| Summary: | We surveyed murres (Uria spp.) annually from 1989 through 1991 at breeding colonies within the trajectory of the oil to determine whether numbers had declined and to evaluate the effects of oil on nesting phenology and reproductive success, following the 1989 Exxon Valdez oil spill (EVOS). The colonies we surveyed contained the majority of the estimated 200,000 murres attending colonies in the affected area, and we found reduced numbers at all study colonies following the spill. In addition, nesting was delayed and productivity rates were far below normal following the spill. In contrast, numbers of murres did not decline and reproductive parameters were normal at 2 colonies we surveyed outside the trajectory. The only indication of recovery since the spill was a slight increase in reproductive success at monitored colonies in 1991. The most likely cause of reduced numbers of murres at cliffs following the oil spill was direct mortality from the oil. Since breeding murres were congregating near colonies at the time of the spill, most murres killed were probably experienced breeders. We concluded that reduced densities and skewed age structures were the most likely causes of abnormal breeding after the spill.
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