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UBC Theses and Dissertations

Factors affecting nest predation artificial and real Sagebrush Brewer’s Sparrow (Spizella breweri breweri) nests in the South Okanagan and Similkameen Valleys, British Columbia Welstead, Kym


Predation is the predominant cause of nest failure for the Brewer's Sparrow (Spizella breweri breweri; BRSP), a provincially red-listed shrub-steppe species that has experienced significant declines throughout most of its range (Breeding Bird Survey). The factors that influence nest survival are often species and context specific. Therefore, it is important to examine the dynamics of Brewer's Sparrow nest survival in British Columbia (BC) so that appropriate management strategies can be devised. I monitored Brewer's Sparrow nests to test if nest density and avian nest predator activity influenced Brewer's Sparrow nest survival. I also conducted an artificial nest experiment to assess potential nest predation risk relative to Brewer's Sparrow abundance and to determine whether increased avian nest predator activity was associated with reduced nest survival. In addition, within site factors such as tree encroachment, timing, and nest concealment were modelled to determine their effects on the survival of real Brewer's Sparrow nests at all nesting stages and for three nest predator categories - avian, mammalian and pooled (all predators combined) using artificial nests. The study was conducted at six sites in the South Okanagan region of BC. My results showed that increased avian nest predator activity was associated with reduced survival of artificial nests. Avian predation of artificial nests was also highest at sites with few Brewer's Sparrows, suggesting that Brewer's Sparrows may avoid nesting at sites with many avian nest predators. Artificial nest predation by small mammals did not vary with Brewer's Sparrow abundance. Survival analysis revealed that nest initiation date was an important predictor of real nest survival, because depredation increased later in the season. Pooled predators and mammalian depredation of artificial nests mirrored this trend. Avian predation risk was reduced through the season but only in combination with less nest cover. Late in the season, artificial nests placed close to trees experienced less mammalian predation and more avian predation. Tree density was related negatively to the survival of real Brewer's Sparrow nests at the nestling and pooled nesting stages. Overall, my results suggest that Brewer's Sparrows nested in sites lower in avian nest predator activity. This trait resulted in the relative importance of other predators, such as small mammals, becoming more apparent. The importance of nest concealment and tree encroachment on real nest survival may be circumvented by Brewer's Sparrows nesting far from trees, in micro-sites with ample cover, and nesting in sites with low avian nest predator activity.

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