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Davis, California 95616
As part of a 3-year comparative study of the foraging ecology and reproductive energetics of four species of antarctic fulmarine petrels, we have been investigating chick growth and development. The overall aim of the study is to develop a better understanding of the population-level energetics in Prydz Bay through an investigation of adult foraging strategies, parental reproductive effort, and nestling energy requirements and how environmental variation affects reproductive performance. This research is providing us with insights into predator-prey dynamics in the antarctic marine ecosystem and the role and impact of fulmarine petrels as top-level predators.
Nestling growth rate is thought to reflect variations in rate and quality of food delivered (Croxall 1984, pp. 533-616). This, in turn, is assumed to be correlated with food availability. Deviations in nestling growth rate from allometric predictions should correspond to interspecific differences in food availability, parental effort, and/or nestling energy demand.
The primary field site is Hop Island (68°53'S 77°50'E) in the Rauer Island group, approximately 40 kilometers south-southwest of Australia's Davis Station. We arrived on station on 22 October 1995 and began the field season on the island on 29 October. We departed the island on 31 March 1996. During the 1995-1996 season, we studied diet composition, chick energetics and growth, adult energetics, and breeding success in populations of snow petrel (Pagodroma nivea), cape petrel (Daption capense), antarctic petrel (Thalassoica antarctica), and antarctic fulmar (Fulmarus glacialoides). The extended nature of the field season allowed us to follow all four species through the entire breeding season, from pre-breeding attendance through to fledging.
Known-age chicks were measured every 3 days throughout the nestling period from the day of hatching. These measurements continued until the chicks fledged. Logistic growth curves were fitted to the growth data for each species. The four petrel species studied grew markedly more rapidly than was predicted for petrels and albatrosses by an allometric equation derived by Croxall and Gaston (1988) ( figure 1). Growth rate, in grams per day, was 169 percent of predicted for cape petrels, 125 percent of predicted for snow petrels, 165 percent of predicted for antarctic petrels, and 180 percent of predicted for antarctic fulmars. Mean adult masses are 469 grams (g) for cape petrels, 258 g for snow petrels, 687 g for antarctic petrels, and 859 g for antarctic fulmars. The nestling periods for cape petrels, snow petrels, antarctic petrels, and antarctic fulmars were 51 percent, 58 percent, 48 percent, and 48 percent, respectively, of the predicted time to fledge based on the predictive equation of Warham (1990).
We used open-circuit respirometry to determine resting metabolic rates of adults and chicks of all four species and to assess chick thermoregulatory capacity. The oxygen consumption of chicks was determined at six ages (3, 8, 15, 28, 35, and 42 days). Figure 2 presents data for 3-day-old and 8-day-old antarctic petrel chicks and illustrates the typical pattern with resting metabolic rate (RMR) increasing as chicks grew and with the thermoneutral zone shifting to lower temperatures. The lower critical temperature (Tlc) of chicks of cape petrel, southern fulmar, and antarctic petrel was initially near 20°C but fell to near 0°C by 28 days age. Snow petrel chicks were less tolerant of low temperatures with Tlcs about 10°C or higher at all ages. Our data permit us to characterize fully the resting metabolic rates of fulmarine petrel chicks throughout the nestling phase.
Growth and respirometry research was augmented by doubly labeled water (DLW) measurements of field metabolic rate (FMR) on nestlings of all four species. We measured FMR of chicks at seven intervals throughout the nestling period. Adult FMR was determined on incubating snow and cape petrels and on snow, cape, and antarctic petrel adults during the chick-rearing period.
Diet samples were collected using the water off-loading technique (Wilson 1984). All four species were sampled at two intervals during the chick period. Preliminary analysis of the samples indicates that diet was similar to the preceding two seasons and was composed of fish (almost exclusively Pleuragramma antarcticum) and krill (Euphausia superba and E. crystallorophias). Snow petrels fed predominantly on fish (approximately 83 percent fish by mass), cape petrels fed primarily on krill (70 percent krill by mass), and antarctic fulmars and antarctic petrels were intermediate in their prey preference (63 percent krill by mass and 40 percent krill by mass, respectively).
Diet was also quantified using stable isotope analysis [delta carbon-13 (13C) and delta nitrogen-15 (15N)] of fecal samples, blood samples, eggshell fragments, and feathers. This technique relies on differential incorporation of isotopes in organisms foraging at different trophic levels. We are investigating the utility of the technique for long-term, minimally invasive dietary monitoring programs.
Different patterns of growth between fulmarine and nonfulmarine procellariiforms reflect different ecological constraints on breeding. Time available for breeding appears to be the primary constraint for antarctic breeding species whereas food is often cited as the primary constraint in more temperate regions. Nestling growth rates are relatively rapid for antarctic fulmarine petrels, suggesting abundant resources and possibly the constraint of a short season in which environmental conditions appropriate for breeding are limited.
We are particularly grateful for the invaluable assistance that Jane Wilson and Catherine Bone provided Peter Hodum in the field. Furthermore, we would like to express our appreciation for the enthusiastic support and assistance provided by the expeditioners at Davis Station.
This work is supported by National Science Foundation grant OPP 92-18536.
Croxall, J.P. 1984. Seabirds. In R.M. Laws (Ed.), Antarctic ecology (Vol. 2). London: Academic Press.
Croxall, J.P., and A.J. Gaston. 1988. Patterns of reproduction in high-latitude Northern and Southern Hemisphere seabirds. Acta XIX Congress of International Ornithologists, 1, 1176-1184.
Warham, J. 1990. Petrels: Their ecology and breeding systems. London: Academic Press.
Wilson, R.P. 1984. An improved stomach pump for penguins and other seabirds. Journal of Field Ornithology, 55, 109-112.