Repeatability of daily energy expenditure in kittiwakes Rissa tridactyla

Abstract

Estimates of repeatability (R) are an important statistic tool in assessing the consistency of individual differences and thus different phenotypes. A high repeatability of estimates of the daily energy expenditure (DEE), allow for better inferences of the long term energetic effort of individuals, as well as for sound correlations of other traits related to individual performance, such as reproductive success. Previously, measurements of DEE have been restricted to 24h, but the ecological relevance of such a time frame has been questioned. Estimates of DEE, using the doubly labelled water (DLW) method, were repeated on individuals of kittiwakes breeding on Svalbard, during four study years. The aim of the study was to investigate the time scale in which kittiwakes balance their energy expenditure. Repeatability of DEE was estimated using two different measurement intervals, measuring over a period of either 24h or 72h, within years as well as between years. In addition, it was investigated whether the less invasive single-sample (SS) DLW protocol could improve repeatability over the more common two-sample (TS) protocol, by allowing for kittiwakes to exhibit a near-natural behaviour. We found that individual DEE was repeatable both within (72h: R = 0.772) and between years (72h: R = 0.430) using the TS protocol and the long measurement interval. The short interval estimates were not significantly different from zero, although applying the SS protocol appeared to have an increasing effect on R. These findings demonstrate, that prolonging the length of the DEE measurement interval, caused a high increase in R, suggesting that kittiwakes budget their energy expenditure over a longer time period than 24 hours. The moderate to high R found between years suggest that kittiwakes are consistently different in their energy expenditure over longer periods of time. However, when adjusting for body mass (BM) only within-season repeatability was significant (72h: R = 0.652), which was mainly due to a high inter-individual variation in BM.