Ecological niche divergence or ecological niche partitioning in a widespread Neotropical bird lineage.

Ecological niche divergence is generally considered to be a facet of evolution that may accompany geographic isolation and diversification in allopatry, contributing to species' evolutionary distinctiveness through time. The null expectation for any two diverging species in geographic isolation is that of niche conservatism, wherein populations do not rapidly shift to or adapt to novel environments. Here, I test ecological niche divergence for a widespread, pan-American lineage, the avian genus of martins (). Despite containing species with distributions that go from continent-spanning to locally endemic, I found limited evidence for niche divergence across the breeding distributions of , and much stronger support for niche conservatism with patterns of niche partitioning. The ancestral had a relatively broad ecological niche, similar to extant basal lineages, and several geographically localized descendant species occupy only portions of the larger ancestral niche. I recovered strong evidence of breeding niche divergence for four of 36 taxon pairs but only one of these divergent pairs involved two widespread, continental species (Southern Martin vs. Gray-breasted Martin ). Potential niche expansion from the ancestral species was observed in the most wide-ranging present-day species, namely the North American Purple Martin and . I analyzed populations of separately, as a microcosm of evolution, and again found only limited evidence of niche divergence. This study adds to the mounting evidence for niche conservatism as a dominant feature of diversifying lineages. Even taxa that appear unique in terms of habitat or behavior may still not be diversifying with respect to their ecological niches, but merely partitioning ancestral niches among descendant taxa.