Coexistence among morphologically similar hummingbird species is promoted via density compensation in a Mexican temperate forest

When limits on the exploitation of shared resources are exceeded among morphologically similar species, competitive exclusion and/or character displacement is expected. However, competition may not be sufficiently intense to engender these phenomena, but it may alter the densities of their local populations, a deterministic phenomenon known as density compensation. Hummingbirds are the most phenotypically specialized nectar-feeding birds, and trait variation within their communities may facilitate finer resource partitioning, which is ideal to test hypotheses related to coexistence. Here, we used simulation models to investigate whether hummingbird ensembles with high morphological similarity to one or several competitors display a reduction in population density as a result of competition in two contrasting habitats (pine forest and second-growth vegetation). In both habitats, four scenarios based on morphological proximity to neighbors were analyzed non-parametrically using multivariate metrics. No evidence of density compensation was found in the pine forest in any scenario. Conversely, evidence of density compensation in the second-growth habitat was found, with the relationship strongest when scenarios represented competition between more proximal neighbors. Our data suggest that the redistribution of hummingbird species between contrasting habitats leads to certain subgroups of morphologically similar species having to pay a population cost to coexist in a given habitat due to density compensation. Thus, competition may not be the leading determinant of hummingbird ensemble structures, which instead appears to be attributable to a combination of mechanisms that operate at local and regional scales, such as source-sink dynamics.