Empirical evidence to assess the hypothesis that nest predation pressure influences avian assemblage composition is mostly lacking. We examined distribution of predation risk for artificial bird nests in the understory of coniferous and deciduous forests in southeastern Alaska and adjacent western Canada to determine whether habitat-specific nest predation pressure could be a factor influencing habitat selection and, in turn, breeding bird diversity. Two sizes of open-cup nests were constructed of natural materials and placed in nest sites representative of those used by local breeding bird species although, on average, artificial nests were more conspicuous than natural nests monitored in a companion study. Artificial nests were exposed to predation during early and late nesting seasons in 1993 and 1994. Principal nest predators identified using automated cameras were red squirrel (Tamiasciurus huclsonicus), Steller's Jay (Cyanocitta stelleri; Alaska only), Gray Jay (Perisoreus canadensis; Canada only), and small mammals. Systematic point-count censuses recorded significantly more red squirrels than jays in coniferous forest, and both jay species reached peak abundances at the border between coniferous and deciduous forest, relative to interior forest of both types. In repeated-measures analysis of variance, predation of artificial nests was significantly higher in coniferous than deciduous forest, and greater for large (thrush size) than for small (warbler size) nests. Nest height (shrub vs. ground) was not an important factor, due to a significant interaction with time (early vs, late nesting season). Natural and artificial nest predation losses were comparable in deciduous forest, but artificial nests were more susceptible than natural nests in coniferous forest understory. Artificial nest losses reflected the distribution of predators, especially red squirrels, and was negatively associated with breeding bird diversity in northwestern forest understory-confirming that nest predation pressure is one (of several) plausible determinants of avian habitat selection and assemblage organization.
