| Summary: | Male and female birds have different interests in reproductive investment, which in turn may increase negative effects of poorer breeding conditions caused by e.g., climate change or ecosystem regime shifts. Using a 33-year time series with resightings of Atlantic puffins Fratercula arctica individually colour-ringed as breeders in previous years, we show that the difference in colony attendance of male and female birds depends on the environmental conditions for raising young, proxied by the average duration of the chick period and size of the herring Clupea harengus fed to the chicks in the colony each year. The longer the chick period, and thus the birds' overall investment in reproduction, the more was the sex ratio of adults sitting out on the colony surface biased in favour of males. An increase in herring size, indicating better feeding conditions for raising chicks, led to more observations of both sexes, and the increase was slightly more prominent for females than males. We discuss the results in relation to general life-history theory on sexual differences in trade-offs between individual investment in breeding and own survival. Our results suggest that females are increasingly more willing than males to invest in provisioning for the chick the longer the chick needs such care. This difference may also prove valuable as an indication of breeding conditions from only a short visit to a colony with colour-ringed birds of known sex. Funding provided by: The Research Council of Norway ROR ID: https://ror.org/00epmv149 Award Number: 192141 The attendance data were collected by capture-mark-resighting (CMR) of adult breeding Atlantic puffins in a colony in North Norway. The birds were sexed by DNA (blood sample), and time since ringing was used as a proxy for their age. Mean dates of chick hatching and chick death or nest departure were estimated from monitoring the content of on average 103 (range 34-284) individually marked active nest-burrows in the same colony throughout the chick period. The length ...
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