Burrowing owl (Athene cunicularia) COSEWIC assessment and status report: chapter 10

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Limiting Factors and Threats

Historically, the ultimate threat to viable populations of Burrowing Owls is thought to have been the conversion and degradation of habitat (Haug et al. 1993, Wellicome and Haug 1995, Clayton and Schmutz 1999). Telfer (1992) reported a 39% decline in unimproved pastureland between 1946 and 1986 in areas of the Prairie Provinces, while dramatic losses of native grasslands have been reported in Alberta (61% of mixed grass prairie), Saskatchewan (81% of mixed grass prairie), Manitoba (99% of both tall and mixed grass prairie), and the western and central U.S. Great Plains (Samson and Knopf 1994).

Mortality of young and adults also affects Burrowing Owl population viability (De Smet 1997, Clayton and Schmutz 1999, King and Belthoff 2001). In Saskatchewan, following years with relatively poor juvenile survival, the local breeding population decreased by 11-48%, while years of high juvenile survival were followed by a population increase as well as relatively strong juvenile recruitment into the breeding population (Todd et al. 2003). The close correlation between nesting productivity in a given year and the size of the population in the following year suggests an important relationship (Franken and Wellicome 2003, R. Poulin, D. Todd, T. Wellicome, unpublished data). De Smet (1997) showed that successful nests in Manitoba were more than three times as likely to be reoccupied the following year than failed nests, and percentages that returned to the same territory were four times greater for successful than for unsuccessful sites.

Figure 10: Pattern of Abundance (number seen per survey hour) of Burrowing Owls Seen on Christmas Bird Counts in Texas (lower figure) and California (upper figure)

Pesticides may also have direct and indirect effects on Burrowing Owls (Fox et al. 1989, Gervais and Anthony 2003, Gervais et al. 2003). Applications of rodenticides (e.g., strychnine) may cause direct mortality of adult and juvenile owls, as well as significantly decreasing breeding success (Butts 1973, James et al. 1990, Sheffield 1997). Insecticides (e.g., carbaryl and carbofuran) have been shown to significantly reduce owl fledging success in Saskatchewan (James and Fox 1987). Heavy use of insecticides and herbicides near Burrowing Owl foraging and roosting areas has been observed (but not quantified) on the wintering grounds in Mexico (G. Holroyd and H. Trefry, pers. comm., October 2004). In California, samples of Burrowing Owl eggs show varying levels of DDE contamination among years, with significant negative effects on reproductive success apparent during years of poor food availability (Gervais and Anthony 2003).

Although the evidence is thus far only correlative, it appears that pesticides may affect reproductive success and survival by significantly decreasing prey abundance. For example, offspring production decreased by up to 83 percent following the application of carbaryl and carbofuran around Burrowing Owl nest burrows (James and Fox 1987). Strychnine-treated grains (used for rodent control) and carbofuran-based insecticides remain widely used in the United States and Mexico and may pose a threat to the Canadian population of Burrowing Owls during migration and on the wintering grounds (McDonald et al. 2004, G. Holroyd, pers. comm., October 2004).

Although poorly quantified, collisions with vehicles are thought to be a significant source of mortality for adult and juvenile Burrowing Owls. Owls often forage near roads where grass conditions are optimal for small rodents and insects, and are thus susceptible to collision with vehicles. Clayton and Schmutz (1999) found that approximately 31% of all known Burrowing Owl fatalities on the Regina Plain, Saskatchewan, were due to collisions with vehicles. In Saskatchewan, 15% of known juvenile mortalities, during the post-fledging/pre-migration period, were attributed to collisions with vehicles (Todd et al. 2003). The author of this report found two dead adult Burrowing Owls on a single morning on small, secondary roads in the Oklahoma panhandle in May 2005, suggesting that vehicle collisions may be a significant mortality factor on the Great Plains.

Predation on Burrowing Owls, especially on recently fledged young, can represent a significant source of mortality in some populations. Todd (2001) found that in one year of a two-year study, 15 of 33 radio-tagged fledglings in Saskatchewan died prior to migration, with the primary cause being predation by avian predators. The high losses to predators were attributed to a combination of habitat fragmentation and reduced availability of mammalian prey (principally Microtus voles; Todd 2001).

Juvenile survival (from fledging to migration), which is largely mediated by local prey availability during the fledging stage, has been shown to have strong effects on subsequent population densities in Saskatchewan (Todd et al. 2003).

In summary, although habitat loss appears to have been the primary historical factor triggering population declines, suitable habitat is currently unoccupied, and lowered demographic factors (adult and juvenile mortality rates, productivity, immigration/emigration) now appear to be the key variables hampering the recovery of Burrowing Owls in the prairie provinces.