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Photo Courtesy of Utah Division of Wildlife Resources
Prairie Falcon
Falco mexicanus
NatureServe conservation status
Global (G-rank): G5
State (S-rank): S4
External links
General information
The prairie falcon, Falco mexicanus, occurs across the western United States, in a small portion of southwestern Canada, and in northern and central Mexico. Typically, this bird is found in open habitats, such as plains and prairies. Populations do not migrate long distances, but some birds move from breeding grounds to lower elevations, where prey availability is higher, during the winter.
The diet of the prairie falcon includes a variety of prey, but mammals, especially ground squirrels, are particularly important during the summer; birds are often captured during winter when small mammals are scarce. Prey may be cached (stored for later use) in vegetation or on a ledge, most commonly during the early periods of brood rearing. Eggs are typically laid on a well-sheltered ledge on a rocky cliff high above the ground. Eggs may be laid in the old nest of a raven or a hawk. Four or five eggs are laid and incubated, mostly by the female, for 29 to 33 days. During this time, the male brings food to the nest. Young are cared for by both parents and remain at the nest for 36 to 41 days. Usually pairs change nest sites within their territory in successive years.
Species range
BREEDING: southeastern British Columbia, southern Alberta, southern Saskatchewan, and northern North Dakota south to Baja California, southern Arizona, southern New Mexico, western and northern Texas, Chihuahua, Coahuila, Durango, and San Luis Potosi (AOU 1983, Lanning and Hitchcock 1991, Steenhof 1998); formerly also northwestern Missouri. NON-BREEDING: from breeding range in southern Canada south to Baja California and central Mexico (AOU 1983, Steenhof 1998). Most abundant in winter in the Great Basin and the central and central-southern latitudes of the Great Plains (Root 1988).
Migration
Some birds winter in breeding range, some migrate south as far as central Mexico, and, in the mountains, some birds migrate to lower elevations. See Palmer (1988) for details.
Habitat
Primarily open situations, especially in mountainous areas, steppe, plains or prairies (AOU 1983). Typically nests in pot hole or well-sheltered ledge on rocky cliff or steep earth embankment, 10 to more than 100 meters above base. May nest in man-made excavations on otherwise unsuitable cliffs (Cade 1982). Vertical cliffs with rock structure overhanging the site are preferred. Nests typically are placed on south-facing aspects, with overhangs offering some protection from solar radiation. May use old nest of raven, hawk, eagle, etc. Commonly changes nest site within territory in successive years (see Palmer 1988). In Mojave Desert, remote nests had higher productivity than did nests that were closer to human activity (Boyce 1988). During winter, falcons use a number of other habitats that are not typical of those used during the breeding season. Dryland wheat fields, irrigated winter wheat and other irrigated croplands also are used for foraging in winter (Enderson 1964, White and Roseneau 1970, Parker 1972, Beauvais et al. 1992). In all cases, large patches with low vegetation stature characterize the habitats used. Depend on Horned Larks (Enderson 1964) and grassland species in general (Schmutz et al. 1991) for prey. Early successional stages, low vegetation height and large percentages of bare ground are an inferred requirement. The use of forested habitat during migration by some Canadian birds (Schmutz et al. 1991) appears to be rare, but use of these habitats is little studied.
Food habits
Primarily feeds opportunistically on mammals (especially ground squirrels), lizards, and birds, generally up to size of quail and rabbits. In southwestern Idaho, reproduction is closely linked to the abundance of the ground squirrel Spermophilus mollis. Even following a prolonged crash in ground squirrel populations, and in the absence of important alternate prey, falcons continued to seek ground squirrels (Steenhof and Kochert 1988). Had a much more specialized diet than other raptors in southwestern Idaho and variation among individuals was low (Steenhof 1998). Ground squirrel populations fluctuate with drought cycles, thus potentially affecting productivity and population trends (Van Horne et al. 1997). In winter, often takes Horned Larks (Eremophila alpestris) on fields of winter wheat. Young may take large insects. Usually captures prey on or near ground; rapidly pursues birds in flight (see Palmer 1988 for many details). May cache prey in vegetation, on ledge, or in small crevice or cavity; caching most common during early brood rearing.
Ecology
Annual mortality estimated at 74% in immatures, 25% in adults (see Evans 1982). Recorded nesting density: 23 pairs on 26 kilometers of cliffs in Colorado, 101 pairs in 72 kilometers along Snake River, Idaho (see Palmer 1988). Defend relatively small areas around the nest site. These may extend 300 - 400 meters around the typical cliff nest and about 100 meters above the site (Ogden and Hornocker 1977, Harmata et al. 1978, Kaiser 1986). Foraging areas are large, overlapping and not defended (Haak 1982, Squires 1986, Hunt 1993). Steenhof (1998) reports nesting season home ranges from six studies that ranged from 59 - 314 square kilometers. Where nesting cliffs are suitable and continuous, will nest at higher densities than most other large North American falcons (Steenhof 1998). At higher densities, nest sites tend to be visually isolated from one another (Anderson and Squires 1997). Densities of nesting falcons ranged from 0.2 pair per kilometer of linear cliff in Montana (DuBois 1984) to 0.66 pair per km in southwestern Idaho, with some stretches of canyon in Idaho having 4.3 pair per kilometer (Steenhof 1988). Winter home ranges are much smaller than breeding season home ranges but still averaged over 30 square kilometers in Colorado (Beauvais et al. 1992). Winter roosts may be far from winter foraging areas, much as nest sites may be far from breeding season foraging areas. Fidelity to breeding territories is very high in some areas. Runde (1987) reports an average 88% return rate in Colorado, Wyoming, and Alberta with Alberta females returning at a very high rate (96%). Return rates in Idaho, where nest sites and mates are at high densities, were substantially lower.
Reproductive characteristics
Laying may begin as early as February in Texas and Mexico; March in California, Washington, Arizona, and Oregon; April in Montana and Wyoming. Clutch size usually is 4-5. Incubation lasts 29-33 days, mostly by female (male brings food). Young are tended by both parents, remain at nest site 36-41 days. First breeds usually at 2 years (sometimes 1 year).
Threats or limiting factors
HUMAN DISTURBANCE: The effect of direct human disturbance depends on a number of factors, including the type of activity, proximity to the nest or roost site, time of year and duration of the activity (Steenhof 1998). Falcons are most sensitive just prior to egg laying. In certain cases, disturbance has negative effects (Platt 1974, Boyce 1982) while in others, there appears to be no significant effect (Edwards 1968, Holthuijzen 1989). Birds also seem able to habituate to aircraft (Harmata et al. 1978, Ellis et al. 1991) and even simulated sonic booms (Ellis et al. 1991). However, large-scale and complex disturbances, such as military tank training, can disrupt foraging behavior and efficiency (Steenhof 1998). Prolonged disturbance is more harmful than periodic, short-term disturbance (Bednarz 1984). GRAZING: The effects of livestock grazing are neither simple nor well understood. The removal of vegetation may impact prey populations, especially in drought years (Steenhof 1998). Grazing also increases the invasion of sites by exotic invasive plants, such as cheatgrass (Bromus tectorum), which increase fire return intervals and accelerate the loss of native vegetation (Steenhof 1998, Wisdom et al. 2000). On the other hand, grazing removes vegetation which in some cases may make prey more available to falcons (Anderson and Squires 1997). However, this short-term, local benefit may be offset by negative effects at larger scales and in longer time frames. INVASIVE EXOTICS: In Idaho, home ranges had a lower cover of exotic annual grasses, primarily cheatgrass, than expected by chance (Marzluff et al. 1997). ENERGY DEVELOPMENT: Prairie Falcons appear to be relatively tolerant of oil and gas (Harmata 1991, Squires et al. 1993) and coal development (Phillips et al. 1990) in foraging areas, except where nest sites are destroyed or direct human disturbance is excessive. Falcons forage in spaces among oil wells where well densities were 1.5 wells per sq km (Anderson and Squires 1997). But the latter site was remote and not frequented by humans. This implies that it is direct human disturbance, not development per se, that is most harmful. RELATIONSHIPS WITH OTHER SPECIES: Falcons are notably tolerant of the Common Raven (Corvus corax) throughout its range (Cade, 1987, Steenhof 1998). Falcons frequently lay eggs in old raven stick nests. As raven populations and distribution are increasing, this may be a management factor deserving more attention. Golden Eagles (Aquila chrysaetos, Red-tailed Hawks (Buteo jamaicensis) and Great Horned Owls (Bubo virginianus) typically are not tolerated by falcons near nesting territories (Platt 1974, Harmata et al. 1978, Kaiser 1986, Holthuijzen 1989). These species prey on Prairie Falcon adults and nestlings. Peregrine Falcons often attack Prairie Falcons that enter a peregrine's territory (Porter and White 1973, Walton 1978). Thus, management actions to benefit these other raptor species may be detrimental to Prairie Falcon populations. SHOOTING: Shooting is the most commonly reported source of adult mortality (Webster 1944, Enderson 1964, van Tighem 1967). Shooting near nests also may cause adults to leave the nests temporarily, exposing eggs or nestlings to additional mortality (Harmata et al. 1978). PREDATORS: Mammalian predators, primarily coyotes (Canis latrans) and bobcats (Lynx rufus), are the main predators of falcon nests where nests are accessible (Steenhof 1998). DISEASE: Rock Doves (Columba livia) infected with trichomoniasis and herpesvirus can spread the infections to falcons when Rock Doves are consumed (Aini et al. 1993, Steenhof 1998). The impact of these diseases on falcon populations is not known. ELECTROCUTION: Electrocution is apparently uncommon (Steenhof 1998). FALCONRY: Legally harvested in 19 states (Conway et al. 1995). Although state agencies set harvest guidelines, these often are established without adequate data or analysis of population impacts. Steenhof (1998) states that the low level of harvest, about 0.2% of the population annually, probably does not affect overall population size. However, adults disturbed by harvest show lower inter-year territory fidelity (Conway et al. 1995). COLLISIONS: Collisions with wires, and fences in particular, cause some mortality, particularly during the fast, low foraging flights (Boyce 1982, Beauvais et al. 1992). Falcons also collide with vehicles. STOCK TANKS: Adults have been known to drown in stock watering tanks (Enderson 1964). ECTOPARASITES: Several ectoparasites contribute to nestling mortality and subsequent reproductive failure (review in Steenhof 1998). PESTICIDES: Susceptible to eggshell thinning from DDE (Noble and Elliot 1990) and may have had more recent reproductive failure as a result of hexachlorobenzene and DDE (Jarman et al. 1996). Although Prairie Falcons eat more mammals than birds, the species may be vulnerable to organophosphates and carbamates where it feeds on birds in agricultural areas (Kirk and Banasch 1996). Heptachlor epoxide and mercury residues also have been detected in falcons. These chemicals are used to treat wheat seeds and were presumable picked up by birds such as Horned Larks. Neither contaminant was thought to be high enough to affect the population level in the study area (Fyfe et al. 1969, 1976).
References
- Steenhof, K. 1998. Prairie falcon (Falco mexicanus). Birds of North America 346: 28 pp.
- Baicich, P. J., and C. J. O. Harrison. 1997. A guide to the nests, eggs, and nestlings of North American Birds, Second Ed. Academic Press, San Diego. 347 pp.
