DAVID G. AINLEY, H.T. Harvey and Associates, Alviso, California 95002
Collaborating with biologists from LandCare Research New Zealand, Ltd. (LCRNZ), we conducted the third year of an investigation into the factors responsible for dramatic growth of Adélie penguin ( Pygoscelis adeliae ) colonies in the Ross Sea during the past two decades (Taylor and Wilson 1990; Blackburn, Taylor, and Wilson 1991). The LCRNZ biologists had collected certain key data during the past two seasons, anticipating our joining the effort in 1996-1997. With our arrival into the project, we increased the effort to work simultaneously at three penguin breeding colonies collecting the same suite of data at each. We hope to continue this project for at least 5 more years.
We are testing the hypothesis that population growth, which has been greater at the smallest colonies (Taylor and Wilson 1990), has resulted from a lessening of pack-ice cover. The smaller pack-ice cover has made food resources more available in a region characterized by concentrated pack ice well through the spring and into the summer; concentrated ice cover increases the effort that penguins must expend during foraging (because they have to walk rather than swim), which, in effect, is a reduction of food availability. In turn, the lessened effort required to find food has led to increased reproductive success and juvenile and adult survival (or so we hypothesize). We ask: Why has growth disproportionately favored the smaller colonies, given that ample space for expansion exists at all colonies? Using the Adélie penguin as a model, this question is of increasing relevance as managers more and more attempt to restore seabird colonies decimated by disasters such as oil spills. Which colonies should be restored: The small ones, which may owe their existence to over-flow of breeders (emigrants) from the larger ones, which have suffered the greatest impact, or the large colonies, where growth is slow and size presents logistical problems and, likely, little in the way of dramatic results to keep the purse managers interested?
In our research, we are comparing the colonies in the Ross/Beaufort Island cluster, in ascending order of size: Cape Royds (about 4,000 pairs), Beaufort Island (45,000), Cape Bird (45,000), and Cape Crozier (180,000). On Beaufort Island and at the three colonies on Ross Island, we are examining rates of colony growth determined from annual counts of breeding pairs in aerial photos. These photos have been taken by LCRNZ biologists beginning in 1981. Soon, we will inspect archived satellite imagery of pack-ice cover during the penguin breeding season to look for correspondence in colony growth and ice cover. In addition to the colony counts, we also
We also banded a sample of chicks and adults to estimate postbreeding survival and emigration when we find these birds in subsequent years. To complement the diet samples, we also collected tissues from chicks killed by south polar skuas ( Catharacta maccormicki ); these will be analyzed for the stable isotopes of nitrogen and carbon to determine the trophic level at which their parents were feeding (low = krill, high = fish). A diet composed of fish is much more energetically rich than one of krill.
The telemetry was conducted by taping small radios to the backs of 15 adults at the three Ross Island colonies in mid-December. Three times daily until mid-January, we then triangulated positions of radios by using directional antennas and receivers to search simultaneously the waters in sight from the colonies from vantage points 15-200 meters (m) above sea level. For 4 days, we tracked also from Hanson Ridge (600 m) on the west side of McMurdo Sound opposite Capes Royds and Bird, for 2 days from the top of Beaufort Island (650 m); and once from Mount Bird (900 m), we assessed overlap in foraging area of birds from Capes Bird and Crozier; and on a 2-day excursion of USCGC Polar Sea , we assessed overlap among all colonies. Owing to the high altitudes and relatively interference-free air waves, our receiving range was on the order of 35 kilometers (km) (as also experienced by Sadleir and Lay 1990); from sea level (ship), range was 5-10 km.
Parental foraging effort was assessed by implanting radio frequency identification (RFID) tags under the skin of 60-70 breeding birds in a breeding group at each colony, encircling the colony with a plastic fence and, thus, channeling the birds to come and go over a computerized scale/RFID tag reader. The weights of birds and tag readings were downloaded daily into a laptop computer for analysis.
Highest chick growth rates and greatest breeding success were experienced by penguins at the smallest colony. These birds also put least effort into procuring food (shortest feeding trips) and ate food of highest caloric content (highest percentage of fish in the diet). Data collected during breeding seasons previous to 1996-1997 indicated similar trends more often than not. The big question, then, is: If breeding conditions are so wonderful at the smallest colony, why does it remain small? We will attempt to answer this question with additional years of research.
Collaborators in the project were my coworkers, Grant Ballard, Sophie Webb, Stephani Zador, and Ian Gaffney; Peter Wilson, Kerry Barton, and Brian Karl, LCRNZ; Mike Beigel and Nathaniel Polish, Beigel Technology Corp.; Christine Ribic, U.S. Geological Survey-Biological Research, University of Wisconsin; Greg Rau, University of California-Santa Cruz; and Nadav Nur, Point Reyes Bird Observatory. We are grateful for the expert logistical support provided by personnel of the National Science Foundation, Antarctic Support Associates and their subcontractors, the New Zealand Antarctic Program, and the USCGC Polar Sea . Our portion of the project was funded by National Science Foundation grant OPP 95-26865.
Blackburn, N., R.H. Taylor, and P.R. Wilson. 1991. An interpretation of the growth of the Adélie penguin rookery at Cape Royds, 1955-1990. New Zealand Journal of Ecology , 15(1), 23-28.
Sadleir, M.F., and K.M. Lay. 1990. Foraging movements of Adélie penguins (Pygoscelis adeliae) in McMurdo Sound. In L.S. Davis and J.T. Darby (Eds.), Penguin biology . San Diego: Academic Press.
Taylor, R.H., and P.R. Wilson. 1990. Recent increase and southern expansion of Adélie penguin populations in the Ross Sea, Antarctica, related to climatic warming. New Zealand Journal of Ecology , 14(1), 25-29.