Reproductive success of south polar skuas at Cape Bird, Ross ...

Furness, R. W. and P. Monaghan. 1987. Seabird ecology. New York: Chapman and Hall. Hunt, G. L., Jr. 1972. Influence of food distribution and human disturbance on the reproductive success of herring gulls. Ecology, 53: 1,051-1,061. Johnstone, B. R. 1971. Skua numbers and conservation at Cape Hallett, Antarctica. Nature, 231:468. Jouventin, P. and M. Guillotin. 1979. Socio-ecologie du skua antarctique a Pointe Geologie. Terra Vie, Revieu Ecologie, 33:109-127. Murphy, E. C., R. H. Day, K. L. Oakley, and A. A. Hoover. 1984. Dietary changes and poor reproductive performance in glaucus-winged gulls. Auk, 101:534-541.

Reproductive success of south polar skuas at Cape Bird, Ross Island GARY D. MILLER

Biology Department University of New Mexico Albuquerque, New Mexico 87131

The south polar skua (Catharacta maccormicki) is an integral part of the antarctic avian community, and as such, some characteristics of their breeding biology may be indicators of general environmental conditions. The recent controversy surrounding the breeding failure of the south polar skua near Palmer Station (subsequent to the Bahia Paraiso incident in January 1989) illustrates its importance (Penhale 1989). The complete reproductive failure of the south polar skuas near Palmer Station that season was blamed on the resulting fuel spill (Eppley and Rubega 1989, 1990a, 1990b; Baringa 1990). Because these skuas exhibit large annual variation in reproductive success (RS), including occasional complete failure, Trivelpiece et al. (1990) suggested that the failure that year was due to factors not directly related to the fuel spill, for example, storms (Ensor 1979). Eppley and

Pierotti, R. 1982. Habitat selection and its effect on reproductive output in the herring gull in Newfoundland. Ecology, 63:854-68. Pierotti, R. and C. A. Annett. 1990. Diet and reproduction output in seabirds. Bio Science, 40:568-574. Risebrough, R. W. and G. M. Carmignani. 1972. Chlorinated hydrocarbons in antarctic birds. In B. C. Parker (Ed.), Conservation Problems in Antarctica. Lawrence Kansas: Allen Press, 63-80. Ryan, P. C. 1988. Intraspecific variation in plastic ingestion by seabirds and the flux of plastic through seabird populations. Condor, 90:446452. Spellerberg,I. F. 1971. Arrival and departure of birds at McMurdo Sound, Antarctica. Emu, 71:167-171.

Rubega (1990a) suggested, however, that the adults were away from their nests more than usual to clean themselves, leaving their chicks vulnerable to predation. This is a reasonable hypothesis, but insufficient data exist on the annual variation in nest attendance and on how nest attendance relates to predation or RS to evaluate it. It is important that we be able to determine whether for any given season food shortages, weather conditions, or unusual occurrences determine RS in avian communities. By understanding the mechanisms behind high annual variation in skua RS, we will better understand what factors control seabird communities in Antarctica. Since the 1986-1987 season, all skua pairs at Cape Bird have been monitored annually and most adults and every nestling in the northern skuary at Cape Bird have been banded each year since 1988-1989. With nearly all the birds in the population marked, more detailed studies of individuals are possible. This population and the monitoring of its RS is an important addition to skua population studies conducted elsewhere on Ross Island (Ainley et al. 1990) and also an important comparison for the populations being studied on the Antarctic Peninsula and at King George Island (Rubega and Eppley 1990a; Trivelpiece et al. 1990). In each year of the study, the field team arrived at Cape Bird during pair formation before any skua egg was laid. The entire area was searched for territorial pairs, and they were monitored to record when they laid eggs. All the nests were monitored daily

Table 1. Reproductive success In south polar skuas at Cape Bird # Eggs Mean Fldg Breed Total laid # Eggs # Chicks cltch success success RS Year Nests' (% hatch) hatch fidge size fldg/chk fldg/egg fldg/pr 19772 79 1986 112 1987 163 1988 173 1989 3 145 1990 168 1991 133

119(24) 28 172(40) 69 319(49) 157 343(53) 182 265(17) 44 320(40) 128 281(43) 121

24 1.51 48 1.54 54 1.96 79 1.98 12 1.83 30 1.90 70 2.59

Mean 139

260 84

45 1.90 25 0.36

SD

2

34

104 58

0.86 0.20 0.30 0.70 0.28 0.43 0.34 0.17 0.33 0.43 0.23 0.46 0.27 0.05 0.08 0.23 0.09 0.18 0.58 0.25 0.53 0.49 0.18 0.33 0.23 0.08 0.16

Total number of nests in 1989-1991 is corrected for experiments. Data from Ensor (1979). Ensor left site on 20 January. Stated AS is probably higher than actual AS. Data from G. S. Court (personal comment). Clutch size is greater than two because some pairs renested.

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Table 2. Reproductive success of control nests (those visited twice during the entire season) and disturbed nests (those checked daily) Breeding Total #Chicks success RS Year Treatment Nests #Eggs fledged fledge/egg fledge/pr 1988 Control 70 128 57 0.45 0.81 Disturbance 118 180 79 0.44 0.67 1990 Control 70 132 26 0.20 0.37 Disturbance 89 152 38 0.25 0.43 1991 Control 66 113 29 0.26 0.44 Disturbance 81 132 51 0.39 0.63 Note: The number of disturbed nests is scaled to the amount in existence on the date of the first check of control nests (19 to 21 December each year). Table 3. Reproductive success of experiementaily fed skua pairs and control pairs. Control sample contains only nests with 2 eggs because that was a requirement of the feeding experiment Breeding Total #Chicks success RS Year Treatment #Nests #Eggs fledged fledge/egg fledge/pr 1990 Fed 30 60 7 0.12 0.23 Control 110 218 36 0.17 0.33 1991 Fed 32 64 28 0.44 0.88 Control 106 212 61 0.29 0.58

until the field team departed in late January or early February of each year. All chicks surviving at the end of the season were considered to have fledged unless their carcasses were discovered in the following season. In each of the 1988, 1990, and 1991 seasons a separate group of nests were visited only twice during the season to compare the RS of nests visited daily with those not disturbed. These control nests were to the south of the primary study area. Finally, as part of the study of the role of food in sibling aggression, 30 and 32 nests were fed in 1990 and 1991, respectively, beginning with the laying of the second egg. The pairs were fed approximately 200 grams of thawed fish each day during the nest visits. All of the pairs readily accepted the supplement. As in other populations, the natural variation in RS of skuas is high (table 1). In seven seasons of monitoring at Cape Bird, skuas had their lowest RS in 1989 and their best year in 1991 (table 1). In contrast to the skuas near Palmer Station no years of complete reproductive failure have been recorded at Cape Bird. The relatively low RS in 1977 was due to storms during the breeding season (Ensor 1979), but none of the other years of low RS can be attributed to storms. Instead, the initial conditions of the rookery (i.e., conditions remaining from the winter) played a major role in the 1989 and 1990 seasons. In both of these seasons, snow covered many skua territories completely, and many eggs were lost early in the season because they were laid directly onto the snow. Also note that hatching success of the eggs is generally low; it exceeded 50 percent in only one of seven seasons (1988) (table 1). The disturbance/ control nests were monitored for three seasons (table 2). Visiting to nests by observers disturbs the adult

1992 REVIEW

skuas at the nest. They fly up and usually attack the observer checking the nest contents or weighing the chick(s). In the course of each field season, one to four eggs were lost because other skuas took advantage of the disturbance to steal an egg. Only one chick is known to have been lost in 6 years of nest checks. Unexpectedly, however, the control nests did better than disturbed nests only in 1988 (table 2). In both the 1990 and 1991 seasons, the disturbed nests fledged more chicks for each pair than did control nests. The feeding experiment was based on the prediction that those nests receiving supplemental food should be more successful than controls. The 200-gram supplement was a substantial increase-about 1.5 times-in the food normally available to the pairs that were fed. Fed nests fledged fewer chicks than controls in 1990, but more chicks in 1991. These contradictory results cannot be easily explained. With the variation in RS over the different seasons and with the results of both the disturbance/ control and the feeding experiments, skua RS is apparently determined by a combination of factors: In some years, storms may be important, in others it may be food supply during chick rearing. In some years the proximity of Adélie penguins (Pygoscelis adeliae) may be a major mortality factor on eggs. Many of these factors are interconnected in ways that are not necessarily obvious. For example, in 1989 there was extensive fast ice during the early season. As a consequence, many Adélie penguins did not breed. Later during the reoccupation phase, an unusually large number of unoccupied (nonbreeding) penguins were in the rookery. That year, aggressive penguins attacking adult skuas, broke many skua eggs in their nests, causing the lowest hatching success of any of the seven seasons monitored. Evidently, detailed studies of the timing and causes of chick and egg mortality must be continued to understand the patterns of RS in skuas. This research was supported by National Science Foundation grants INT 86-03504 (1986 and 1987 seasons) and DPP 89-16353 (1990 and 1991 seasons). I also acknowledge the support of the New Zealand Ross Dependency Research Committee, the University Grants Committee, the Golden Kiwi Lottery Board (1988 season), and the information furnished by Gordon Court (1989 season). lalso thank Lloyd Davis for starting me at Cape Bird, and G. Court, E. Speirs, D. Hutchinson, P. Carey, A. McKechnie, K. Nordin, B. Keimel, P. Martin, and G. Wallace for their assistance and companionship in the field.

References Ainley, D. G., C. A. Ribric, and R. C. Wood. 1990. A demographic study of the south polar skua Cat haracta maccormicki at Cape Crozier. Journal of Animal Ecology, 59:1-19. Baringa, M. 1990. Eco-quandary: What killed the skuas? Science, 249:24. Ensor, P. H. 1979. The effect of storms on the breeding success of south polar skuas at Cape Bird, Antarctica. Notornis, 26:349-53. Eppley, Z. A. and M. A. Rubega. 1989. Indirect effects of an oil spill. Nature. 340:513. Eppley, Z. A. and M. A. Rubega. 1990a. Reproductive failure in a population of south polar skuas following the 'Bahia Paraiso' oil spill in Antarctica. Marine Ecology Progress Series, 67:1-6. Eppley, Z. A. and M. A. Rubega. 1990b. Reply to Trivelpiece et al. Nature., 345:211-212. Penhale, P. A. 1989. Research team focuses on environmental impact of oil spill. Antarctic Journal of the U.S., 24(5):9-12. Trivelpiece, W. Z., D. C. Ainley, W. R. Fraser, and S. G. Trivelpiece. 1990. Skua survival, letter. Nature, 345:211.

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