Breeding-season frugivory by Harris' sparrows (Zonotrichia querula) and white-crowned sparrows (Zonotrichia leucophrys) in a low-arctic ecosystem

摘要

We studied breeding-season frugivory by Harris' sparrows (Zonotrichia querula) and white-crowned sparrows (Z. leucophrys) in the forest-tundra ecotone of the Northwest Territories during a 3-year period. Density of fruits produced during previous summers and preserved beneath the snow averaged between 83 and 94/m(2) during June. Mass of alpine bearberry (Arctostaphylos alpina) fruits collected during June was significantly greater than that of crowberry (Empetrum nigrum) or mountain cranberry (Vaccinium vitis-idaea), while the relative sugar content of mountain cranberry fruits in June was greater than that of alpine bearberry or crowberry fruits. The protein content of all fruits was relatively low (less than or equal to 3.1%). Harris' and white-crowned sparrows maintained body mass and fat condition while feeding heavily on fruits during the early breeding season and shifted to arthropods when feeding nestlings. The results of our study, plus the widespread distribution of arctic and subarctic subshrubs that produce fleshy fruits consumed by other species, particularly ptarmigan (Lagopus spp.) and shorebirds, suggest that community-wide studies are needed to evaluate the importance of-over-winter and late-summer fruits in the diets of terrestrial arctic and subarctic birds.