Nest Site Characteristics and Nest Predation in Harris' Sparrows and White-Crowned Sparrows in the Northwest Territories

Abstract

I examined the relationship of nest-site and nest-patch characteristics to nest success in ground-nesting Harris' Sparrows (Zonotrichia querula) and Gambel's White-crowned Sparrows (Z. leucophrys gambelii) in the forest-tundra ecotone of the Northwest Territories, Canada. I found 34% of all Harris' Sparrow nests depredated, primarily by arctic ground squirrels (Spermophilus parryii), while no White-crowned Sparrow nests were disturbed by predators. White-crowned Sparrow nests appeared to be less susceptible to predation than Harris' Sparrow nests because the former were placed in areas with more shrubs and ground cover, and denser vegetation, than were Harris' Sparrow nests. Comparison of successful and depredated Harris' Sparrow nests supported the idea that interspecific differences in rates of nest predation were due to differences in concealment rather than to density-dependent nest predation. Successful Harris' Sparrow nests were placed in areas with more shrub cover and more dense vegetation within 5 m of the nest than were depredated nests. Orientation of the nest entrance did not differ between Harris' and White-crowned sparrow nests, nor between successful and depredated Harris' Sparrow nests. However, nest entrances of both species were nonrandomly oriented, with mean orientation vectors 135 degrees to 170 degrees from pre-vailing storms. Reasons for the tendency of Harris' Sparrows to select sites where chances of predation are relatively high are unclear, but could be related to a lack of suitable nest sites in the study area.