Breaking bonds: maternal and offspring state shape constraint-based brood adoption in a seaduck

The adaptiveness of adoption for donors, recipients, and offspring remains unsettled despite long-standing interest. Using decade-long data on individually marked female and duckling common eiders (Somateria mollissima), displaying frequent alloparenting, we examined factors influencing the likelihood of females donating young and ducklings being adopted. We explored how donor traits, including maternal body condition, relative head size (a proxy of relative brain size), and relative hatching date, and offspring characteristics such as body condition relative to siblings, affect these processes. At least one offspring was permanently adopted in 34.7% of brood observations. Females in better body condition and larger relative head size were less likely to donate offspring, while the likelihood of offspring transfer was higher in larger natal broods. The odds of offspring donation peaked just before the population’s hatching peak, suggesting that recipient availability influences adoption. Ducklings in poorer body condition than their natal broodmates and those whose mothers were in lower body condition were, respectively, significantly and marginally significantly more likely to be adopted. Offspring transfers may thus result from physiological and cognitive constraints, rather than reflecting a fitness-maximizing strategy. Donating young and becoming adopted ultimately align with a salvage strategy for poor-condition donors and poor-condition offspring, but are likely driven by constraints rather than active tactics. Multiple tenders in broods prevented unique recipient identification, yet prior research suggests that recipients may accrue fitness benefits. Future research quantifying the fitness consequences for all parties in different environmental contexts is required for a more comprehensive understanding of alloparental behavior. Keywords: alternative reproductive tactics, body condition, cognitive constraints, common eider, conspecific brood parasitism, salvage strategy