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Understanding why certain species occupy wider climate niches than others is a fundamental pursuit in ecology with important implications for conservation and management. However, existing synthesis on this topic has focused on the consequences rather than the causes of climate niche expansion, leading to significant gaps in our understanding of the possible evolutionary drivers of this important ecological property. Here we leverage species distribution models powered by millions of citizen science sightings of birds to determine how a comprehensive suite of parameters influences the breadth of climate niches. Our analyses show that migration and more central locations in climate space are directly associated with wider climate niches. Additionally, they indicate that larger brains, smaller bodies, and broader dietary requirements are indirectly associated with narrower niches, presumably because they enable the occupancy of geographically widespread habitats that occupy narrow areas in climate niche space. Through follow-up analyses we further clarify how the different factors considered in this study help shape niche breadth by affecting the colonization of more versus less frequently used habitats. Overall, our findings shed light on critical, yet highly under-appreciated properties of climate niches, underscoring the complexity and interconnectivity of the factors that shaped their evolution among birds. 

Abstract The processes that allow some lineages to diversify rapidly at a global scale remain poorly understood. Although earlier studies emphasized the importance of dispersal, global expansions expose populations to novel environments and may also require adaptation and diversification across new niches. In this study, we investigated the contributions of these processes to the global radiation of crows and ravens (genusCorvus). Combining a new phylogeny with comprehensive phenotypic and climatic data, we show thatCorvusexperienced a massive expansion of the climatic niche that was coupled with a substantial increase in the rates of species and phenotypic diversification. The initiation of these processes coincided with the evolution of traits that promoted dispersal and niche expansion. Our findings suggest that rapid global radiations may be better understood as processes in which high dispersal abilities synergise with traits that, like cognition, facilitate persistence in new environments.