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Burrowing Owl

Limiting Factors

Burrowing OwlLimiting factors for Burrowing Owls are considered to be those factors that increase mortality of juveniles or adults, decrease either productivity of breeders or the proportion of individuals that breed, or reduce habitat quality. Some of these limiting factors, such as Habitat Loss and Degradation, Mortality on Migration or Wintering Grounds, Pesticides, Predation, Collision with Vehicles, and Shooting can be directly related to human activity.

Habitat Loss and Degradation: During this century on the Canadian prairies, the amount of pasture has declined substantially within the Burrowing Owl's range. Every five years, Agriculture Canada tallies land use associated with farming and ranching. The agricultural census data show that native pasture habitat within the Burrowing Owl's range declined over the past quarter century in Alberta. From 1966 to 1991, the amount of total farm area allocated as pasture within the owl's range decreased by approximately eight percent. The most striking loss of grassland habitat occurred between 1976 and 1986, the decade that followed a peak in wheat prices. Nevertheless, Alberta has retained much of its pasture (46%) compared to Saskatchewan (26 %) and Manitoba (19 %). It should be stressed that pasture represents Burrowing Owl habitat only at a very coarse level because this category also contains land not used by Burrowing Owls for nesting ( for example: rocky soil, hilly terrain, and low land that occasionally floods). The land systems that Burrowing Owls select, such as flat lacustrine land with few rocks are also favoured for farming, and thus have undoubtedly declined at a faster rate than has pasture in general. Levels of fragmentation are also likely to be highest in heavily cultivated areas and, thus, in areas preferred by Burrowing Owls for nesting. The decline in availability of nesting habitat, and the simultaneous increase in fragmentation, appeared to cease about a decade ago, yet the Burrowing Owl population continues its steady decline.

At present, the evidence for habitat limitation in Alberta's Burrowing Owls is equivocal. Many researchers point out that the owl decline has caused habitat, which until recently contained owls, to become unoccupied, suggesting that habitat is no longer saturated. In Alberta, 71% of Burrowing Owl sites - all of which had, at some time since 1989, been occupied by owls - were unoccupied as of 1995. The condition of this habitat has a reportedly remained unchanged.

Mortality on Migration or Wintering Grounds: Winter mortality is extremely difficult to measure in long-distant migrants especially given the fact that most Burrowing Owls do not return to the same nest sites from year to year. Mortality cannot be accurately estimated by the recovery of Burrowing Owl carcasses or aluminum bands because the owls and their bands are quite small and many birds winter in the tropics. Mortality must therefore be deduced from survival, based on resightings of live, banded birds on breeding grounds. Unless site fidelity is 100 %, this method invariably overestimates mortality because birds emigrating from the study area are assumed to be dead. 

Pesticides: Pesticides can affect populations both directly, by causing adult mortality and lowering the ability to reproduce, and indirectly, by lowering the availability of prey or by eliminating burrowing mammals. Nothing is known about mortality of migrating or wintering Burrowing Owls caused by pesticide exposure. Similarly, no research has been conducted in Alberta to investigate the direct effects of pesticides on breeding Burrowing.

Carbofuran is a broad-spectrum insecticide that may be sprayed on cereal and forage crops to reduce damage caused by grasshoppers. In Saskatchewan, exposure to carbofuran within 50 metres of the Burrowing Owl's nest burrows was associated with a 54% reduction in the number of young per nest and a 50% reduction in the proportion of pairs raising one or more young.

Research on potential indirect effects of pesticide use in southern Alberta suggests that food availability may be lowered by the application of carbofuran. A considerable reduction in the number of invertebrate prey occurred immediately following carboftiran spraying. In addition, survival of tagged deer mice and meadow voles in grassland sprayed for grasshopper control was 40% and 33% less, respectively, than that of unsprayed populations.

Predation: Agricultural practices and the declining numbers of Wolves on the prairies have encouraged increases in many predator populations. Populations of Red Foxes, Coyotes, and Striped Skunks are thought to have grown considerably since historic times. These species thrive in agricultural areas despite disturbances by humans. The increased number of trees in the prairies, which resulted from fire suppression and deliberate planting, has also allowed some avian predator populations to increase. Great Homed Owls, Swainson's Hawks, and Red-tailed Hawks are found in greater abundances than they were on the previously treeless plains of the prairies. The provision of den sites in farm buildings allowed the range of another potential predator, the raccoon, to expand northward well into the Canadian prairies, where they were absent in the past.

Collision with Vehicles: Available data suggest that effects of vehicle collisions on Burrowing Owl populations in Alberta are small. Of 44 adults and recently fledged owls fitted with radio transmitters in the Hanna study area, none were hit by vehicles. During II years of research (1986 to 1996, inclusive) conducted on the same study site during and prior to the telemetry work, five owls were found dead on roads within the study area, and of 780 owls banded on the site between 1986 and 1996, none have been returned as vehicle mortalities off of the study site. A grader operator near Hanna, whose route covered roads in an area of 2300 kilometres, found three dead Burrowing Owls in 1992. This represents one percent of the total number of owls expected to be in the area that year, considering the density of owls and their estimated productivity.

Shooting: At a distance, Burrowing Owls can easily be mistaken for ground squirrels. Consequently, it is possible that some owls are shot accidentally each year. However, given that such incidents probably occur infrequently, shooting mortality likely has little or no direct effect on the overall owl population in Alberta. The shooting of burrowing mammals may have indirect negative effects on Burrowing Owls, by lowering the availability of nest and roost burrows.

Reprinted from Alberta Wildlife Status Report No. 11 (1997), with permission from Alberta Sustainable Resource Development.

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