Aspects of the population dynamics of Antarctic blue-eyed shags phalacrocorax atriceps king
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 av...
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| Format: | Thesis |
| Language: | unknown |
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1992
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| Online Access: | http://etheses.dur.ac.uk/6267/ http://etheses.dur.ac.uk/6267/1/6267_3622.PDF |
| Summary: | 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 ... |
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