American Kestrel

American Kestrel

Scientific Name: Falco sparverius
Alternate names: Kestrel, Sparrow Hawk, American Sparrowhawk
Size: Length 22-27 cm; wingspan 52-61 cm; mass 97-150 g
Type migrant: partial

Introduction

The American Kestrel is one of the best known, most frequently observed, and
readily identifiable raptors in North America. Kestrels are conspicuous, colorful,
open-habitat birds of prey about the size of a Mourning Dove. They occur
throughout the Western Hemisphere from Alaska and Canada to southernmost South
America. They are the smallest and most widespread falcons in North America.
Kestrels can be found in most open habitats with adequate cavities for nesting and
perches for hunting. The species readily adapts to human-modified environments,
and is frequently seen in pastures and parklands perched along the road.
Historically, American Kestrels have benefited from agricultural development.
However, recent habitat changes including urbanization, suburbanization, and
reforestation have the potential to reduce the amount of available habitat for the
species.

Identification

Like other falcons, American Kestrels have long, pointed wings and long tails.
Compared with its larger cousins the Merlin and Peregrine Falcon, the American
Kestrel has less powerful wingbeats, and appears more buoyant in flight. Unlike
many other raptors, American Kestrels exhibit sexually dimorphic plumage. A unique
characteristic of the American Kestrel is that individuals acquire adult, sexually
dimorphic plumage even before they fledge.
American Kestrels have reddish-brown backs and tails, blue-gray crowns with
variable amounts of rufous, and two dark vertical stripes on the sides of their heads.
They have two dark “eyespots” on the back of their head. Male kestrels have bluegray
wings. Females have reddish-brown wings with black barring. Males have
rufous tails with one wide, black sub-terminal band and a white tip. Females have
rufous tails and many black bars. The light-colored underparts of females typically
are heavily streaked with brown; those of males have variable amounts of dark
spotting or streaking. Females are about 10-15% larger than males.

Breeding Habits

American Kestrels are secondary cavity nesters that nest in existing natural and
man-made cavities. The species prefers nest sites that are surrounded by suitable
hunting grounds and that have unobstructed entrances. In some areas, the lack of
available nest cavities limits the number of breeding pairs. Kestrels often use
cavities excavated by woodpeckers. Crevices in rocks and cavities in stream cut
banks function as potential nest sites as well. Kestrels also nest in buildings and
other man-made structures including nest boxes erected for the species. Nest-box
programs for kestrels allow populations to grow in areas where nest sites are
limiting.

American Kestrels exhibit territory fidelity and many nest in the same territory year
after year. Pairs reuse nest sites particularly if they have successfully raised a brood
there previously. Kestrels typically are monogamous and some pairs remain
together across years. In sedentary populations, kestrels often remain at the nest
site for the entire year. In migratory populations, males return to the breeding
grounds first and when females arrive they associate with territorial males. Pairs
bond using aerial displays and courtship feeding. Aerial displays incorporate a series
of sequential dives and ascents, during which the male calls several times. Males
play the primary role in searching for suitable nest sites. After finding a potential
nest site, the male seeks out the female and leads her to it. In some cases the male
carries food, “flutter-glides,” (i.e. flys with short, quick wingbeats in slow, buoyant
flight) and calls to entice the female to follow him to a nest site.
American Kestrels are fairly vocal during the breeding season. Their most common
call is a rapid, high-pitched klee-klee-klee-klee. The male often “flutter-glides” and
calls as he approaches the nest site when delivering prey. When he does, the female
flies out of the nest cavity and “flutter-glides” with him. Both fly together to a perch
and the male transfers food to the female. Males do most of the hunting until the
young are two weeks old, thereafter both male and female supply food to their
young.

Females often create a depression or “scrape” in the substrate on the floor of the
cavity in which to lay their eggs. Pairs usually raise a single brood each year, but
kestrels lay replacement clutches if the first clutch is lost early in the season. On
rare occasions pairs raise two broods in a single season. Kestrels usually lay four to
five eggs, and incubation begins shortly before the last egg is laid. The youngest
chick often is unable to compete for food with its dominant, older siblings and
sometimes does not survive if food is scarce. Incubation takes about 30 days, and
the female does most of it. After the eggs hatch, the female broods the nestlings
continually until they are about nine days old. Thereafter, she broods only at night
and during periods of inclement weather. As her brooding time decreases, her time
spent hunting and her role in food provisioning increases. When the nestlings are
two weeks old, the adults begin to leave intact prey at the nest. Fledging occurs
about 30 days after they hatch, often over a period of several days. Young kestrels
depend on their parents for food for two to three weeks after they fledge. During
this time, the young sometimes return to the nest cavity to roost, and remain close
to their siblings.

Feeding Habits

American Kestrels are opportunistic hunters that forage in open areas with short
vegetation. The species is primarily a “sit-and-wait” perch-hunter, and elevated
perches that afford good visibility of the surrounding area are an important
component of suitable habitats. Hovering flight is a conspicuous, yet less frequently
used hunting method. Kestrels typically hover-hunt where perches are lacking,
usually in moderate winds and updrafts. Although most prey items are caught on
the ground, insects and birds sometimes are taken in flight. Kestrels catch their
prey with their feet and thereafter administer a killing bite to the back of the head.

Small prey items sometimes are eaten on the ground or in flight, but larger items
usually are brought to a perch. Caching of surplus food occurs throughout the year,
but the frequency of this behavior tends to be highest in autumn and winter and
lowest in summer.

The species preys mainly on insects and small mammals. Diets vary geographically
and seasonally. American Kestrels are more proficient at capturing insects than other
prey types. Kestrels usually consume small insects whole, but sometimes only eat
the head and internal parts of larger insects. Commonly taken insects include
grasshoppers, cicadas, beetles, dragonflies, butterflies and moths. Spiders and
scorpions are eaten as well. American Kestrels also take small rodents including
voles, mice, and shrews, as well as small birds, reptiles, and amphibians. The
species rarely feeds on carrion except for prey that it has previously killed and
cached.

Migration

The American Kestrel is one of 26 North American raptors that are partial migrants.
Some, but not all, populations of kestrels are migratory. American Kestrels breeding
in northern portions of their range are more migratory than those breeding farther
south, and birds in northern areas migrate farther than those in southern areas.
Many southern populations are sedentary. The species exhibits a “leap-frog” pattern
of migration in which northern birds winter south of southern birds. In comparison
to Merlins and Peregrines, which often fly to the tropics to overwinter, most
American Kestrels breeding in North America overwinter in the United States.
Like many other raptors, migrating American Kestrels concentrate along leading lines
while migrating, particularly along the Atlantic Coast, the shorelines of the Great
Lakes, and the ridges of Appalachian Mountains in the East and the Rocky Mountains
in the West. In general, migration counts of American Kestrels are higher at coastal
watchsites than at inland watchsites. Kestrels, like other falcons, are chiefly selfpowered
migrants that only occasionally soar on migration. Even so, American
Kestrels often capitalize on favorable soaring conditions, such as mountain updrafts
and thermals, while traveling. The species avoids large water-crossings. At Hawk
Mountain Sanctuary, kestrel flights are greatest on days when a cold front has past
through the area, presumably because of the strong updrafts that occur at such
times.

In autumn, juvenile and female kestrels tend to migrate earlier than do adult males,
probably because males take longer to complete their pre-migratory molt than do
females. At Hawk Mountain Sanctuary, the median date of female passage precedes
that of males by 11 days. Late arrival on the wintering grounds may force males to
spend the winter in sub-optimal habitat if more favorable habitats already are
occupied. In southern North America, sexes appear to winter in different habitats,
with females occurring in more open habitats and males occurring in more wooded
areas.

Bibiliography and Credits:

“Raptor Life Histories.” <span style=”text-decoration: underline;”>Raptor Life Histories</span>. 01 Jan. 2007. Hawk Mountain Sanctuary. 29 June 2009 <http://hawkmountain.org/index.php?pr=Raptor_Life_History>

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