Nonrandom Nest-Site Selection in Evening Grosbeaks

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Abstract
Nest-site selection was studied in Evening Grosbeaks (Coccothraustes vespertinus) living in two areas (Eldora and Wild Basin) of the Front Range of the Rocky Mountains of Colorado[USA]. One hundred and twenty-nine original variables were reduced to compare nesting (NS; n = 49) and nonnesting control (NNS; n = 10) sites using multivariate and univariate statistics. Overall, four of the five variables best discriminating between NS and NNS were measures of habitat density (deciduous stem intersect, deciduous canopy cover) or shrub dispersion (large clumps, irregular matrix). Although there were distinct differences between the two NS with respect to shrub dispersion, canopy cover, and the relative proportion of major vegetation, NS were more open than NNS within the same area. In both Eldora and Wild Basin, grosbeaks showed a preference for ponderosa pines (Pinus ponderosa) in which to build high camouflaged nests, close to the main tree trunk, predominantly with southern or northern exposures. We suggest that these nest locations provide (1) high visibility for detection of potential predators, (2) the opportunity for grosbeaks to see their nests from a distance, (3) easy access for departure and arrival, (4) aid in thermoregulation, and (5) protection from wind and rain and increased nest stability.