SummaryPopulations of the recently split Northern Rockhopper PenguinEudyptes moseleyiare restricted to Tristan da Cunha and Gough Island in the South Atlantic, and Amsterdam and St Paul in the Indian Ocean. The majority of the population is in the Atlantic (> 80%), but population trends at Tristan da Cunha and Gough are uncertain. Early records indicate “millions” of penguins used to occur at Tristan da Cunha and Gough Island. The most recent estimates indicate declines in excess of 90% for both Gough and the main island of Tristan that have occurred over at least 45 and 130 years, respectively. Numbers breeding at Inaccessible and Nightingale islands (TDC) also may have declined since the 1970s, albeit modestly, whereas numbers on Tristan appear stable over the last few decades. Current population estimates are 32,000–65,000 pairs at Gough, 18–27,000 at Inaccessible, 19,500 at Nightingale, and 3,200–4,500 at Tristan. Numbers and trends at Middle Island (TDC) are unknown. Middle Island supported an estimated 100,000 pairs in 1973, and recent observations suggest this colony is being impacted by competition for space with recently recolonising Subantarctic Fur SealsArctocephalus tropicalis. Past human exploitation and the impact of introduced predators may be responsible for the historical decline in numbers at Tristan, but these factors cannot explain the sharp decrease (since the 1950s) at Gough Island. Overall, declines at Gough, Tristan, Nightingale and Inaccessible islands indicate a three-generation decline of > 50%. Taken in combination with recent decreases in Indian Ocean populations, the Northern Rockhopper Penguins is now categorised as globally ‘Endangered’. Determining the causal factors responsible for these recent declines is an urgent priority.

In comparison to the mainland, populations of rodents on islands are often characterized by a suite of life history characteristics termed the “island syndrome.” Populations of rodents introduced to islands are also well known for their impacts on native species that have evolved in the absence of mammalian predators. We studied the ecology and behavior of introduced house mice Mus musculus on Gough Island where they are the only terrestrial mammal and where their predatory behavior is having a devastating impact on the island’s burrowing petrel (order Procellariiformes ) population and the Critically Endangered Tristan albatross Diomedea dabbenena . Mice on Gough exhibit extreme features of the island syndrome, including: a body mass 50–60% greater than any other island mouse population, peak densities among the highest recorded for island populations, and low seasonal variation in numbers compared to other studied islands. Seasonal patterns of breeding and survival were linked to body condition and mass, and mice in areas with high chick predation rates were able to maintain higher mass and condition during the winter when mouse mortality rates peak. Within-site patterns of chick predation indicate that proximity to neighboring predated nests and nesting densities are important factors in determining the likelihood of predation. We conclude that selection for extreme body mass and predatory behavior of mice result from enhanced overwinter survival. Small mammal populations at temperate and high latitudes are normally limited by high mortality during the winter, but on Gough Island mice avoid that by exploiting the island’s abundant seabird chicks.

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