Abstract
ABSTRACT: Varying the timing of reproduction in response to local environmental conditions is akey factor influencing reproductive success. We used 19 yr (1991 to 2009) of data on breedingchronology and reproductive success of Adelie penguins Pygoscelis adeliae and gentoo penguins P. papua at King George Island, Antarctica to compare clutch initiation dates (CIDs), hatch rates,and creche rates at population and individual levels. The CIDs of both species occurred earlierwith warmer October temperatures, but gentoo penguins adjusted CIDs twice as much per °C oftemperature as Adelie penguins. Gentoo penguins also exhibited lower variances in hatch andcreche rates than Adelie penguins, suggesting that greater ability to respond to ambient condi-tions can buffer reproductive success. Mixed-effects models revealed that individual- and year-specific random effects on the overall population mean best accounted for observed variation inCIDs, with some individuals routinely breeding earlier than others. However, individual differ-ences in the relative timing of breeding provided no advantage for long-term reproductive suc-cess. The results suggest that, among gentoo and Adelie penguins, plasticity in CIDs with respectto environmental conditions is primarily a population-level rather than an individual-levelresponse. Energetic constraints of contrasting migratory and fasting behaviors likely contribute tothe different abilities to respond to warming spring temperatures. Overall, larger responses ofgentoo penguin CIDs to October temperature and lower variability in reproductive success,despite reduced reproductive success with delayed breeding, suggests that gentoo penguins arebetter equipped than Adelie penguins to adjust to rapid warming in the Antarctic Peninsulaecosystem.KEY WORDS: Phenology · Adelie · Gentoo · Climate change · Migrant
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