In this project, we examined various hypotheses that address one of the fundamental questions in ecology and evolution: what determines the range of a species? We used demographic data for saltmarsh sparrows (Ammodramus caudacutus) collected over the majority of the global breeding range. Saltmarsh sparrows are considered threatened by climate change, specifically sea level rise, which is predicted to result in loss of the tidal marsh habitat where saltmarsh sparrows live across their entire life cycle. For my dissertation, I investigated the reproductive biology of saltmarsh sparrows both to provide vital information for wildlife managers and to explore broad questions in ecological and evolutionary theory. We examined the spatial variation in risks to fecundity, vital rates, and niches across the global range of a species. We were thus able to investigate some of the most fundamental concepts in ecology, the drivers of species’ distributions and spatial and temporal variation in niches. Specifically, I 1) explored competing risks to saltmarsh sparrow fecundity across their global range; 2) quantified saltmarsh sparrow fecundity across the range and tested whether fecundity decreases from the range center to its periphery; 3) characterized the nesting niche of saltmarsh sparrows across a large spatial scale to determine whether niche conservatism holds in this system; and 4) investigated differences in nesting niches between saltmarsh and sympatric Nelson’s sparrows and the fitness consequences of those differences. The results of these chapters suggest that though saltmarsh sparrow fecundity is influenced by large-scale factors such as global predation gradients, the saltmarsh sparrow range is not determined by large-scale trends in demographic rates or habitat marginality with latitude or between sister species
