The role of social and ecological processes on phenotypic evolution in birds
Keywords: Birds
Macroecology
Diversification
Biodiversity
Macroevolution
Sexual selection

The broad concept of “biodiversity” can be roughly separated into two related components: trait diversity and species richness. Despite the fact that one or both of these types of biodiversity underlie much of ecology, evolution, and conservation, however, it remains largely unknown how traits and speciation dynamics can interact, particularly at a large scale. My thesis uses modern phylogenetic comparative methods and a new global database of avian morphological traits to quantify and predict the drivers of biodiversity across the world’s birds, focusing particularly on the relative roles of ecological and social traits to understanding broad evolutionary patterns.
In Chapter 2 I present a survey of avian functional traits, focusing on eight measurements of the beaks, wings, tails, and tarsi of 42,334 individuals representing 10,023 extant and recently extinct species. The global trait distribution of avian communities is consistent with a competition-based model of community assembly, and I find no evidence of environmental filtering at the biome level. The traits exhibited within avian orders tend to become more dissimilar as species richness increases, with the notable exception of the Passeriformes, an order containing around 60% of the total avian species richness but occupying a region of morphospace expected of a clade two orders of magnitude smaller. The Passeriformes also possess remarkable vocal morphology and behavior, and thus I spend the next three chapters of this thesis focused on the role of social processes in avian evolution.
In Chapter 3, I use detailed morphological and vocal trait data from the suboscine family Furnariidae to demonstrate that social traits evolve faster and with less regularity than ecological traits. I then in Chapter 4 examine the social and ecological drivers of female song in birds, a widespread trait whose persistence challenges traditional sexual selection theory. I find that the separate drivers of male and female song can be explained by social selection theory, a framework which encompasses all social interactions above and beyond competition for mating opportunities. In Chapter 5 I apply this concept of social selection to macroevolutionary studies, using sex-specific song behaviors to separately investigate the roles of social and sexual selection on speciation and extinction rates. I demonstrate that lineages with male-only song (sexual selection) diversify faster than lineages with both male and female song (social selection). This result suggests that social selection theory may inform the controversial relationship between sexual selection and diversification.
Finally, in Chapter 6 I look at the role of dispersal in modulating these speciation and extinction rates. Using data from the wing morphologies of 26,043 individuals from 6,028 species, I test the “intermediate dispersal hypothesis,” hitherto only examined at small scales, across the order Passeriformes, revealing that the highest rates of diversification are indeed found in lineages with an intermediate capacity for flight. When birds that defend year-round territories are considered separately from non-territorial birds, however, the intermediate dispersal hypothesis only holds for territorial birds, demonstrating the importance of considering the ecological context of traits in macroevolutionary studies.
Together, my results present evidence that both social and ecological processes facilitate the generation and maintenance of biodiversity in birds.