Assessing indirect effects of oil in the presence of natural variation: The problem of reproductive failure in South Polar Skuas during the Bahia Paraiso oil spill

A population-wide mortality of South Polar Skua chicks occurred during a small diesel-oil spill at Palmer Station, Antarctica in 1989. It was hypothesized that sublethal oiling of adult South Polar Skuas temporarily disrupted parental guarding of chicks. Unattended chicks were preyed on by other skuas, and all known chicks within the local population were lost. Subsequently, other researchers have proposed that this reproductive failure was unrelated to the presence of oil. Natural variation producing food shortages or storms in critical periods occasionally may result in reproductive failure among seabirds. Here, evidence for these alternative hypotheses is examined. This controversy illuminates the general problem of determining the effects of an oil spill on natural populations, whose numbers and reproductive success vary both in time and space.