in Florida". The Wilson Bulletin (Wilson Ornithological Society) 100 (1): 91–107. JSTOR 4162520.
  50. 1 2 "American Kestrel Partnership". The Peregrine Fund. 2012. Retrieved 27 May 2012.
  51. ↑ Mullenix, Matthew (2002). American Kestrels in Modern Falconry. Western Sporting Publications. pp. 82–84. ISBN 1-888357-05-3.
  52. ↑ "Legal Requirements for Raptor Possession". Bureau of Land Management. 15 July 2008. Archived from the original on 21 August 2010. Retrieved 29 October 2010.

Cited books

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