Determinants of Survival for Common Tern Chicks Sterna hirundo: The Importance of Hatch Order, Hatch Date and Other Factors across Three Decades

In birds, chick survival to fledging is a commonly used index of reproductive success and challenges during the prefledging period can predict performance at subsequent life stages. Prefledging survival of Common Tern chicks was studied at a North American breeding colony in 1989–2022 (4647 chicks in 32 years) to elucidate the relative importance of factors influencing chick survival and interactions between them. Study plots were fenced so that surviving chicks could be followed to fledging age. Mean productivity during the study was lower than at other coastal North American colonies and the main cause of chick mortality was starvation. Chick survival was related most strongly to hatch date and hatching order. Survival was highest among the earliest-hatched chicks and showed a curvilinear decline with increasing hatch date. Within a brood, first-hatched chicks were more than twice as likely to survive as second-hatched chicks and over 11 times more likely to survive than third-hatched chicks. To a lesser extent, chick survival depended on brood size, being higher in larger broods during ‘good’ years but lower in larger broods in ‘poor’ years. These results indicate that studies of chick survival or productivity need to consider hatch date (which may be an index of parental quality). Survival of second-hatched (B-)chicks showed the most variability, so that this parameter may serve as a single index of reproductive performance. Annual colony-wide productivity was correlated with mean clutch size earlier in the same year.