: Found primarily in eastern Colorado as a summer resident (Andrews and Righter 1992). Casual winter resident on the eastern plains (ibid.)
Population Status: Locally uncommon to fairly common on the eastern plains (Andrews and Righter 1992). In western Colorado, it is rare to uncommon (ibid.)
Populations are presently declining in Colorado, and in some areas, they have been nearly or completely extirpated (ibid.) However, Colorado BBS sample sizes are insufficient to reliably predict population trends (CBO 1995).
Historically, burrowing owls were common wherever there were prairie dog towns in northeastern Colorado (CPW WSDB). As of 1974, the U.S. Dept. of the Interior listed the owl as a "status-undetermined species (Colorado; Zarn 1974). In Boulder County, Colorado, the history of the owl has been one of steady decline (Thompson and Strauch 1987). From 1972 to 1981, the species has been classified as decreasing in abundance in Colorado and was listed on the Blue List from 1972 to 1981 (Marti and Marks 1989, Pezzolesi 1994). From 1966 to 1968, the species was Red Book listed (considered rare) by the United States Department of the Interior (Colorado; Pezzolesi 1994).
In other parts of its breeding range, populations also appear to be declining and the burrowing owl has been listed as Endangered (Minnesota and Iowa) or a species of "special concern" in 12 states. In parts of Canada, it is designated as Endangered (Manitoba and British Columbia) or Threatened (Saskatchewan and Alberta) (Haug et al. 1993).
USFS Region Two Sensitive Species. Colorado Natural Heritage Program S3/S4—Rare to Uncommon/Common.
Threats: Loss of habitat is a major factor in the decline of burrowing owls (Andrews and Righter 1992). Loss of habitat to development and the loss of nesting sites due to poisoning campaigns against ground squirrels and prairie dogs have led to decreasing burrowing owl populations in the west (Wesemann and Rowe 1986). The lack of nesting burrows limits burrowing owl numbers, hence control of prairie dog towns is harmful (Haug et al. 1993). Suitable habitat does remain unoccupied, however, indicating that there are other factors involved (Andrews and Righter 1992).
The burrowing owl is very vulnerable to mammalian and avian predators (Haug et al. 1993). Disturbance at the nest site by humans and dogs caused a drop in reproductive success (Thomsen 1971, Millsap and Bear 1988 fr. Haug et al. 1993). In some areas, vehicle collisions are a major source of mortality (Haug et al. 1993).
Intensive agriculture has taken a major toll on the bird and its habitat, destroying nesting burrows in grasslands and native prairies, increasing vulnerability to predators and exposing breeding owls and their young to the toxic effects of pesticides (see "Response to Agriculture" and "Response to Pesticides" below for further information) (Haug et al. 1993). Insecticide appears to result in both direct mortality and indirect mortality (ibid.) A significant negative impact on the survival and reproductive success was found when Carbofuran, a carbamate insecticide, was sprayed over nest burrows (James and Fox 1987 fr. Haug et al. 1993). This was believed to be due to direct toxicity. When strychnine-coated grain was used to control ground squirrels, the weights of breeding owls declined (James et al. 1990 fr. Haug et al. 1993). This implies either a sub-lethal effect of the strychnine or a reduction in prey availability.
Eradication of colonies through mammal damage control activities and habitat destruction as a result of urbanization will continue to determine burrowing owl populations in the western U.S. (Colorado; Zarn 1974, Collins 1979, Butts and Lewis 1982, Marti and Marks 1989, Plumpton 1992, Pezzolesi 1994). Other factors that may threaten populations include predation, starvation, diseases, parasites, highway mortality, shooting, and accidental poisoning during rodent control operations (Colorado; Zarn 1974, Collins 1979, Butts and Lewis 1982, Marti and Marks 1989, Plumpton 1992, Pezzolesi 1994).
Low reproduction may result from food limitation (Utah; LeClerc 1990). Adverse effects caused by human environmental change are undetermined, but changes in vegetation structure and tolerance of human activity can cause problems (Marti and Marks 1989). Because owls perch outside their burrows in the daytime, they are highly vulnerable to human disturbance including shooting (ibid.) Drastic increases or decreases in vegetation structure resulting from grazing regimes could reduce the suitability of an area for burrowing owls (ibid.) Loss of riparian areas poses a potential threat (ibid.)
Gleason and Johnson (Idaho; 1985) determined badgers were the major predators of nestlings prior to emergence. Mortality resulting from motor vehicles and predation (by some hawks) was reported in North Dakota (Konrad and Gilmer 1984). With increasing development, there is an increased chance of disturbance to owls by humans through burrow vandalism, harassment by pets, and increased mortality from collisions with automobiles, as well as a decrease in sufficient amounts of open space (Wesemann and Rowe 1986, Colorado; Plumpton 1992).
Two aspects of the biology of the western burrowing owl appear to influence both its regional and local abundance: 1) it prefers areas of short vegetation, and 2) it rarely, if ever, digs its own burrows (Wesemann and Rowe 1986). Burrow availability affects owl abundance (Coulombe 1971 and Butts and Lewis 1982 fr. Plumpton 1992). Towns traditionally used by prairie dogs, but where prairie dogs were eliminated and vegetation was uncropped, went unused by nesting burrowing owls (Colorado; Plumpton 1992). However, periodic mowing may maintain vegetation at levels acceptable for nesting owls (ibid.) Pezzolesi (1994) found a significant difference (P = 0.0021) between the average size of available prairie dog towns with a mean equal to 17 acres (6.93 hectares) and those of towns used by owls with a mean equal to 45 acres (18.32 hectares), as well as a positive relationship between the average area of available prairie dog towns and nest attempts (P = 0.0033). Larger towns may provide security by decreasing the probability of predation (ibid.)
Owls are susceptible to parasitism by species of biting owl louse and Mallophaga such as Philopterus speotyi, Philopterus syrnii, and Kurodaia sp. (Stoner 1932 fr. USFS 1994).
Threats to breeding habitat are considered extensive (CBO 1995). However, very few factors affecting population size and distribution are known—several nonspecific references are available (ibid.) Threats to wintering habitat are also considered extensive (ibid.) However, even less information is available. Factors affecting population size and distribution are unknown or unsubstantiated—no major, and few nonspecific references on breeding ecology are available (ibid.) The burrowing owl is ranked very high (26) for conservation priority in Colorado (ibid.)
Primary Breeding Habitat Associations and High Value Habitats
Primarily found in grasslands and mountain parks, usually in or near prairie dog towns (Andrews and Righter 1992). Semi-desert shrub lands are used rarely (ibid.)
The burrowing owl also uses well-drained, steppes, deserts, prairies and agricultural lands (Haug et al. 1993). Macracken et al. (1985 fr. Dobkin 1992) found this species to favor well-grazed, early successional grasslands with soils having significant sand content. Openness, short vegetation, and burrow availability are essential (Colorado; Zarn 1974, Pezzolesi 1994). Preferred habitat surrounding the burrow is open, heavily grazed, native mixed-grass prairie (Konrad and Gilmer 1984). This owl sometimes breeds and forages in vacant areas within cities and towns (Marti and Marks 1989). It may also be associated with colonies of large rodents, such as prairie dogs or Richardson’s ground squirrels (Johnsgard 1979).
Territory Size/Density/Site Fidelity/Dispersal
Territory sizes have been estimated to be 1.98 acres (0.8 hectares) (Thomsen 1971 fr. Zarn 1974). The range = 0.1 acres to 4.0 acres (0.04 hectare to 1.6 hectares) (CPW WSDB) with the smallest, 0.1 acre (0.04 hectare), in Oklahoma (Zarn 1974).
The mean home range for male owls in Saskatchewan was 0.9 square miles (2.41 square km) with a range of 0.05 square mile to two miles (0.14 square km to 4.81 square km) (Haug and Oliphant 1990), with these values being considered as minimum breeding home ranges. Owls with the largest home ranges fledged the most young, and owls with smaller home ranges lost all or most of their young to predators (ibid.) Mean home-range size appeared inversely proportional to grasshopper availability (ibid.)
Owls will abandon nests when distance between nest sites is less than 361 feet (110 m) (Oregon and Washington; Green 1993). Spacing of 544 feet (166 m) between burrows in California is assumed to be the result of territory defense (Sharp 1992). In Imperial Valley, California, density was reported to be three owls per 100 acres (40 hectares) in optimal habitat while in Oklahoma 543 owls in 2,009 square miles (5,100 square km) were reported (Sharp 1992). Foraging grounds are not defended and are not considered part of an owl’s territory (USFS 1994).
The mean dispersal distances of juveniles in Manitoba was nine miles (14.4 km) with a range of 1.5 miles to 16.4 miles (2.4 km to 26.4 km) (n = 6); in Alberta, they moved a range of 984 feet to 18.6 miles (300 m to 30 km). Females usually disperse shorter distances than males (ibid.) Burrows may be re-used for many years (Ehrlich et al. 1988, Lutz and Plumpton 1994).
In Colorado, of the 38 percent of banded adults that returned, 66 percent reused the same prairie dog town from the prior year (Plumpton 1992). Overall, prairie dog towns had 90 percent reuse in 1991 (ibid.) Martin (1973 fr. Pezzolesi 1994) found every male nested in the same burrow that it had previously nested, unless the burrow had been destroyed. In such cases, owls would choose burrows in close proximity to those used previously (ibid.) Only five percent of juveniles returned: males always returned in the following year, whereas females always skipped a year and returned at age two (Lutz and Plumpton 1994). Pezzolesi (Colorado; 1994) found a significant difference between the proportion of individuals returning that had successful (84 percent) and unsuccessful (16 percent) nests the previous year, but did not find such differences between sexes or age classes. However, males had a return rate almost three times that of females (ibid.) Overall, only nine percent of banded individuals in her study exhibited nest-site fidelity (ibid.) Other authors have described the burrowing owl as demonstrating mild site tenacity (Martin 1973 and Aufforth 1981 fr. Wesemann and Rowe 1986, Idaho; Rich 1984).
Nest Sites: The presence of a nest burrow is critical. Burrowing owls can excavate their own burrows, but usually depend on burrows that have been started by colonially burrowing mammals, especially ground squirrels and prairie dogs (Colorado; Zarn 1974, Johnsgard 1979, Konrad and Gilmer 1984, Marti and Marks 1989, Plumpton 1992, Ehrlich et al. 1988, Sharp 1992, Pezzolesi 1994). Burrows excavated by badgers are also used (Plumpton 1992, Sharp 1992, Pezzolesi 1994). Burrows, tunnels, and nest cavities are characteristically lined with shreds of dried cow or horse dung, and sometimes weed stalks, grass, feathers, or portions of prey items (Plumpton 1992, Sharp 1992, Pezzolesi 1994).
Butts (1973 fr. Pezzolesi 1994) noted that burrowing owls primarily utilize only active portions of prairie dog towns—an observation supported by experimental results of Pezzolesi’s (1994) study. Burrowing owls appeared to prefer older, larger prairie dog towns at Rocky Mountain Arsenal (Pezzolesi 1994).
At Rocky Mountain Arsenal in Colorado, nesting owls occupied burrows with a shorter distance to the nearest road, and shorter grass and forb height (less than 0.005) than available (Plumpton 1992). They also select burrows with greater burrow density, nearest-perch distance (less than 0.004), and percentage of bare ground (less than 0.05) than available (Colorado; Plumpton 1992). Nesting sites are characterized by a high percentage (40 to 50 percent) of bare ground where prey is abundant (Oregon and Washington; Green 1993). A high density of burrows may be preferred (Todd and James 1989, Plumpton 1992) because it provides more escape routes for young owls (Haug et al. 1993). High perches nearby may also be a factor in nest site selection (Green fr. Haug et al. 1993). Owls selected burrows with good horizontal visibility (short vegetation or, when vegetation is greater than two inches (five cm), elevated perches) to avoid badger predation (Oregon and Washington; Green 1993). There is no observed preference for orientation of the burrow entrance (Todd and James 1989 fr. Haug et al. 1993, Plumpton 1992).
The distribution of burrows within colonies was highly variable in Oklahoma (Butts and Lewis 1982). Burrows in which the surrounding vegetation was greater than four inches (ten cm) were not used (ibid.) The owls did not show preference for any type of soil when selecting burrows (ibid.) Burrow dimensions were relatively consistent: tunnel size = 5.5 inches to six inches (14 cm to 15.2 cm) wide and = 4.3 inches to 5.1 inches (eleven cm to 13 cm) in height and nest chambers were roughly circular or oblong ten inches (25.4 cm) wide and four inches to six inches (ten cm to 15.2 cm) in height) (ibid.) Nest chambers averaged 27 inches (69 cm) below the ground surface and 59.1 inches (150 cm) from the burrow entrance (ibid.)
Burrows are used year-round in non-migratory populations (Haug et al. 1993); however, most of Colorado’s burrowing owls are migratory (Andrews and Righter 1992, Lutz and Plumpton 1994). Burrowing owls (n = 555) banded at Rocky Mountain Arsenal National Wildlife Refuge, Denver, Colorado were completely migratory (Lutz and Plumpton 1994). All burrows were in black-tailed prairie dog (Cynomys ludovicianus) towns (ibid.)