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{{leftlegend|#936aa2|outline=gray|Year round}}
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[[File:American Kestrel, Colorado.jpg|thumb|Kestrel resting in an apple tree.]]
The '''American
The American kestrel usually hunts in energy-conserving fashion by perching and scanning the ground for prey to ambush, though it also hunts from the air. It sometimes hovers in the air with rapid wing beats while homing in on prey. Its diet typically consists of [[grasshopper]]s and other insects, [[lizard]]s, mice, and small birds (e.g. [[American sparrow|sparrows]]). This broad diet has contributed to its wide success as a species. It nests in cavities in trees, cliffs, buildings, and other structures. The female lays three to seven eggs, which both sexes help to incubate.
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The American kestrel is a common bird used in [[falconry]], especially by beginners. Though not as strong a flyer as many other, larger falcons, proper training and weight control by the falconer allows many American 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.<ref>[[#Mullenix|Mullenix]]</ref>
==Description==▼
Under traditional classification, the American kestrel is the smallest raptor in America.<ref>[[#Wauer|Wauer]]</ref> The American kestrel is sexually dimorphic, although there is some overlap in plumage coloration between the sexes. The bird ranges from {{convert|22|to|31|cm|in|abbr=on}} in length<ref>Davis, Kate, ''Falcons of North America'', 2008, Mountain Press Publishing Company, {{ISBN|978-0-87842-553-2}}, Kindle Edition, Location 2232.</ref> with a wingspan of {{convert|51|-|61|cm|in|abbr=on}}. The female kestrel is larger than the male, though less so than larger falcons, being typically about 10% to 15% larger within a subspecies. The more northern subspecies tend to larger sizes (northern hemisphere), with a large northern female being about twice the size of a small southern male. The male typically weighs {{convert|80|-|143|g|oz|abbr=on}}, and the female {{convert|86|-|165|g|oz|abbr=on}}. In standard measurements, the wing bone is {{convert|16|-|21|cm|in|abbr=on}} long, the tail is {{convert|11|-|15|cm|in|abbr=on}} and the tarsus is {{convert|3.2|-|4|cm|in|abbr=on}}.<ref name="ADW">{{cite web|url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Falco_sparverius.html|title=American Kestrel ''Falco sparverius''|publisher=Animal Diversity Web. University of Michigan Museum of Geology|author=McCollough, Kathryn|year=2001|access-date=13 September 2010| archive-url= https://web.archive.org/web/20100913074940/http://animaldiversity.ummz.umich.edu/site/accounts/information/Falco_sparverius.html| archive-date= 13 September 2010 | url-status= live}}</ref><ref>{{cite book |title=Raptors of the World |last1=Ferguson-Lees |first1=James |last2=Christie |first2=David A. |publisher=Houghton Mifflin |year=2001 |isbn=978-0-618-12762-7}}</ref><ref>[http://www.allaboutbirds.org/guide/american_Kestrel/lifehistory American Kestrel, Life History, All About Birds – Cornell Lab of Ornithology]. Allaboutbirds.org. Retrieved 2013-02-25.</ref>▼
Physically, American kestrels are leaner and less muscular than larger falcons.<ref name="merlinfalconry.com">Mullenix, Matt (2003) [http://www.merlinfalconry.com/#!merlins-and-kestrels-compared Merlins and American Kestrels Compared]. merlinfalconry.com.</ref> 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.<ref>Davis, Kate, Kindle locations 225–234.</ref> The wings are moderately long, fairly narrow, and taper to a point. 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. For their size, they have strong talons and beaks, and can swiftly dispatch 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 success of this body style and hunting strategy is reflected in the high success of the species in densely populating a large range throughout the Americas. The flight of the American kestrel is not so dramatic and swift as more muscular falcons such as [[merlin (bird)|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.▼
In contrast to many other raptor species, the sexes differ more in plumage than in size. Males have blue-grey [[wing]]s 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.<ref name="Cornell">{{cite web|url=http://www.allaboutbirds.org/guide/American_Kestrel/id|title=American Kestrel, ''Falco sparverius''|publisher=Cornell Lab of Ornithology|access-date=13 September 2010}}</ref> 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'.<ref name="Cornell"/> 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.<ref>{{Cite book|last1=Tveten|first1=John L.|last2=Tveten|first2=Gloria A.|title=Our life with birds: a nature trails book|publisher=Texas A&M University Press|location=College Station, TX|year=2004|chapter=Our Smallest Falcon—American Kestrel: 198/1996|isbn=978-1-58544-380-2|page=210}}</ref> Two black spots (ocelli) can be found on each side of the white or orangish nape.<ref>{{Cite book|last1=Clark|first1=William S.|last2=Wheeler|first2=Brian K.|title=A field guide to hawks of North America|publisher=Houghton Mifflin Harcourt|location=New York, NY|year=2001|isbn=978-0-395-67067-5|page=252}}</ref> 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.<ref>{{Cite journal|doi=10.1111/j.1095-8312.2007.00735.x |last1=Negro|first1=Juan José|last2=Bortolotti|first2=Gary R.|last3=Sarasola|first3=José Hernán|year=2007|title=Deceptive plumage signals in birds: manipulation of predators or prey?|journal=Biological Journal of the Linnean Society|volume=90|issue=3|pages=467–477|doi-access=free|hdl=10261/33914|hdl-access=free}}</ref>▼
===Vocalizations===▼
The American kestrel has three basic vocalizations – the "klee" or "killy", the "whine", and the "chitter".<ref>{{Cite journal |last1=Mueller |first1=Helmut C. |year=1971 |title=Displays and Vocalizations of the Sparrow Hawk |url=https://sora.unm.edu/node/128778 |journal=The Wilson Bulletin |volume=83 |issue=3 |pages=249–254 |jstor=4160099}}</ref> 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 (zoology)|copulation]]. The "chitter" is used in activities that involve interaction between male and female birds, including [[courtship feeding]], copulation, and the feeding of nestlings.<ref>[[#Wauer|Wauer]], pp. 11–12</ref> Nestlings can produce calls similar to those of adults at 16 days old.<ref>{{Cite journal |last1=Smallwoood |first1=John A. |last2=Dudajek |first2=Valerie |year=2003 |title=Vocal Development in American Kestrel (''Falco sparverius'') Nestlings |url=https://sora.unm.edu/node/54014 |journal=Journal of Raptor Research |volume=37 |issue=1 |pages=37–43}}</ref>▼
==Taxonomy==
The American kestrel was [[Species description|formally described]] in 1758 by the Swedish naturalist [[Carl Linnaeus]] in the [[10th edition of Systema Naturae|tenth edition]] of his ''[[Systema Naturae]]'' under the current [[binomial nomenclature|binomial name]] ''Falco sparverius''.<ref>{{cite book | last=Linnaeus | first=Carl | author-link=Carl Linnaeus | year=1758 | title= Systema Naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis | volume=1 | edition=10th | page=90 | publisher=Laurentii Salvii | location=Holmiae (Stockholm) | language=Latin | url=https://www.biodiversitylibrary.org/page/726997 }}</ref> 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.<ref>{{ cite book | last=Catesby | first=Mark | author-link=Mark Catesby | year=1729–1732 | title=The Natural History of Carolina, Florida and the Bahama Islands | volume=1 | location=London | publisher=W. Innys and R. Manby | language=English, French | page=5, Plate 5 | url=https://www.biodiversitylibrary.org/page/40753133 }}</ref> Linnaeus specified the [[type locality (biology)|type locality]] as America but this was restricted to South Carolina based on Catesby.<ref>{{ cite book | editor1-last=Mayr | editor1-first=Ernst | editor1-link=Ernst Mayr | editor2-last=Cottrell | editor2-first=G. William | year=1979 | title=Check-List of Birds of the World | volume=1 | edition=2nd | publisher=Museum of Comparative Zoology | location=Cambridge, Massachusetts | page=402 | url=https://www.biodiversitylibrary.org/page/16109042 }}</ref> The genus name is from [[Late Latin]] ''falco'' meaning a "falcon". The specific epithet ''sparverius'' is [[Medieval Latin]] for a "sparrowhawk".<ref>{{cite book | last=Jobling | first=James A. | year=2010| title=The Helm Dictionary of Scientific Bird Names | publisher=Christopher Helm | location=London | isbn=978-1-4081-2501-4 | pages=[https://archive.org/stream/Helm_Dictionary_of_Scientific_Bird_Names_by_James_A._Jobling#page/n157/mode/1up 157], [https://archive.org/stream/Helm_Dictionary_of_Scientific_Bird_Names_by_James_A._Jobling#page/n361/mode/1up 361]}}</ref>
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''. 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.<ref name="Wauer4">[[#Wauer|Wauer]], p. 4</ref>
As noted in the introduction, DNA analysis shows the American kestrel to actually be genetically more closely related to the larger American falcons<ref name="Wink, M. 2004, pp. 483-498"/><ref name="Griffiths, C. 1999"/> than to the true kestrels. However, based on its physical similarity to the kestrels and the established nature of the name American kestrel, there has been little impetus to change its name. This could change in the future if continued genetic research more precisely determines the evolutionary history of the American kestrel within the genus ''Falco''. The entire genus is actually a set of species so closely related that most or all can be hybridized by artificial insemination. Significant natural hybridization of species has occurred in the past during the evolution of this closely related set of species, such that precise evolutionary genetic analysis as to which species are more basal to other species or to the genus as a whole is difficult to render.
Seventeen subspecies of the American kestrel are recognized, generally based upon plumage, size, and vocalizations:<ref>{{cite web|url=http://bna.birds.cornell.edu/bna/species/602/articles/systematics|title=American Kestrel: Systematics|last1=Smallwood|first1=John A.|last2=Bird|first2=David M.|publisher=Cornell Lab of Ornithology|year=2002|access-date=4 September 2010}}</ref>
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*''F. s. cinnamominus'', described by Swainson in 1837, is found in Peru, Chile and [[Argentina]].
*''F. s. cearae'', described by Cory in 1915, is found from northeastern Brazil south to eastern [[Bolivia]].
▲==Description==
▲Under traditional classification, the American kestrel is the smallest raptor in America.<ref>[[#Wauer|Wauer]]</ref> The American kestrel is sexually dimorphic, although there is some overlap in plumage coloration between the sexes. The bird ranges from {{convert|22|to|31|cm|in|abbr=on}} in length<ref>Davis, Kate, ''Falcons of North America'', 2008, Mountain Press Publishing Company, {{ISBN|978-0-87842-553-2}}, Kindle Edition, Location 2232.</ref> with a wingspan of {{convert|51|-|61|cm|in|abbr=on}}. The female kestrel is larger than the male, though less so than larger falcons, being typically about 10% to 15% larger within a subspecies. The more northern subspecies tend to larger sizes (northern hemisphere), with a large northern female being about twice the size of a small southern male. The male typically weighs {{convert|80|-|143|g|oz|abbr=on}}, and the female {{convert|86|-|165|g|oz|abbr=on}}. In standard measurements, the wing bone is {{convert|16|-|21|cm|in|abbr=on}} long, the tail is {{convert|11|-|15|cm|in|abbr=on}} and the tarsus is {{convert|3.2|-|4|cm|in|abbr=on}}.<ref name="ADW">{{cite web|url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Falco_sparverius.html|title=American Kestrel ''Falco sparverius''|publisher=Animal Diversity Web. University of Michigan Museum of Geology|author=McCollough, Kathryn|year=2001|access-date=13 September 2010| archive-url= https://web.archive.org/web/20100913074940/http://animaldiversity.ummz.umich.edu/site/accounts/information/Falco_sparverius.html| archive-date= 13 September 2010 | url-status= live}}</ref><ref>{{cite book |title=Raptors of the World |last1=Ferguson-Lees |first1=James |last2=Christie |first2=David A. |publisher=Houghton Mifflin |year=2001 |isbn=978-0-618-12762-7}}</ref><ref>[http://www.allaboutbirds.org/guide/american_Kestrel/lifehistory American Kestrel, Life History, All About Birds – Cornell Lab of Ornithology]. Allaboutbirds.org. Retrieved 2013-02-25.</ref>
▲Physically, American kestrels are leaner and less muscular than larger falcons.<ref name="merlinfalconry.com">Mullenix, Matt (2003) [http://www.merlinfalconry.com/#!merlins-and-kestrels-compared Merlins and American Kestrels Compared]. merlinfalconry.com.</ref> 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.<ref>Davis, Kate, Kindle locations 225–234.</ref> The wings are moderately long, fairly narrow, and taper to a point. 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. For their size, they have strong talons and beaks, and can swiftly dispatch 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 success of this body style and hunting strategy is reflected in the high success of the species in densely populating a large range throughout the Americas. The flight of the American kestrel is not so dramatic and swift as more muscular falcons such as [[merlin (bird)|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.
▲In contrast to many other raptor species, the sexes differ more in plumage than in size. Males have blue-grey [[wing]]s 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.<ref name="Cornell">{{cite web|url=http://www.allaboutbirds.org/guide/American_Kestrel/id|title=American Kestrel, ''Falco sparverius''|publisher=Cornell Lab of Ornithology|access-date=13 September 2010}}</ref> 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'.<ref name="Cornell"/> 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.<ref>{{Cite book|last1=Tveten|first1=John L.|last2=Tveten|first2=Gloria A.|title=Our life with birds: a nature trails book|publisher=Texas A&M University Press|location=College Station, TX|year=2004|chapter=Our Smallest Falcon—American Kestrel: 198/1996|isbn=978-1-58544-380-2|page=210}}</ref> Two black spots (ocelli) can be found on each side of the white or orangish nape.<ref>{{Cite book|last1=Clark|first1=William S.|last2=Wheeler|first2=Brian K.|title=A field guide to hawks of North America|publisher=Houghton Mifflin Harcourt|location=New York, NY|year=2001|isbn=978-0-395-67067-5|page=252}}</ref> 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.<ref>{{Cite journal|doi=10.1111/j.1095-8312.2007.00735.x |last1=Negro|first1=Juan José|last2=Bortolotti|first2=Gary R.|last3=Sarasola|first3=José Hernán|year=2007|title=Deceptive plumage signals in birds: manipulation of predators or prey?|journal=Biological Journal of the Linnean Society|volume=90|issue=3|pages=467–477|doi-access=free|hdl=10261/33914|hdl-access=free}}</ref>
[[File:American Kestrel, Barr Lake State Park.png|thumb|American Kestrel, Barr Lake State Park, Colorado]]
▲===Vocalizations===
▲The American kestrel has three basic vocalizations – the "klee" or "killy", the "whine", and the "chitter".<ref>{{Cite journal |last1=Mueller |first1=Helmut C. |year=1971 |title=Displays and Vocalizations of the Sparrow Hawk |url=https://sora.unm.edu/node/128778 |journal=The Wilson Bulletin |volume=83 |issue=3 |pages=249–254 |jstor=4160099}}</ref> 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 (zoology)|copulation]]. The "chitter" is used in activities that involve interaction between male and female birds, including [[courtship feeding]], copulation, and the feeding of nestlings.<ref>[[#Wauer|Wauer]], pp. 11–12</ref> Nestlings can produce calls similar to those of adults at 16 days old.<ref>{{Cite journal |last1=Smallwoood |first1=John A. |last2=Dudajek |first2=Valerie |year=2003 |title=Vocal Development in American Kestrel (''Falco sparverius'') Nestlings |url=https://sora.unm.edu/node/54014 |journal=Journal of Raptor Research |volume=37 |issue=1 |pages=37–43}}</ref>
<gallery>
File:Male American Kestrel.jpg|Male upperparts pattern
Line 89 ⟶ 91:
File:20200707-DSC 1815.jpg|alt=Just fledged male|Just fledged male – Maine
</gallery>
American kestrels in Canada and the northern United States typically migrate south in the winter, some of them converging with resident kestrels of smaller size in Mexico,<ref>{{Cite journal |last1=Frixione |first1=Martín G. |last2=Rodríguez-Estrella |first2=Ricardo |date=2020-12-01 |title=Genotoxicity in American kestrels in an agricultural landscape in the Baja California peninsula, Mexico |url=https://doi.org/10.1007/s11356-020-10392-0 |journal=Environmental Science and Pollution Research |language=en |volume=27 |issue=36 |pages=45755–45766 |doi=10.1007/s11356-020-10392-0 |pmid=32803597 |bibcode=2020ESPR...2745755F |s2cid=221132575 |issn=1614-7499}}</ref><ref>{{Cite web |url=https://meridian.allenpress.com/rapt/article-abstract/57/2/275/492370/Factors-Influencing-Prevalence-and-Intensity-of?redirectedFrom=fulltext |access-date=2023-09-22|title=The Journal of Raptor Research |website=meridian.allenpress.com}}</ref> 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.<ref>[[#Wauer|Wauer]], pp. 23–24</ref> American Kestrels breeding at lower latitudes – below 48ºN to be precise – arrive earlier after warmer springs, whereas birds from higher latitudes return to their breeding grounds at the same time each year. These patterns suggest that short-distance migrants are better able to cope with climate change.<ref>{{Cite journal |last1=Powers |first1=Breanna F. |last2=Winiarski |first2=Jason M. |last3=
The American kestrel is not long-lived, with a lifespan of <5 years for wild birds.<ref name="BNA Demography">{{cite web|url=http://bna.birds.cornell.edu/bna/species/602/articles/demography|title=American Kestrel: Demography and Populations|last1=Smallwood|first1=John A.|last2=Bird|first2=David M.|publisher=Cornell Lab of Ornithology|year=2002|access-date=27 May 2012}}</ref> The oldest [[bird banding|band]]ed wild bird was 11 years and 7 months,<ref>{{Cite journal|last1=Clapp|first1=Roger B.|last2=Klimkiewicz|first2=M. Kathleen|last3=Kennard|first3=John H.|year=1982|title=Longevity Records of North American Birds: Gaviidae through Alcidae|journal=Journal of Field Ornithology|volume=53|issue=2|pages=81–124|jstor=4512701|url=https://sora.unm.edu/node/50944}}</ref> while captive kestrels can live up to 14–17 years.<ref name="BNA Demography"/> 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.<ref name="BNA Demography" />
===Feeding===
American kestrels feed largely on small animals such as [[grasshopper]]s,<ref>{{Cite journal |last1=Orozco-Valor |first1=Paula M. |last2=Grande |first2=Juan M. |date=2021-08-20 |title=Diet Variation of a Generalist Predator, the American Kestrel Falco sparverius, in a Gradient of Agricultural Intensification in Central Argentina |url=https://bioone.org/journals/acta-ornithologica/volume-56/issue-1/00016454AO2021.56.1.008/Diet-Variation-of-a-Generalist-Predator-the-American-Kestrel-Falco/10.3161/00016454AO2021.56.1.008.full |journal=Acta Ornithologica |volume=56 |issue=1 |doi=10.3161/00016454AO2021.56.1.008 |s2cid=237247221 |issn=0001-6454}}</ref> [[Cricket (insect)|cricket]]s, [[butterflies]], [[moth]]s,<ref name="animaldiversity.org">{{Cite web|url=https://animaldiversity.org/accounts/Falco_sparverius/|title=Falco sparverius (American kestrel)|first=Sutton|last=Townes|website=Animaldiversity.org|access-date=16 March 2022}}</ref> [[Dragonfly|dragonflies]], [[beetle]]s,<ref>{{cite web|url=https://sta.uwi.edu/fst/lifesciences/sites/default/files/lifesciences/documents/ogatt/Falco_sparverius%20-%20American%20Kestrel.pdf|title=American kestrel (Falco sparverius)|website=Sta.wi.edu|access-date=16 March 2022}}</ref> [[lizard]]s, [[Mouse|mice]], [[vole]]s, [[shrew]]s, [[frog]]s,<ref name="animaldiversity.org"/> and small birds. The kestrel has also been reported to have killed [[scorpion]]s, [[snake]]s,<ref name="doi.org">{{Cite journal |last1=Frixione |first1=Martín G. |last2=Rodríguez-Estrella |first2=Ricardo |date=2020-11-01 |title=Trophic segregation of the Burrowing Owl and the American Kestrel in fragmented desert in Mexico |url=https://doi.org/10.1080/00222933.2020.1865470 |journal=Journal of Natural History |volume=54 |issue=41–42 |pages=2713–2732 |doi=10.1080/00222933.2020.1865470 |bibcode=2020JNatH..54.2713F |s2cid=234998691 |issn=0022-2933}}</ref> [[bat]]s,<ref>{{cite journal|last1=Mikula|first1=P.|last2=Morelli|first2=F.|last3=Lučan|first3=R. K.|last4=Jones|first4=D. N.|last5=Tryjanowski|first5=P.|year=2016|title=Bats as prey of diurnal birds: a global perspective|journal=Mammal Review|volume=46|issue=3|pages=160|doi=10.1111/mam.12060}}</ref> and [[squirrel]]s.<ref>{{Cite journal|last=Sherrod|first=Steve K.|year=1978|title=Diets of North American Falconiformes|journal=Journal of Raptor Research|volume=12|issue=2|pages=103–106|url=https://sora.unm.edu/node/52680}}</ref> 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 line]]s, 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.<ref>{{Cite journal|last1=Collopy|first1=Michael W.|last2=Koplin|first2=James R.|year=1983|title=Diet, Capture Success, and Mode of Hunting by Female American Kestrels in Winter|journal=The Condor|volume=85|issue=3|pages=369–371|jstor=136708|doi=10.2307/1367081|url=https://sora.unm.edu/node/103373}}</ref>
Prey is most often caught on the ground, though occasionally they take birds in flight. Before striking, the kestrel characteristically bobs its head and tail, then makes a direct flight toward the prey to grab it in its [[Talon (anatomy)|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.<ref name="merlinfalconry.com"/> 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.<ref>{{Cite journal|doi=10.2307/1938854|last=Rudolph|first=Seri G.|year=1982|title=Foraging Strategies of American Kestrels During Breeding|journal=Ecology|volume=63|issue=5|pages=1268–1276|jstor=1938854|bibcode=1982Ecol...63.1268R }}</ref>
===Reproduction===
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The young adult kestrels may breed from a year old, and the species has approximately a three to five-year life expectancy in the wild.
In ecological terms, the reproductive pattern of the American kestrel leans towards a small bird "''r'' selection" strategy.<ref>Davis, Kate, Kindle location 948.</ref> In [[r/K selection theory|''r''/''K'' selection theory]], selective pressures are [[hypothesis|hypothesiz]]ed to drive [[evolution]] in one of two generalized directions: ''r'' or ''K'' selection.<ref name="Pianka, E.R 1970">{{cite journal | last1 = Pianka | first1 = E.R. | author-link = Eric Pianka | year = 1970 | title = On r and K selection | journal = American Naturalist | volume = 104 | issue = 940| pages = 592–597 | url=https://www.researchgate.net/publication/275142242 | doi = 10.1086/282697 | s2cid = 83933177 }}</ref> ''R''-selected species are those that place an emphasis on a high growth rate, typically exploiting less-crowded [[ecological niche]]s, and produce many [[offspring]], each of which has a relatively low probability of surviving to adulthood (i.e., high ''r'', low ''K''). By contrast, ''K''-selected species display traits associated with living at densities close to [[carrying capacity]], and typically are strong competitors in such crowded niches that [[Parental investment|invest]] more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood (i.e., low ''r'', high ''K''). Between these two extremes, the American kestrel is one of the few raptor species that lean towards being ''r''-selected. They are able to breed at one year old, have few non-breeding adults in the population, and have larger broods. Their population growth rate is high relative to larger raptors, which typically lean towards being ''K''-selected.
==Stress physiology==
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===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.<ref name="Strasser" /> Thus, high levels of traffic disturbance and human development surrounding American kestrel nests are found to increase stress hormones leading to reproductive failure.<ref name=":2">{{Cite journal|last1=Strasser|first1=Erin H.|last2=Heath|first2=Julie A.|date=2013-08-01|title=Reproductive failure of a human-tolerant species, the American kestrel, is associated with stress and human disturbance|journal=Journal of Applied Ecology|language=en|volume=50|issue=4|pages=912–919|doi=10.1111/1365-2664.12103|bibcode=2013JApEc..50..912S }}</ref> 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.<ref name="Strasser" />
===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 [[Homeostasis|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 the growth and development of the spleen and bursa in American kestrels.<ref>{{Cite journal|last1=Fernie|first1=Kim J.|last2=Mayne|first2=Greg|last3=Shutt|first3=J. Laird|last4=Pekarik|first4=Cynthia|last5=Grasman|first5=Keith A.|last6=Letcher|first6=Robert J.|last7=Drouillard|first7=Ken|date=2005-12-01|title=Evidence of immunomodulation in nestling American kestrels (''Falco sparverius'') exposed to environmentally relevant PBDEs|journal=Environmental Pollution|volume=138|issue=3|pages=485–493|doi=10.1016/j.envpol.2005.04.008|pmid=15951077|bibcode=2005EPoll.138..485F }}</ref>
While PBDEs can affect immune response and suppress growth of certain organs, they can also affect the thyroid system of American Kestrels. Exposure to PBDEs ''in vivo'' can alter the thyroid system and retinol concentrations in kestrels. This leads to oxidative stress, lipid peroxidation, and changes in glutathione metabolism.<ref>{{Cite journal |title=Exposure to Polybrominated Diphenyl Ethers (PBDEs): Changes in Thyroid, Vitamin A, Glutathione Homeostasis, and Oxidative Stress in American Kestrels (Falco sparverius) |url=https://academic.oup.com/toxsci/article/88/2/375/1691148/Exposure-to-Polybrominated-Diphenyl-Ethers-PBDEs |access-date=2022-03-06 |journal=Toxicological Sciences |year = 2005|doi=10.1093/toxsci/kfi295|pmid = 16120752|last1 = Fernie|first1 = K. J.|last2 = Shutt|first2 = J. L.|last3 = Mayne|first3 = G.|last4 = Hoffman|first4 = D.|last5 = Letcher|first5 = R. J.|last6 = Drouillard|first6 = K. G.|last7 = Ritchie|first7 = I. J.|volume = 88|issue = 2|pages = 375–383|doi-access = free}}</ref> These systems are important in early development, growth, regulation of metabolism, thermoregulation, and reproduction. Oxidative stress is also known to contribute to cancers and neurological diseases.<ref>{{Cite journal |last1=Pizzino |first1=Gabriele |last2=Irrera |first2=Natasha |last3=Cucinotta |first3=Mariapaola |last4=Pallio |first4=Giovanni |last5=Mannino |first5=Federica |last6=Arcoraci |first6=Vincenzo |last7=Squadrito |first7=Francesco |last8=Altavilla |first8=Domenica |last9=Bitto |first9=Alessandra |date=2017 |title=Oxidative Stress: Harms and Benefits for Human Health |journal=Oxidative Medicine and Cellular Longevity |volume=2017 |pages=8416763 |doi=10.1155/2017/8416763 |issn=1942-0900 |pmc=5551541 |pmid=28819546|doi-access=free }}</ref>
Exposure to PCBs might also affect American Kestrel reproduction. It was found that PCBs affect the function of carotenoids in kestrels. This led to changes in coloration, especially during breeding season for adults.<ref name=":0">{{Cite journal |last1=Bortolotti |first1=Gary R. |last2=Fernie |first2=Kimberly J. |last3=Smits |first3=Judit E. |date=2003 |title=Carotenoid Concentration and Coloration of American Kestrels (Falco sparverius) Disrupted by Experimental Exposure to PCBs |url=https://www.jstor.org/stable/3599160 |journal=Functional Ecology |volume=17 |issue=5 |pages=651–657 |doi=10.1046/j.1365-2435.2003.00778.x |jstor=3599160 |bibcode=2003FuEco..17..651B |issn=0269-8463}}</ref> PCB-exposed males where duller and contributed less to egg incubation than unexposed males. PCB-exposed females kept their color longer than they should have. Normally, loss in color is associated with carotenoids being directed to ovaries to help in egg development. PCB-exposed females retained their colors longer, suggesting the PCBs made them less prepared for reproduction. The same females also had significant delays in egg laying. Offspring also showed higher incidence of developmental problems and decreased reproductive success.<ref name=":0" />
PCBs have also been found to affect eye color in American Kestrels. Eye color in kestrels is known to vary with age and sex, however, when exposed to PCBs, color patterns were suppressed regardless of age and sex.<ref>{{Cite journal |last1=Bortolotti |first1=GaryR. |last2=Smits |first2=JuditE. |last3=Bird |first3=DavidM. |date=January 2003 |title=Iris Colour of American Kestrels Varies with Age, Sex, and Exposure to PCBs |url=http://dx.doi.org/10.1086/345485 |journal=Physiological and Biochemical Zoology |volume=76 |issue=1 |pages=99–104 |doi=10.1086/345485 |pmid=12695990 |s2cid=37168509 |issn=1522-2152}}</ref> 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.
PDBEs were linked to changes in breeding behavior in kestrels as well. Different levels of PDBE exposure were linked to different changes in behavior as well. Overall, PDBE exposure led to changes in behaviors that strengthen the bond between a breeding pair. Such behaviors include frequent copulation, food transfers, male posturing, nest box inspection, and specific mating calls (7). High exposure levels led to increases in some behaviors and decreases in some, whereas low exposure caused decreases in almost all behaviors observed. PDBE exposure also altered the timing of these behaviors, often delaying them by several days when compared to the control group.<ref>{{cite journal |last1=Fernie |first1=Kim J. |last2=Shutt |first2=John L. |last3=Letcher |first3=Robert J. |last4=Ritchie |first4=James I. |last5=Sullivan |first5=Katrina |last6=Bird |first6=David M. |title=Changes in Reproductive Courtship Behaviors of Adult American Kestrels (Falco sparverius) Exposed to Environmentally Relevant Levels of the Polybrominated Diphenyl Ether Mixture, DE-71 |journal=Toxicological Sciences |date=March 2008 |volume=102 |issue=1 |pages=171–178 |doi=10.1093/toxsci/kfm295|pmid=18065774 }}</ref>
American Kestrels have also been used extensively in toxicology research. Fenthion is a common pesticide that is used to kill insects such as flies and gnats.<ref>{{Cite web |last=PubChem |title=Fenthion |url=https://pubchem.ncbi.nlm.nih.gov/compound/3346 |access-date=2022-03-06 |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref> It was also found that kestrels are highly susceptible to secondary fenthion poisoning. When 14 kestrels were presented with live sparrows who had come into contact with a fenthion solution, all 14 died within 3 days after consuming the sparrows.<ref>{{Cite journal |last1=Hunt |first1=K. A. |last2=Bird |first2=D. M. |last3=Mineau |first3=P. |last4=Shutt |first4=L. |date=1991-07-01 |title=Secondary poisoning hazard of fenthion to American kestrels |url=https://doi.org/10.1007/BF01055561 |journal=Archives of Environmental Contamination and Toxicology |language=en |volume=21 |issue=1 |pages=84–90 |doi=10.1007/BF01055561 |pmid=1898122 |bibcode=1991ArECT..21...84H |s2cid=30935069 |issn=1432-0703}}</ref>
Diphacinone is another common pesticide that is often used to kill rodents and is thought to be related to secondary poisoning in birds of prey.<ref>{{Cite journal |last1=Rattner |first1=Barnett A. |last2=Horak |first2=Katherine E. |last3=Warner |first3=Sarah E. |last4=Day |first4=Daniel D. |last5=Meteyer |first5=Carol U. |last6=Volker |first6=Steven F. |last7=Eisemann |first7=John D. |last8=Johnston |first8=John J. |date=2011-03-11 |title=Acute toxicity, histopathology, and coagulopathy in American kestrels (Falco sparverius) following administration of the rodenticide diphacinone |url=https://setac.onlinelibrary.wiley.com/doi/10.1002/etc.490 |journal=Environmental Toxicology and Chemistry |volume=30 |issue=5 |pages=1213–1222 |doi=10.1002/etc.490 |pmid=21337606 |s2cid=9840733 |issn=0730-7268}}</ref> When kestrels were orally dosed with diphacinone, blood clotting rates significantly decreases. 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.
American kestrels can also be significantly influenced by air contaminants. When exposed to common gaseous pollutants such as benzene, toluene, nitrogen dioxide, and [[sulfur dioxide]], significant changes to the thyroid systems were observed. Higher levels of thyroid activity indicate that exposure to these gases leads to a loss of inhibition of thyroid glands in kestrels.<ref>{{Cite journal |last1=Fernie |first1=Kim J. |last2=Cruz-Martinez |first2=Luis |last3=Peters |first3=Lisa |last4=Palace |first4=Vince |last5=Smits |first5=Judit E.G. |date=2016-10-18 |title=Inhaling Benzene, Toluene, Nitrogen Dioxide, and Sulfur Dioxide, Disrupts Thyroid Function in Captive American Kestrels (Falco sparverius) |url=https://doi.org/10.1021/acs.est.6b03026 |journal=Environmental Science & Technology |volume=50 |issue=20 |pages=11311–11318 |doi=10.1021/acs.est.6b03026 |pmid=27646166 |bibcode=2016EnST...5011311F |issn=0013-936X}}</ref> However, there were no changes to immune function or food consumption.
==Status and conservation==
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