Burrowing owl (Athene cunicularia) COSEWIC assessment and status report: chapter 8
Biology
Burrowing Owls are summer residents in the northern half of their breeding range, including the Canadian Prairie Provinces. Some individuals released in British Columbia in captive-breeding reintroduction programs do not migrate, but this is likely an artifact of captive-breeding. Burrowing Owls arrive on their prairie breeding grounds in April and May, lay an average of 9 eggs, typically fledge 3-5 young, and then begin fall migration sometime in late August or September (Wellicome 1997, Wellicome 2000, Todd et al. 2003). Most pairs are monogamous (Wellicome 2005), although polygyny is occasionally reported (Haug 1985).
Male owls typically defend a nest site and display for prospecting females (Haug et al. 1993). Both sexes may renovate and maintain the nest burrow, but only females incubate eggs and brood young. Males provision the female with food during the 28-30 day incubation period and while nestlings are brooded (Haug et al. 1993, Poulin 2003). The nestling period lasts approximately 44 days (Landry 1979), after which time juveniles disperse to nearby satellite burrows (Green 1983; Todd 2001b). Normally only a single brood is raised but a pair will re-nest if the first clutch is lost early in the season (Haug et al. 1993).
Estimates of adult survival have ranged from 37% to 51% over six years in Saskatchewan (James et al. 1997), 47% to 58% in Alberta (unpublished data cited in Haug et al. 1993), and 24% to 40% during a population decline in Manitoba (De Smet 1997). Dyer (cited in Haug et al. 1993) reported a 37% return rate for adults in British Columbia. Juvenile survival rates are lower, but are more difficult to estimate due to the lower site-fidelity of juvenile owls (De Smet 1997, Wellicome et al. 1997). Dyer (loc. cit.) found a 14% return rate for juvenile owls in B.C., while Hoyt et al. (2001) found a 6% return rate in Saskatchewan, and De Smet (1997) measured 3.5% return rate in Manitoba. Johnson (1997) found a minimum juvenile survival rate of 23% in California, but lower juvenile survival rates were noted in Colorado, where only 5% of fledglings were seen the following year (Lutz and Plumpton 1999). Clearly, these return rates underestimate survival rates, given that many adults and juveniles typically disperse from breeding and natal areas (see Movements/dispersal below). The lack of an accurate measure of adult and juvenile survival rates hampers attempts to accurately predict local population viability (McDonald et al. 2004).
Juvenile mortality after fledging apparently varies with predation pressure and local food availability. In Saskatchewan, a study involving radio-tagged juveniles found that mortality during the period between fledging and migration averaged 42% during normal years, but that no tagged juveniles died in a year of high food availability (Todd et al. 2003). In Alberta, post-fledging juvenile survival ranged from 45% during 1995–1996 (n = 21; Clayton and Schmutz 1999) to 61% during 1999–2000 (n = 52; Shyry 2005). In addition, juvenile mortality tended to be higher in relatively fragmented habitat patches (Todd 2001a), suggesting that habitat fragmentation may be negatively affecting juvenile survival in Great Plains habitats (see also Clayton and Schmutz 1999).
The degree to which adult and juvenile Burrowing Owls show fidelity to breeding (and natal) sites is difficult to assess as resightings of banded birds within finite study areas may underestimate dispersal. However, information from stable isotope studies suggests significant dispersal and genetic exchange among neighbouring populations (Duxbury 2004).
In Alberta, observations of individually-marked returning juveniles showed that nests were established 300 m to 30 km from their natal sites, with females moving farther than males (J. Schmutz, cited in Haug et al. 1993). Natal dispersal on the Regina Plain ranged from 0 to 295 km (Wellicome et al. 1997). De Smet (1997) reported that returning juveniles nested 1 to 77 km from their natal sites in Manitoba.
Between-year movements of adults were significantly lower. On the Regina Plain, movements ranged between 0−45 km for females, with no dispersal (i.e., 100% site-fidelity) shown by males (Wellicome et al. 1997). Adult males in Manitoba moved an average of 3.0 km among years and females moved an average 10.9 km.
Recovery of Burrowing Owls banded in Canada suggests that most of the prairie population migrates directly south through the Great Plains and winters in central Mexico (James 1992, Hertaas 1995, Duxbury 2004).
Aside from anecdotal observations of Burrowing Owls being harassed by songbirds (e.g., Martell 1990), there are few observations of interspecific interactions aside from predation events.
Adult and juvenile Burrowing Owls are taken by a wide range of predators, with raptors and badgers (Taxidea taxus) being the most common (Wellicome et al. 1997, Todd et al. 2003, McDonald et al. 2004). Predation has been cited as a significant source of mortality in local populations. For example, an entire wintering population on Santa Barbara Island, California was eliminated due to predation by Barn Owls (Tyto alba; Drost and McCluskey 1992). Reintroduction efforts in British Columbia have been hampered by heavy predation from Northern Harriers (Circus cyaneus), Great Horned Owls (Bubo virginianus), Red-tailed Hawks (Buteo jamaicensis), and coyotes (Canis latrans; Leupin and Low 2001). In Alberta and Saskatchewan, avian predation accounted for almost half of all mortality of juvenile owls between the fledging and migration periods (Clayton 1997, Todd 2001). Badgers have been noted as serious predators of Burrowing Owls in Saskatchewan (Wellicome et al. 1997), in Oregon (Green 1983), and in Nebraska (Desmond 1991). Finally, near human habitations, domestic cats and dogs are known to prey heavily on eggs and young (Haug 1985, Millsap and Bear 1988, Sleno 2000).
Burrowing Owls are generally tolerant of minor human disturbance around nest sites. In many areas of their range, Burrowing Owls are often victims of shooting programs aimed at prairie dogs (James and Espie 1997). Even when such shooting is not directed at owls, it typically results in reduced owl reproductive success (Woodard 2002).
Most studies of Burrowing Owl foraging behaviour have noted the flexibility in the species’ diet, depending on time of day, season, and local fluctuations in different prey species (McDonald et al. 2004). On the Canadian prairies, voles (Microtus spp.), mice, (Peromyscus spp.), grasshoppers (Acrididae), and beetles are common prey items (Haug et al. 1993). During the summer, Burrowing Owls typically forage during the day around their nest sites for insects, but feed on small mammals in nearby grasslands at night (Schmutz et al. 1991, Haug et al. 1993, Sissons et al. 2001). During the breeding season in Saskatchewan, over 90% of the prey biomass is composed of small mammals and such prey is typically captured at night (Poulin 2003). A similar ratio of insects to mammals is consumed during the winter in Mexico (Valdez Gomez et al. 2002).
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