THERESA L. BUCHER* and CAROL M. VLECK, Department of Zoology and Genetics, Iowa State University, Ames, Iowa 50011
*Present address: University of California-Los Angeles, Department of Biology, Tarzana, California 91356
The Adélie penguin breeds at rookeries distributed along the coast of Antarctica and on islands south of 60°S latitude. Numerous investigators have documented reproductive success and causes of breeding failure of banded birds at various latitudes, most often in colonies on Ross Island, Antarctica, all located south of 77°S latitude ( see Ainley, LeResche, and Sladen 1983) but also at King George Island (62°10'S 58°30'W) (Trivelpiece et al. 1990) and Signy Island in the South Orkney Islands (60°43'S 45°36'W) (Croxall et al. 1988). We studied the breeding biology of a colony of Adélie penguins on Torgersen Island, west of the Antarctic Peninsula (64°46'S 64°05'W). We documented the pattern of nest attendance and reliefs of banded birds and here compare the pattern we found with that described for colonies at higher and lower latitudes.
A fairly consistent pattern of nest attendance and reliefs has been described and strongly linked to breeding success or failure in the most southerly populations studied on Ross Island (Davis 1982, 1988; Davis and Miller 1990). After females lay their second eggs, they go to sea to forage. Males take the first and longest incubation bout; females take the second bout while the males forage at sea; males take the third bout; and the females return from sea at approximately the time of hatching of the first egg. Therefore, there are usually three attendance switches before hatching, and the total time of the first three incubation bouts approximately equals the incubation period (table). After hatching, the parents shorten the length of their alternate foraging bouts to feed the chicks regularly. Incubation from the laying of the second (last) egg until the first egg hatches usually is between 32 and 34 days.
In contrast to the Ross Island birds, Trivelpiece et al. (1990) reported a different pattern for birds nesting at Admiralty Bay on King George Island (table). Both male first bouts and female second bouts were shorter than the bouts reported for Ross Island, and the average number of attendance switches until hatching ranged from 3.4 to 6.7 over five different seasons. The year with the lowest average number of switches also had the lowest number of chicks hatched per pair and the lowest number of chicks fledged per pair.
At the most northerly latitude on Signy Island, South Orkney Islands, Croxall et al. (1988) reported first and second bout lengths of 13.7 and 12.8 days, respectively, for an "average" year between 1976 and 1987. This first bout length is similar to those reported at Admiralty Bay, but the second bout length is similar to those reported for the more southerly Ross Island colonies. The total number of days for the first two bouts suggests that there are probably more than three attendance switches during incubation until hatching.
At the midlatitude site on Torgersen Island, we found that the mean number of attendance switches varied significantly between years [ANOVA, F=94.2(3,533)]. It averaged from five to nine in different years and ranged from as few as one to as many as 19 among nests within different years (table). First incubation bouts of males and second bouts of females were shorter on Torgersen than at any of the other sites.
There is not a single pattern of coordination of incubation bouts and nest reliefs that leads to successful hatching either within colonies or across colonies in widely separated locations. However, there are consistent differences between locations. For example, Ross Island colonies consistently have longer bouts and fewer switches than do Torgersen Island colonies.
Davis and Miller (1990) suggest that food per se does not determine the length of foraging trips, at least during the incubation period. Ice conditions, including heavy ice and late breakup and lack of easy access to open water, affect the length of the first foraging trip and this change in turn affects the length of the second and third foraging trips because of the change in the length of time available until hatching (Davis 1988). Desertions are more prevalent (Ainley and LeResche 1973), and reproductive success is lowest in the years when ice persists the longest (Croxall et al. 1988).
Although social behavior within a species has traditionally been viewed as rather static, recently this view has changed. Social interactions, as well as life history, mating systems, and morphology, vary intraspecifically and correlate with environmental heterogeneity. Such a situation has been reported by Waas (1990) in cave- and burrow-dwelling little blue penguins ( Eudyptula minor ). Incubation behavior may vary with environmental heterogeneity as well. In Adélie penguins, the length of foraging trips during incubation may be related to the distance to open water and/or to the extent and type of ice near the colony, both of which will vary temporally and spatially and may affect the presence and accessibility of food.
We thank our field assistants, Asrun Kristmundsdottir, Wendy Reed, David Vleck, Lori Ross, and David Lott for their able assistance with all aspects of this research, and the support staff at Palmer Station. This research was supported by National Science Foundation grant OPP 93-17356 to C.M. Vleck.
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Ainley, D.G., R.E. LeResche, and W.J. Sladen. 1983. Breeding biology of the Adélie penguin. Berkeley: University of California Press.
Croxall, J.P., T.S. McCann, P.A. Prince, and P. Rothery. 1988. Reproductive performance of seabirds and seals at South Georgia and Signy Island, South Orkney Islands, 1976-1987: Implications for southern ocean monitoring studies. In D. Sahrhage (Ed.), Antarctic ocean and resources variability . Berlin: Springer-Verlag.
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