Ecology, behavior, and trophic adaptations of the salt marsh song sparrow Melospiza melodia samuelis: The importance of the tidal influence gradient

Residence in the semi-marine salt marsh habitat strongly affects the ecology, behavior, and trophic adaptations of the San Pablo Song Sparrow ( Melospiza melodia samuelis), a terrestrial songbird. The gradient of tidal influence, which differentiates the habitats within the marsh, has particular importance for Song Sparrow behavioral ecology. The first chapter of this dissertation documents a biogeographic pattern of convergent bill morphology across ten sparrow taxa endemic to tidal marshes of North America. Relative to their body weight, tidal marsh sparrows have longer, thinner bills than their non-tidal-marsh counterparts, which is likely a trophic adaptation for consuming more invertebrates and fewer seeds, as well as for probing in sediment crevices to capture prey. The second chapter presents a conceptual model of the salt marsh food web based on stable isotope data from a marsh in northern San Francisco Bay, California. This model shows that, unlike results from other studies on Atlantic coast marshes, the food web was divided into compartments by habitat, with the marsh plain being trophically separate from the low marsh and slough areas. This compartmentalization implies that little trophic exchange occurs between the marsh plain and the adjacent estuary via the food web. The third chapter shows that this sparrow subspecies has an unusual territorial system that relates to differences between marsh habitats. Sparrow territories are areas of exclusive use centered on slough habitat, while home ranges extend well beyond the territories into shared feeding areas on the marsh plain. Approximately one-third of nests are located a few meters out side of territory boundaries. Sparrow population density, the physical structure of the marsh habitats, and the distribution and abundance of food resources may explain this unusual use of space. Chapter four explores the relationship between sparrow fitness and the type of slough habitat the territory occupies. Although reproductive success measures did not vary by slough size, sparrows on small sloughs had higher adult survival. Rates of extra-pair paternity were highest on sloughs of medium size. Therefore, sparrow fitness was highest on the smaller sloughs, which may be of interest in marsh restoration.