Earlier hatching date rather than chick phenotypic quality enhances first-year survival in the black-headed gull.

Abstract

BACKGROUND Favourable conditions experienced during early ontogeny are expected to positively influence individual performance at later stages of the life cycle and determine lifetime fitness. In this study, we investigated whether early-life (chick) phenotypic quality (body condition and immunocompetence) and hatching date are associated with post-fledging survival in the black-headed gull Chroicocephalus ridibundus. To this end, we quantified size-corrected body mass, total blood haemoglobin concentration, and PHA-induced immune response in more than 500 colour-marked gull chicks in Poland. We then analysed their first-year survival rates using capture-recapture models based on extensive resighting data collected across Europe. RESULTS Neither better body condition (body mass and haemoglobin concentration) nor higher immunocompetence during rearing significantly improved post-fledging (first year of life) survival rate. In contrast, post-fledging survival was primarily determined by hatching date, as early-hatched chicks showed enhanced survival rates than those hatched late in the season. The positive effect of early hatching on post-fledging survival was apparent despite lower phenotypic quality of those early-hatched chicks. CONCLUSIONS Our study identifies hatching date as the key determinant of post-fledging survival in the black-headed gull. We suggest that the cumulative negative effects of delayed (suboptimal) hatching and fledging in black-headed gulls may outweigh any benefits of elevated condition and immunocompetence at the natal stage, ultimately compromising post-fledging survival. We propose that our findings may be explained by varying levels of temporal (mis)synchronization between peak food availability and key stages of the annual cycle, particularly the early post-fledging period. The study adds to the understanding of the carry-over effects between the successive stages of the life cycle in wild colonial birds.