Comparative Breeding Biology of Harris' Sparrows and Gambel's White-Crowned Sparrows in the Northwest Territories, Canada

Abstract

I studied the breeding biology of Harris' Sparrows (Zonotrichia querula) and Gambel's White-crowned Sparrows (Z. leucophrys gambelii) in the forest-tundra ecotone of the Northwest Territories, Canada from 1989 to 1991. Adult sex ratio was 1:1 for Harris' Sparrows, and approximated 1:1 for the smaller number of White-crowned Sparrows captured. Nesting cycles of Harris' Sparrows and White-crowned Sparrows were similar, and appeared to be affected by patterns of snow melt and arthropod abundance. Adults began arriving during the last week in May; most initiated nesting during the second or third week in June, after breeding habitat was at least 60% snow-free. During all years of the study, the peak in fledging occurred just prior to maximum arthropod abundance. Clutch size and egg mass as a percentage of adult mass did not differ between the species. Only female Harris' Sparrows and White-crowned Sparrows incubated eggs and brooded nestlings, but males and females of both species fed young. Mean length of the incubation period for Harris' Sparrows (12.8 days) was shorter than previously reported. Length of the nestling period and patterns of nestling growth and development were very similar in the two species. Breeding densities of White-crowned Sparrows averaged about 30% that of Harris' Sparrows, but egg success (young fledged/eggs laid) and productivity (young fledged/pair) was higher for White-crowned Sparrows. Lower productivity for Harris' Sparrows was caused by predation on eggs and nestlings by arctic ground squirrels (Spermophilus parryii) and shorttailed weasels (Mustela erminea). Lower breeding density of White-crowned Sparrows probably was due to a lack of suitable breeding habitat, perhaps combined with a lower return rate for adult White-crowned Sparrows during some breeding seasons. I observed little evidence ofinterspecific competition between the species.