The influence of cover on the breeding biology of western gulls on Santa Barbara Island, California

The breeding biology of Western Gulls (Larus occidentalis) on Santa Barbara Island is strongly influenced by the distribution and abundance of certain forms of cover, especially the shrub, Suaeda californica (seablite). Early in the breeding season, nesting gulls concentrated in areas where cover was relatively abundant; higher breeding success was associated with these same areas. Analysis of offspring mortality in relation to several characteristics of the nesting environment -- general cover in the nesting area, concealment, shade, cover provided by the incubating parent and artificial cover -- indicated that cover enhanced survival by providing protection from two major sources of mortality, conspecific predation and heat exposure, and by influencing chick movements. While other gulls constituted the major threat to egg and chick survival on this colony, observations made during this study demonstrated that heat stress, also, is a potential danger to gull offspring. Eggs containing very young embryos heated within a few hours to dangerously high, if not lethal, internal temperatures when left exposed in the sun, while 50% of all chick mortality in 1976 occurred during a heat wave in June of that year. Interpretation of results showing a higher proportion of undeveloped eggs in areas containing little cover is complicated by the fact that the eggs may have been either infertile or dead at very early stages in development. Other results presented here provide indirect evidence that younger, inexperienced breeding gulls may contribute to greater hatching failure in the poorer habitats. The importance of cover to gulls nesting on this island has serious implications in light of a severe, long-term decline of Suaeda and other native island vegetation.