Post-fledging ecology of borrowing owls in Alberta and Saskatchewan: Dispersal, survival, habitat use and diet
Clayton, Kort M
The burrowing owl (Speotyto cunicularia) occupies an extensive range throughout North and South America. In Canada, at the northern end of its range, this species is listed as endangered. Landscape alterations resulting in direct loss, degradation, and fragmentation of habitat have likely hampered the survival and productivity of burrowing owls. Research and management efforts employed thus far have failed to reveal critical limiting factors or reverse the decline of this species in Canada. This study focused on monitoring the movements, habitat use, diet, and survival of burrowing owls during late summer and autumn in southern Alberta and Saskatchewan. The Alberta study area was primarily native rangeland. Less than 10% of the original native prairie remained on the Saskatchwan study area. Over two years, 44 radio-transmitters were deployed on burrowing owls in Alberta, and 33 in Saskatchewan. Alberta juveniles dispersed significantly earlier and further from nest sites, and moved more frequently than juveniles on the Saskatchewan study area. Directional analysis of post-fledging or pre-migratory movements revealed a southerly focus in both populations, but was more variable among juveniles than among adults. Owls consistently migrated from both study areas in late September or early October. Adult females exhibited the highest mean survival (0.83), whereas adult male (0.46) and juvenile (0.48) rate were similar. Most mortality occurred during the post-fledging period when owl activity peaked around the nest. Mortality from vehicle collisions was higher, and predation lower on the study area in Saskatchewan. Population modeling revealed that average productivity from the Alberta study area (3.5 young/nest attempt), coupled with mortality for the study period, was not high enough to cause the annual rate of decline found in this population (0.67). This suggests about 32% of annual mortality occurs off the breeding grounds. Habitat use/availability analysis revealed that owls preferred pastures with shorter grass for both roosting and nesting. Tame and native grass pastures were used about equally by owls for both roosting and nesting. A relationship was found between abundance of burrowing owl prey and vegetation structure. Deer mouse (Peromyscus maniculatus) and grasshopper (Acrididae) abundance were negatively correlated with vegetation height and density. In a high vole year, microtine vole numbers were positively correlated with vegetation height and density.