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Contributions to nesting ecology of forest birds. Horvath, Otto

Abstract

An approximately 228 acres large forested area near Hope, B.C., has been investigated in the mahonietosum subassociation of Pseudotsugeto-Gaultherietum mesic association of the Coastal Douglas-fir Zone. Botanical survey and bird census work by the nest (or territory) mapping method were carried out during two summer seasons. Fifteen ecosystem types are described and their bird population, expressed in average densities per area unit as well as by average biomass is tabulated. Physical environmental conditions, especially pertaining to microclimate, were measured at 55 nests of the eight most common passerine species, and nest placing and construction has been considered at 167 nests of the same species. It has been found that the nesting of these species in their habitats is the function of the present vegetation, physical environment and the adaptive range of the species. In certain species the birds apparently chose nest sites with equable microclimate. In others it was found that the insulation of the nest varies according to the needs indicated by the extremes of the microclimate; again in others the nest material chosen provided the best available resistance to mechanical stresses. While other essential environmental requirements were not studied the results point toward an assumption that microclimate strongly influences the site, height, position, and material of the nests, and ultimately, the selection of nesting habitat of the birds studied.

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