Constraints on foraging and their consequences for emperor penguins

Abstract

Emperor penguins are the largest living penguin and the only bird species that breeds during the Antarctic winter, on sea-ice around the Antarctic continent. They are fundamentally adapted to the high Antarctic sea-ice environment, showing radical breeding and foraging behaviour. High latitude-breeding exposes emperor penguins to extreme changes in sea-ice cover and light conditions over the course of the year, both factors that are likely to affect foraging success. This thesis examines some of the ecological and physiological constraints on emperor penguin foraging behaviour using birds from Pointe GÃ ?Ã ©ologie, Antarctica and considers how these birds modulate behaviour to cope with their extraordinary environment. The trophic impact of emperor penguins on squid communities at Pointe Geologie was examined by analysing squid beaks obtained from the stomach of moribund chicks. Emperor penguins consumed primarily four squid species (Psychroteuthis glacialis, Kondakovia longimana, Gonatus antarcticus, Alluroteuthis antarcticus). Beak nitrogen stable isotope ratios indicated that squids consumed by the emperor penguin cover about two trophic levels. The trophic position of emperor penguins from different breeding colonies considerably differs because the principal components in the emperor penguin's diet; fish, krill and squid, differ in their average trophic level and also in their composition. The foraging areas and depths used by 20 breeding emperor penguins at Pointe GÃ ?Ã ©ologie during winter, spring and summer in 2005/2006 (corresponding to incubation, early chick-brooding, late chick-rearing and the adult pre-moult period, respectively) were investigated using satellite telemetry and pressure transducers. Chick-provisioning penguins foraged over the Antarctic shelf in areas covered by winter pack-ice, using fissures in the ice although winter-foraging females compensated for limited water access by expanding their horizontal search component underwater. During the spring ice break-up, foraging ranges rarely exceeded the shelf slope and direct access to the sea was apparently almost unlimited. Spring-foraging birds showed higher area-restricted-search than birds in winter indicating more patchy prey abundance. During pre-moult trips, emperor penguins ranged much farther offshore than breeding birds, which augurs for particularly profitable oceanic feeding areas. These areas are presumably exploited because the time constraints imposed by having to return to a central place to provision the chick no longer apply.Dive depth analyses were performed to determine how depth affects foraging efficiency. Penguins increased overall dive duration with increasing dive depth due to increasing transit (descent and ascent) durations as well as increasing duration of the bottom phase of the dive. Post-dive duration during which birds recovered at the surface was also positively correlated with both dive depth and dive duration, with accelerating recovery periods for dives in excess of 455 s. Consideration of efficiency, as the bottom duration divided by the dive cycle duration, showed that emperor penguins are less efficient during deeper dives, as predicted by physiological theory. Deeper dives are tenable if prey densities are appropriate, however, Application of an index of prey capture using the number of undulations in the bottom phase of the dive indicated that the most lucrative dive depths for the equipped birds were between 50 and 225 m, which were also the most frequently visited.Since emperor penguins are considered to be visual hunters their foraging ability is believed to depend critically on light conditions. The influence of light on the foraging ability of breeding emperor penguins was examined during winter and spring, using light-recording depth loggers and combining results with models of light availability as a function of depth and time of year. Logger-measured light intensity decreased with increasing water depth while depth-specific values were higher at greater sun elevation angles. The virtually continuous daylight in spring/summer appears to provide penguins with more than double the depth-time availability of birds foraging in winter. A simple energetic model shows how reduced light levels led to increased foraging trip lengths for breeding birds in winter compared to spring-foraging birds.This work highlights the dependence of the emperor penguin on particular conditions to survive but also alludes to sensitivity to environmental change. Further work is suggested to refine proposed models so that the effects of climate change may be postulated for this enigmatic and unique species.