Sex-specific growth of nestlings of the Whiskered Tern Chlidonias hybrida, a species with sexual size dimorphism and female brood desertion

The growth of a nestling during the first few weeks after hatching is crucial for its further life and is sensitive to the conditions experienced during this period. Among species exhibiting Sexual Size Dimorphism (SSD), one of the most important factors influencing growth parameters is the sex of the hatchlings. In this study, we tested whether sex and other factors (hatching date, egg volume, brood size and sex ratio in the brood) were related to sex on growth parameters in Whiskered Tern (Chlidonias hybrida) chicks. In this species, males are 3%–10% larger than females when adult. At hatching, only the total head length was greater in males than females, but values close to fledging were greater for most of the measured parameters (3%—tarsus, 4%—total head length, 13%—body mass). Moreover, the maximum body mass growth rate was higher in males but females entered the rapid growth phase somewhat earlier and achieved the final total head length sooner. Nestling growth was also significantly correlated with brood size and hatching date, though mostly negatively (e.g. lower tarsus and total head length asymptote, lower maximum growth rate and growth rate constant of total head length in bigger and later clutches). The occurrence of SSD during the nestling period and higher maximum growth rate of body mass in males indicates that the costs of raising sons may be higher than of raising daughters. However, we did not find evidence of either sex-biased nestling mortality or skewed sex ratio.