Aspects of the population dynamics of Antarctic blue-eyed shags phalacrocorax atriceps king

Abstract

Blue-eyed shag chicks have been ringed in almost every year since 1969 at two colonies on Signy Island. In this study, over 40% of breeders (801 pairs in 1987) were ringed and of known age, and during the breeding season adults were readily recaptured. Annual adult survival rates during the study averaged 0.86, although the mean annual survival rate between the early 1980's and 1987 was lower, averaging 0.76. There was no difference in the annual survival rate in relation to sex or breeding experience, but birds older than 11 years showed evidence of decreased survival rates, of about 0.04 annually. The first year survival rate averaged 0.36 but varied widely between 0.03 (in 1979) and 0.69 (in 1984). Low first year survival rates occurred about every four years. Only mean monthly temperature in April and the duration of sea ice were selected as explaining variation in first year survival rates and together they accounted for over two-thirds of the variation. Post-fledging survival rates were independent of hatching order, although only a few last-hatched chicks survived the nestling period. Although movements to and from neighbouring colonies could not be monitored thoroughly, no ringed birds were seen during searches of the two nearest colonies, and the rate of colony exchange between the two Signy colonies suggested that established breeders exhibited a high degree of faithfulness to their colony, only 0.1% moving each year. Recruits tended to return to their natal colony, and only 2.4% moved away between the two Signy colonies. Blue-eyed shags showed deferred breeding and whilst a few individuals began breeding at two years, most waited until their third and fourth year and some delayed breeding until six years old. The modal age of first breeding varied between years being earliest in 1987 (3 years) when recruitment was also high. Over the age range at which most birds recruited, there was no difference between the sexes in the age of first breeding. Prior to the year in which they first bred, young shags visited the colony to which they subsequently recruited and the number of visits per bird increased with age. Older pre-breeding birds also made visits earlier in the season. Low mate fidelity was normal, only 39% retained the same mate in the following breeding season. Of those which changed mate, divorce accounted for 46%, and the annual rate of divorce declined in relation to increasing age and breeding experience. A comparison of the divorce rates of similar aged birds with different breeding experience showed that recruits were more likely to divorce, suggesting that breeding experience was more important in divorce than age alone. Pairs which divorced in the current year had had lower mean clutches, broods and fledged fewer chicks in the previous year compared with those which kept the same mate. However, pair stability during the previous year had no effect on the breeding performance in the current year. Divorce was associated with further divorce in the following year. Both sexes showed a high tendency to re-nest within 12 m of the previous year's nest, although this was less marked in females. Re-use of the previous year's nest was low. A high proportion of pairs had partners of similar age, 46% being of equal age and 70% differing by a maximum of one year. Nearly twice as many equal aged pairs arose compared with the number expected if mating was random with respect to age, and the proportion of equal aged pairs declined among older birds. With respect to breeding experience and pair stability, experienced pairs and those which changed mates did not form more equal aged partnerships than expected by chance, and only for recruits did more equal aged pairs form than expected. Breeding experience was weakly related to breeding performance and the effect was stronger in 1987 than in 1986. Younger males built poorer quality nests than older birds, but it was not possible to resolve the effects of age and nest quality independently on breeding success.