Selective Importance of Heat Stress in Gull Nest Location

A brief but intense heat wave on 9 June 1979 caused catastrophic chick mortality in a population of Western Gulls on Santa Barbara Island, California, USA. Mortality ranged from O to 90% in different areas of the colony. Mortality was not a function of the amount of vegetation cover available to chicks, not did chick age or size affect the probability of mortality during the heat wave. In the absence of a heat wave, chick mortality due to heat stress is rare in this population. Microclimate analysis in 1980 showed that the area of high chick heat stress mortality in 1979 was characterized by higher ground and air temperatures and lower wind velocities than the area of low mortality. Taxidermic thin metal chick models had a higher integrated thermal load, or operative environmental temperature, in the area of previous high mortality. The persistent nesting by gulls in areas of high potential chick mortality cannot be explained by habitat shortage or by the competitive inferiority of those birds. Enhanced access to food resources may explain the persistence of one area, but not of several others. Analysis of long—term weather records for the southern Channel Islands region showed that heat waves during the breeding season are rare: of nine possible heat waves in 62 yr of data, five have occurred since 1973. The present nesting distribution of gulls on the island has probably not evolved in a regime of frequent heat waves, but rather in response to a long period of highly climatic conditions.