About 150 rare, adult, yellow-eyed penguins died over a short period during the summer of 1989-1990 on the Otago Peninsula. These were from a total mainland population estimated at 240 breeding pairs. Penguin chicks and non-breeding birds were not affected, but there were indications of shortages in feed supply for birds that bred successfully. Thirteen dead penguins were examined. In ten birds, the cause of death was not established. Although it was commonly found that adult birds had little or no food in their gut, none were considered to have starved to death. No consistent pathological lesions were found, nor were any viruses, Chlamydia or significant bacteria recovered from selected tissues. Toxicology tests ruled out poisoning by copper, zinc, iron, lead, arsenic, selenium, cadmium, mercury, organophosphates, organochlorines, polychlorinated biphenyls, hexachlorobenzene or the toxins of Clostridium perfringens, Clostridium botulinum and dinoflagellates. The problem did not recur during the following summer.

Migratory behavior can differ markedly amongst individuals within populations or species. Understanding the factors influencing this variation is key to understanding how current environmental changes might influence migratory propensity and the distribution and abundance of migratory species across their range. Here, we investigate variation in migratory behavior of the partially migratory Eurasian oystercatcher (Haematopus ostralegus) population breeding in Iceland. We use the resightings of color-ringed adults and stable isotopes to determine whether individuals migrate or remain in Iceland during winter and test whether individual migratory strategies vary in relation to sex, body size, and breeding location. We also explore individual consistency in migratory strategy and test whether assortative mating with respect to strategy occurs in this population. The proportion of migrants and residents varied greatly across breeding locations but not with respect to sex or body size. Individuals were consistent in migratory strategy between years and there was no evidence of assortative mating by migratory strategy. We use these findings to explore factors underlying the evolution and maintenance of partial migration at high latitudes.