Like clockwork - nest attendance patterns and foraging behaviour of Snares penguins Eudyptes robustus as a function of daylength and oceanography

The breeding routines and foraging behaviour of many pelagic seabird species is influenced by environmental factors. Seasonality greatly affects the temporal prey availability for many marine species while the spatial distribution of prey often correlates to oceanographic features. We examined the influence of such environmental factors on the nesting routines and the foraging behaviour of Snares penguins Eudyptes robustus that is endemic to the Snares island group south of New Zealand. Nest attendance and foraging patterns were studied during the incubation stage of three consecutive breeding seasons (2002-2004). Nesting patterns observed in one of the biggest colonies (ca 1200 nest) were highly synchronised, with male penguins leaving the colony to forage within a five-day period around 13 October each year. The males stayed at sea for a mean 11 days before most males returned within a 7-day period around 24-26 October which also marked the main departure period of the females. The females’ foraging trips were considerably shorter and ranged from 5-8 days. The females’ return occurred around the same dates in 2002 and 2003 (late October) but was markedly later in 2004 (early November). Nevertheless, the female’s return was always in sync with egg hatching. Foraging ranges and dive behaviour of six male and two female penguins was examined with GPS dive loggers and time depth recorders. Four of the six males foraged mainly in the cooler waters south of the subtropical front (STF), some 200 km east of the Snares. Dive behaviour of all males indicates primarily travelling behaviour during the first two days at sea. Two males remained in warmer Central Tasman Water (CTW). Movements of three birds determined from GPS suggest that the penguins targeted sea areas with elevated chlorophyll a concentration. Dive behaviour was also related to water mass with dive depths being on average deeper in the cooler waters of the STF. Both females remained only within warmer CTW; temperature data suggests that both birds foraged north to north-east of the island. Dive data indicates that females travelled continuously throughout their trips. The Snares penguin’s foraging behaviour is dictated by oceanic productivity which in turn depends on environmental factors such as day length. Thus, foraging and, consequently, nesting patterns of incubating Snares penguins are also to a great extent a product these factors.