Offspring sex-ratio and environmental conditions in a seabird with sex-specific rearing costs: a long-term experimental approach

Abstract

Sex allocation studies among birds and mammals are notoriously inconsistent with theoretical predictions. One explanation is the difficulty of collecting data on costs and benefits of sex-ratio adjustments, which prevents the investigation of underlying assumptions. Some predictions may thus have been tested in species where they should not have been expected. Here, we focus on the “cost of reproduction hypothesis”, which states that parents with low investment capacity should avoid producing the most expensive sex to minimise the decrease in their residual reproductive value. In the black-legged kittiwake (Rissa tridactyla), sons are energetically more expensive than daughters. Using 10\xa0years of data (1172 chicks from 790 broods) from a long-term feeding experiment, we predicted a stronger decrease in the probability of producing a son with deteriorating environmental conditions among Control than among supplementally Fed parents. To test this prediction, we used three proxies of environmental conditions and a recent sliding window approach. We found no support for our prediction. Hence, we investigated between-year sex-ratio variation in relation to feeding status to detect a response to an unmeasured environmental variable. There was no interaction between year and feeding status, nor any effect of feeding status itself. However, the probability of producing a male increased with time, which could be a response to an oceanic regime shift that occurred around our colony, but that our proxies failed to capture. Our study further highlights the difficulty of explaining sex-ratio variation in long-lived species with complex life-histories where multiple selective pressures can occur simultaneously.