Convergence in foraging guild structure of forest breeding bird assemblages across three continents is related to habitat structure and foraging opportunities

Comparisons of community structure across sites allow for the detection of convergent patterns and the selective forces that have produced them. In this study, we examined the foraging guild structure of birds breeding in forests on three continents – Europe, North America, and Australia, with largely phylogenetically distinct avifaunas. We examined two hypotheses: (1) the bird assemblages in the three geographically separated forested study sites should have similar foraging guild patterns to the extent to which environmental resources of these forests are similar, and (2) if bird assemblages in structurally similar forest habitats have undergone adaptive evolution, then radiation of species into guilds should have been caused by analogous selective resource gradients (factors). Bootstrapped cluster analysis (UPGMA) and bootstrapped principal coordinate analysis (BPCoA) of chord distances were employed to determine foraging guild structure for each assemblage, and to extract the significantly different factors responsible for segregation of species into guilds. Cluster analyses identified three analogous foraging guilds (ground and litter foragers, foliage gleaners, and trunk foragers) in each of the bird assemblages, supporting the first hypothesis of guild structure convergence. The BPCoA determined that two environmental factors (vertical resource allocation and spatial tree morphology gradients) were primarily responsible for segregation of species into guilds in these three geographically distant but structurally similar forests. These findings support the hypothesis that guild structures in forest bird assemblages largely reflect the similarities and differences in forest structure and the distribution and abundance of foraging resources, and result from largely adaptive evolution.