Passerine sister clade comparisons reveal variable macroevolutionary outcomes of interhemispheric dispersal.

Dispersal events offer a unique window into macroevolutionary processes, especially with respect to the effects of competition on diversification. Empirical studies testing alternative predictions of competitive effects are often limited in either geographic or phylogenetic scale. Here, we tested some of these hypotheses by comparing an assemblage of 16 oscine passerine clades, representing independent dispersal events into the Western Hemisphere, to their sister clades in the Eastern Hemisphere. We also compared the diversity of this assemblage of clades to an older, incumbent passerine clade in the Western Hemisphere, the suboscines. Specifically, we tested for ecological opportunity and incumbency-mediated constraints by analysis of clade-specific morphological disparities and rates of evolution relative to dispersal history. While there was no consistent outcome of oscine dispersal and macroevolution in the Western Hemisphere relative to their Eastern Hemisphere sister groups, most clades supported a role for ecological opportunity or incumbency effects, and such effects were better explained by differences in species accumulation than by differences in rates of trait evolution or colonization timing. This general pattern was not evident when comparing the entire oscine assemblage of the Western Hemisphere to the incumbent suboscine radiation; oscines and suboscines occupy comparable regions of functional trait diversity and, despite higher rates of trait evolution in oscines, these observations were consistent with simulated null expectations. This result suggests that oscine and suboscine assemblages may have evolved in relative isolation for a significant fraction of their history.

Songbirds of the Americas show uniform morphological evolution despite heterogeneous diversification.

Studying the relationship between diversification and functional trait evolution among broadly co-occurring clades can shed light on interactions between ecology and evolutionary history. However, evidence from many studies is compromised because of their focus on overly broad geographic or narrow phylogenetic scales. We addressed these limitations by studying 46 independent, biogeographically delimited clades of songbirds that dispersed from the Eastern Hemisphere into the Americas and assessed (1) whether diversification has varied through time and/or among clades within this assemblage, (2) the extent of heterogeneity in clade-specific morphological trait disparity and (3) whether morphological disparity among these clades is consistent with a uniform diversification model. We found equivalent support for constant rates birth-death and density-dependent speciation processes, with notable outliers having significantly fewer or more species than expected given their age. We also found substantial variation in morphological disparity among these clades, but that variation was broadly consistent with uniform evolutionary rates, despite the existence of diversification outliers. These findings indicate relatively continuous, ongoing morphological diversification, arguing against conceptual models of adaptive radiation in these continental clades. Additionally, they suggest surprisingly consistent diversification among the majority of these clades, despite tremendous variance in colonization history, habitat valences and trophic specializations that exist among continental clades of birds.

Relationships of song structure to phylogenetic history, habitat, and morphology in the vireos, greenlets, and allies (Passeriformes: Vireonidae).

Acoustic signals show immense variation among passerines, and several hypotheses have been proposed to explain this diversity. In this study, we tested, for the first time, the relationships of song structure to phylogeny, habitat type, and morphology in the vireos and allies (Vireonidae). Every measure of song structure considered in this study had moderate and significant phylogenetic signal. Furthermore, two song-constraining morphological traits, bill shape and body mass, also exhibited significant phylogenetic signal. Song length showed the largest within-clade similarity; longer songs were highly conserved in part of the greenlet (Hylophilus) clade, whereas shorter songs characterized the remaining seven genera. We found no differences in song structure among vireonids living in different habitat types. However, vireonids with shorter, stouter bills and larger bodies sang songs with lower minimum and maximum peak frequency, compared with species with longer, thinner bills and smaller bodies. We conclude that Vireonidae song evolution is driven partially by phylogenetically conserved morphological traits. Our findings support the phylogenetic signal and morphological constraints hypotheses explaining structural diversity in avian acoustic signals.