Boreal forest songbird diversity and reproductive success : roles of vegetation, predators, and competitors

Abstract

The hypothesis that predators diversify songbird communities, by preying more heavily on individuals and species with greatest similarity in nest site use, was evaluated using field studies in boreal forests of Alberta and Saskatchewan and artificial nest experiments in Alberta. Positive relationships between predator diversity or abundance and diversity, of birds were detected, after controlling effects of vegetation. Limited competition may have also contributed to patterns of bird diversity because there were more positive than negative associations between ecologically similar congeners. Predator responses to artificial songbird nests that varied in nest-site placement, vegetation features and nest spacing patterns were also examined. Predators were expected to destroy nests characterized by similar vegetation features or nest types. Variability among nest sites was achieved by deploying nests throughout a gradient of vegetation and by deploying nests to simulate two- and three-species assemblages. However, predation did not increase as variance in vegetation surrounding nest sites decreased across plots nor did variance in vegetation surrounding successful nests increase among plots as predation level increased. The addition of another species' nest type to assemblages did riot result in lower predation rates, nor did predators destroy more clumped than randomly distributed nests. Data from these and other artificial nest experiments were used to identify habitat attributes associated with successful nests versus those destroyed by specific predators. Successful nests and those visited by mice tended to be ground nests well-concealed by dense Shrubs. Squirrels and birds usually visited above-ground nests at sites with few shrubs and high tree densities. Reliability of patterns of songbird productivity was tested using artificial nests (visual and olfactory cues) and indices of reproductive success in mixedwood forest of Alberta. Avian predators did riot discriminate between wicker nests dipped in mud and wicker nests covered by, a camouflage fabric, whereas mammalian predators showed a weak tendency to depredate camouflaged nests. Nests containing plasticine eggs in field experiments and egg assortments containing plasticine eggs in laboratory experiments with captive deer mice (Peromyscus maniculatus) were depredated more than those only containing finch and quail eggs. Young and old forest stands were used to compare differences in reproductive effort and success, using songbird behavior and number of fledglings. Some species differed in density (17%), indices of reproductive behavior (33%), and number of fledglings observed (13%) between stand-ages. Using behavioral indices or fledgling numbers to estimate nesting success in forested habitats did not appear feasible. Also, a presumed positive relationship between indices of reproductive success density were obtained for only 40-45% of species. Overall, results were consistent, in part, with evidence that mechanisms other than competition or vegetation structure may contribute to forest songbird community structure. My findings also suggest that predators diversify songbird communities, though discrepancies exist between results from natural songbird communities versus artificial nests possibly because artificial nests do not portray reliable predation events. Further experimental research is required to clarify the role predators play in structuring songbird communities and to refine methodology used to detect patterns of avian reproductive success.