ORIGINAL PAPER Is natural hatching asynchrony optimal? An experimental investigation of sibling competition patterns in a facultatively siblicidal seabird Thomas Merkling & Lena Agdere & Elise Albert & Romain Durieux & Scott A. Hatch & Etienne Danchin & Pierrick Blanchard Received: 19 April 2013 /Revised: 17 October 2013 /Accepted: 18 October 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract In unpredictable environments, any tactic that en- ables avian parents to adjust brood size and, thus, energy expenditure to environmental conditions should be favoured. Hatching asynchrony (HA), which occurs whenever incuba- tion commences before clutch completion, may comprise such a tactic. For instance, the sibling rivalry hypothesis states that the hierarchy among chicks, concomitant to HA, should both facilitate the adjustment of brood size to environmental con- ditions and reduce several components of sibling competition as compared to synchronous hatching, at both brood and individual levels. We thus predicted that brood aggression, begging and feeding rates should decrease and that older chick superiority should increase with HA increasing, leading to higher growth and survival rates. Accordingly, we investigat- ed the effects of an experimental upward and downward manipulation of HA magnitude on behaviour, growth and survival of black-legged kittiwake (Rissa tridactyla ) chicks. In line with the sibling rivalry hypothesis, synchronous hatch- ing increased aggression and tended to increase feeding rates by parents at the brood level. Begging rates, however, in- creased with HA contrary to our expectations. At the individ- ual level, as HA magnitude increased, the younger chick was attacked and begged proportionally more often, experienced a slower growth and a higher mortality than its sibling. Overall, the occurrence of energetic costs triggered by synchronous hatching both for parents and chicks, together with the lower growth rate and increased mortality of the younger chick in highly asynchronous broods suggest that natural HA magni- tude may be optimal. Keywords Aggression . Begging . Hatching asynchrony . Kittiwake . Sibling competition . Sibling rivalry hypothesis Introduction In unpredictable environments, female birds often lay more eggs than the number of chicks the pair would be able to fledge under average environmental conditions (Lack 1954; Ricklefs 1965). This strategy allows pairs to produce more chicks than average when environmental conditions turn out to be good. These occasional fitness gains are supposed to exceed costs of caring for extra chicks doomed to die under average or poor environmental conditions (the facultative brood reduction hypothesis, Lack 1947, 1954). Hence, any tactic favouring the facultative elimination of the weakest chick(s) at low costs according to environmental conditions should be advantageous for parents. Hatching asynchrony (HA) is common among birds. It occurs whenever incubation commences before the clutch completion and may have evolved, for instance, in response Communicated by I. R. Hartley T. Merkling (*) : L. Agdere : E. Albert : R. Durieux : E. Danchin : P. Blanchard UMR5174 EDB (Laboratoire Évolution & Diversité Biologique), CNRS, ENFA, Université Toulouse 3 Paul Sabatier, 118 Route de Narbonne,, 31062 Toulouse, France e-mail: thomasmerkling00@gmail.com T. Merkling : L. Agdere : E. Albert : R. Durieux : E. Danchin : P. Blanchard CNRS, UMR5174 EDB, Université Paul Sabatier, 31062 Toulouse, France S. A. Hatch U.S. Geological Survey, Alaska Science Center, 4210 University Drive, Anchorage, AK 99508, USA Present Address: S. A. Hatch Institute for Seabird Research and Conservation, 12850 Mountain Place, Anchorage, AK 99516, USA Behav Ecol Sociobiol DOI 10.1007/s00265-013-1646-y