Multiple routes to interspecific territoriality in sister species of North American songbirds

Behavioral interference between species can influence a wide range of ecological and evolutionary processes. For example, aggressive and reproductive interference can lead to competitive exclusion, facilitate invasions, and limit how species ranges shift in response to climate change. While empirical examples of these diverse outcomes abound, a general understanding of when these behaviors arise is necessary to improve predictions of species coexistence and patterns of biodiversity. One type of behavioral interference, interspecific territoriality, has been found to generate important ecological and evolutionary patterns across numerous taxonomic groups, yet still unknown are the ecological circumstances associated with interspecific territoriality and its likelihood to persist through time. Here I present the first comparative study of interspecific territoriality among all sister species of North American songbirds. I tested core hypotheses regarding the origins and maintenance of interspecific territoriality and evaluated the role of interspecific territoriality and hybridization in shaping species distributions and in determining the likelihood of closely related species transitioning from parapatry to sympatry. To do so, I conducted extensive literature searches to identify sister species pairs that exhibit interspecific territoriality and assessed whether ecological traits and morphological divergence predict interspecific territoriality. Among closely related North American songbirds, interspecific territoriality is pervasive, and interspecifically territorial species pairs have more recently diverged than non-interspecifically territorial pairs. Analysis of the ecological correlates of interspecific territoriality indicates that misdirected intraspecific aggression and resource competition can each lead to interspecific territoriality. These patterns have implications for the ability of species pairs to coexist in sympatry: time since divergence between closely related songbird species does not predict breeding range overlap, nor does interspecific territoriality enable closely related species to transition from parapatric to sympatric distributions. Instead, the combination of interspecific territoriality and hybridization appears to be an unstable state associated with parapatry, whereas species that are interspecifically territorial and do not hybridize are able to achieve fine-scale and coarse-scale breeding range overlap. In sum, these results suggest that interspecific territoriality has multiple ecological origins and that interspecific territoriality and hybridization together can have striking impacts on species ranges. Our work highlights the value of comparative analyses for identifying ecological causes and evolutionary outcomes of this important interspecific behavioral interaction