The concept of the ecological niche is fundamental to understanding constraints on species distributions in space and time, and in explaining the origin and maintenance of biodiversity. A niche can be broadly defined to include all of the biotic and abiotic conditions that a species requires to persist. Niche breadth, or the degree of specialization, may influence how labile a species niche is, which can have broad implications for species ability to adapt to environmental change, and for explaining patterns of diversification. I investigated mechanism that facilitate or constrain niche evolution at multiple scales. First, I developed an index of specialization in bill morphology using museum specimens across a diverse New World Passerine clade. I used this index of specialization to evaluate the relative influence of geographic and ecological niche partitioning on speciation rates across islands and continents. I then examined evolutionary transition rates among generalist and specialist bill morphotypes to determine if specialization constrains further evolution over long time scales, thus creating an evolutionary dead end. My results suggest that specialization increases speciation rates, and that niche expansion allowing transitions from specialist back to a more generalist bill morphology were common. I further explored mechanisms that drive these broad scale patters by examining patterns of intraspecific niche partitioning in closely related tidal marsh passerines. I found that habitat characteristics that reflected a salinity gradient best explained parallel patterns of bill size divergence among populations of two closely related sparrow species. Lastly, I examined if the definition of specialization varies across niche axes. We found that niche breadth, or the degree of specialization, is correlated among functional, environmental, and competition axes among five species of Passerelid sparrows. By examining the influence of specialization on macroevolutionary patterns of diversification and patterns of niche partitioning within species we gain a more comprehensive understanding of how niches evolve across different temporal and taxonomic scales. I found specialization is associated with increased speciation rates that influence continental-scale patterns of diversification. I also provide evidence that specialists retain the potential for niche expansion at the species and population scale. Patterns of intraspecific niche partitioning along habitat gradient presented here also increase our understanding of how species might adapt to change at scales that are applicable to local conservation. My results suggest strategies to incorporate a diversity of habitat characteristics may be beneficial for conserving intraspecific variation and adaptive capacity of specialist species
