The American kestrel (Falco sparverius) is a small, vibrantly colored falcon found throughout North and South America. As cavity nesters, kestrels do not build nests themselves but rather use the abandoned nests and holes of other animals. Kestrel nests can be found in a variety of natural and man-made sites, with certain characteristics that make them attractive to these falcons. Understanding the typical features of a kestrel’s nest can provide insight into their breeding biology and ecology.
Natural Nest Sites
In the wild, American kestrels nest in tree cavities excavated by woodpeckers, most commonly Northern Flickers. Kestrels will also use natural crevices in cliffs and rocks, as well as holes in cacti in the southwestern United States. Desirable natural sites are generally high off the ground, with entrances just large enough for the kestrels to pass through. The height provides security from predators, while the small opening helps the nest stay protected from the elements. Inside, the cavity needs to be large enough to accommodate the kestrels, eggs, and nestlings.
Tree Cavities
Tree cavities make up the majority of natural American kestrel nest sites. When claiming an old woodpecker hole, kestrels favor those with entrance holes about 3 inches wide and interiors about 4-8 inches wide. The cavity needs to be deep enough so the nest, eggs, and young are well protected. Kestrels rarely excavate cavities themselves and will choose nest sites where the hard excavation work has already been done. The benefits of nesting in mature trees include insulation from temperature extremes, reduced threat from ground predators, and greater exposure when locating prey.
Cliff Crevices
Kestrels will also nest in gaps and cracks in cliffs, canyons, and rock outcroppings. Ideal crevices will have an opening just large enough for the falcons to slip through and a roomy inner cavity to house the nest. A north-facing crevice helps keep the nest cool in hot weather. The rough cliff exterior deters predators from climbing up and provides numerous outlooks for scanning the surroundings. The elevated location gives parent kestrels a wide vantage point for spotting prey.
Cacti Holes
In desert environments, kestrels will nest in cavities within large cacti like saguaro and organ pipe. The thick cactus walls provide insulation from the extreme heat. Spines surrounding the outer surface help deter snakes and other predators. The cactus holes are high above the desert floor, giving kestrels a commanding view to hunt from. However, cacti holes can overheat if located on the sun-exposed side of the cactus during hot weather. Kestrels prefer west or north-facing holes to avoid overheating.
Man-Made Nest Sites
American kestrels readily utilize artificial cavities provided by humans, including nest boxes specifically for kestrels. Other popular man-made sites are openings in buildings, pockets on highway signs, and holes in utility poles. Artificial sites are most attractive to kestrels when they replicate key features of natural cavities.
Kestrel Nest Boxes
Special kestrel nest boxes can be an excellent conservation tool, helping provide safe nesting options in areas lacking natural cavities. Nest boxes should be made of wood or similar insulating material with a small, round or oval entrance hole. Common designs include an external perch below the entry hole and internal perches for the falcons. Boxes are mounted on tree trunks, poles, or buildings usually 15-30 feet high, with southeast to northwest exposures away from prevailing winds. Kestrels prefer boxes with appropriate cavity dimensions and will avoid using undersized or oversized boxes.
Buildings
Kestrels frequently nest in openings of buildings and other structures. Popular spots include gaps in walls, spaces under roof tiles or shingles, pockets on cornices and eaves, holes in siding, and cavities in ventilation systems. The nooks and crannies of barns, sheds, abandoned houses, and silos offer numerous attractive options. As with natural sites, kestrels favor cavities high off the ground with entrances just large enough to enter. Suitable building cavities provide similar protective and insulating benefits to natural nest holes.
Highway Signs and Utility Poles
The small hollow spaces on the backside of highway signs or within utility poles make convenient nesting spots for kestrels, especially across open country. These artificial cavities tend to be more exposed than natural sites or nest boxes, however, making eggs and young more prone to adverse weather and predators. The metal and wooden substrates also provide less insulation than tree cavities, although some poles contain chambers filled with insulating materials. The height of road signs and poles supplies desirable elevation above ground for kestrel nests.
Nest Contents
Inside their nest cavity, American kestrels arrange various materials to form a nest bowl. Kestrels do not transport nesting material themselves but rather rely on already available resources within the cavity. Typical nest contents include:
Substrate
The nest base starts with a substrate layer of dirt, sawdust, wood shreds, pine needles, gravel, or other debris. This forms the foundation for the nest bowl and helps prevent eggs from rolling around. Kestrels usually use materials left over from previous occupants like woodpeckers. In natural and man-made cavities, substrate provides a firm, flat surface for the nest.
Grasses
Atop the base, kestrels line the nest with grasses and other fine plant materials. Common grass species include brome, fescue, and moor. The greenery provides a soft, smooth cup to cradle the eggs and hatchlings. The grasses help insulate eggs from temperature fluctuations while allowing ventilation. Kestrels may frequently add new grasses throughout incubation and brooding.
Feathers
The falcons often decorate the nests with feathers, including downy feathers plucked from their own breasts. The feathers provide supplementary padding and help further regulate nest temperature and humidity. Both male and female kestrels contribute feathers, with the hormone prolactin triggering feather shedding before breeding season. Feathers also serve as nest sanitation by absorbing moisture from droppings.
Nest Site Competition
Kestrels sometimes compete for nesting cavities with other secondary cavity nesters. Species known to vie with kestrels include:
European Starlings
The larger and more aggressive starlings are major competitors for kestrel nest sites. Starlings often take over kestrel nests or physically displace them from coveted cavities. Kestrels can be excluded from ideal nesting habitat if starlings saturate the area. Providing kestrel nest boxes can help mitigate such competition.
Screech Owls
Screech owls may seize cavities from kestrels, even enlarging small openings to accommodate their larger size. The nocturnal owls have the advantage of claiming holes at night after kestrels finish daytime activity. Kestrels sometimes successfully co-nest near owls by choosing concealed cavities.
Woodpeckers
Woodpeckers prefer excavating their own cavities but may re-carve old holes inhabited by kestrels. Woodpeckers pose less direct competition but can harass kestrels away from desirable nests. Kestrels benefit long-term from woodpeckers continually creating new habitats.
Eastern Bluebirds
Bluebirds are secondary cavity nesters too, often competing with kestrels for natural holes and nest boxes. The earlier-nesting bluebirds can claim prime sites first, forcing kestrels to pick from leftovers. Adjusting box placements and sizes can allow both species to successfully co-exist.
Ideal Nesting Habitat
When choosing breeding areas, American kestrels favor locales featuring:
Open Areas
Kestrels thrive in open country including grasslands, meadows, and pastures. The unobstructed views give them wide hunting ranges to spot prey. Open fields near woods or wetlands provide ideal combinations.
Scattered Trees & Shrubs
Trees scattered over open terrain supply essential cavities for nesting. The sporadic perches also provide hunting vantage points. Shrubs add protection and an abundance of insects.
Woodland Edges
Forest edges create transition zones dotted with snags and fallen trees. These marginal woods offer prime nesting and perching spots at the border of open hunting grounds.
Rural Human-Use Areas
Kestrels prosper near rural human habitation. Farmyards, roadsides, and hayfields contain prey-attracting habitat. Barns, silos, and other structures provide artificial nesting cavities.
Grazing Lands
Active or fallow grazing lands support rodent populations on which kestrels feed. The short vegetation supplies excellent hunting visibility as well.
Nest Site Fidelity
American kestrels exhibit nest site fidelity, returning to the same general nesting areas year after year. Kestrels may switch between nearby cavities but are loyal to particular territories. Familiar areas offer proven food resources and nesting experiences. Fidelity reduces annual search efforts and conflicts with competitors. Once a good nesting spot is found, kestrels will come back to reuse it.
Benefits
Site fidelity provides kestrels with the following advantages:
– Increased nesting success in proven cavities
– Decreased risks of competition with other cavity users
– Reduced energy expenditures to find/claim nests
– Enhanced familiarity with local prey sources
– Multi-year occupation can enlarge cavity openings
Disadvantages
Potential drawbacks to fidelity include:
– Nest predation if sites are discovered by predators
– Reduced adaptation if habitat conditions change
– Exposure to parasites that accumulate in reused cavities
– Constraints on dispersal to potential new habitat
Timing of Nesting
American kestrels time breeding to match peak food availability for their young. Nesting dates vary across their range:
Northern Populations
Kestrels in Canada and the northern U.S. nest from May through July, with peak activity in June. The later spring allows prey like voles and insects to fully emerge.
Southern Populations
In the southern U.S. states like Texas and Florida, kestrels begin nesting as early as February. The longer warm season supports multiple broods.
Mountain Populations
At higher mountain elevations, kestrels delay nesting until July or August due to colder temperatures and snowpack. The bird’s seasonal adaptations allow breeding across diverse regions.
Duration of Use
American kestrels may use nesting cavities for varying lengths of time:
– Kestrels may switch nests within a single breeding season for second broods.
– Favored cavities may be reused annually for multiple breeding seasons.
– Some nest boxes have been continuously occupied by kestrels for over a decade.
– Holes in live trees can be used for many generations if the tree remains healthy.
– Old woodpecker holes last around 5 years on average before decaying.
– Nest longevity depends on cavity material and exposure to elements.
Ideal cavities will be utilized repeatedly each year until no longer suitable. Kestrels exhibit strong nest fidelity, but poor habitat conditions or nest competition will force relocation. Long-lived nests in live trees and high-quality nest boxes allow generations of kestrels to benefit. Careful maintenance of such artificial structures is vital for maintaining productive nest sites. With appropriate habitat management, essential nesting cavities can be provided continuously over many kestrel lifespans.
Conclusion
The American kestrel is highly adapted for utilizing cavities for breeding across a variety of North American ecosystems. Kestrel nests can be found in diverse natural and man-made sites that share key features: insulating and protective materials, small openings, elevated heights, and proximity to open hunting areas. Tree cavities, rock crevices, cactus holes, nest boxes, and building gaps all reflect suitable nesting habitats. Inside, kestrels fashion nests out of available plant materials and feathers to create warm, secure bowls for eggs and young. Kestrels favor nesting in open country and readily return to reuse nests in subsequent years. Understanding kestrel nest requirements provides important guidance for conservation efforts. Promoting intact, healthy forests facilitates natural cavity abundance. Installing raptor nest boxes in appropriate habitat supplies alternative nesting opportunities. Targeting nesting habitat management helps ensure stable breeding opportunities for American kestrels. With supportive conditions, kestrel nests can successfully produce new generations of these charismatic raptors.