ABSTRACT Differential hatching timing among siblings may result in hierarchical resource distribution, leading to disadvantages for later‐hatched nestlings. Later‐hatched chicks may need to invest more in developing their skeletal structures that are related to locomotion to catch up with first‐hatched chicks and better survive after leaving the nest, resulting in greater fitness for the breeding pair. We analyzed chick growth patterns during the guarding period according to hatching order for Gentoo ( Pygoscelis papua ) and Chinstrap ( P. antarcticus ) Penguins at Narebski Point, King George Island, in Antarctica. We fitted the four‐parameter Unified‐Richards growth curve to chick measurement data. For Gentoo Penguins, growth curves showed that first chicks reached a maximum growth rate earlier than did second chicks in all body traits, leading to the growth rate of the second chicks overtaking that of first chicks from mid or late guarding period. The maximum growth rate for second chicks of Chinstrap Penguins was achieved later than that for the first chicks, only in body mass. Our results indicate that developmental strategies could differ between sympatric species with a close phylogenetic relationship, and that second chicks prioritize energy allocation to reduce asymmetry of skeletal traits during the early life stages. For field applications, our growth models provided reliable age estimates regardless of hatching order. Flipper length was the most accurate estimator for Gentoo Penguin chicks, while body mass and total head length were more reliable for Chinstrap Penguins.
Jeong et al. (Fri,) studied this question.