Habitat and Area Effects on Forest Bird Assemblages: Is Nest Predation an Influence?

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Abstract
The ability of nest predation to explain patterns of covariation in species numbers with area and habitat was examined for forest birds. Numbers of individuals and species of birds were counted in 23 drainages in high elevation (2300 m) forest in central Arizona for 3 yr. Variation in bird species numbers was compared to variation in area and habitat structure among drainages. Birds were grouped into foraging (bark, understory foliage, canopy foliage, aerial) and nesting (cavity, understory foliage, canopy foliage) guilds based on heights and substrates used. Numbers of species increased with area for all guilds, but species—area slopes differed among nesting guilds. Differences in slopes cannot be explained by passive sampling because source pool size was the same or similar between guilds compared. Differences in slopes are consistent with a prediction that slopes should be greater for guilds that are more susceptible to nest predation. Variation in numbers of species among drainages was positively correlated with variation in the density of foraging and nesting substrates. However, correlations were greater when based on nesting than foraging heights and substrates. The results are consistent with a prediction that birds are selecting habitat sites based in part on the availability of nest sites that minimize risk of nest predation, and that these nest sites increase with density of foliage at nest height. These results ar also consistent with a hypothesis that availability of suitable nest sites is one of the bases for the relationship between species numbers and foliage density for foliage—nesting species. This conclusion is supported by a re—analysis of the data of Willson (1974), where foliage—nesting birds were grouped into nest—height guilds using data from the Cornell Nest Record Program; more of the birds that were added with vegetation layers tended to nest in those layers than forage in them. In short, multiple activities (foraging and nesting) can allow multiple processes to act simultaneously.