Breaking bonds: maternal and offspring states relate to constraint-based brood donation in a seaduck

The adaptiveness of alloparenting for donors, recipients and both natal and transferred offspring remains unsettled despite long-standing interest. Using decade-long data on individually marked female and duckling common eiders (<i>Somateria mollissima</i>), which frequently transfer offspring between broods, 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 validated proxy of relative brain size, potentially associated with risk assessment and reproductive decision-making) and relative hatching date, and offspring characteristics such as body condition relative to siblings, relate to 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 offspring transfer was more likely from larger natal broods. Offspring donation was most likely 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. Taken together, duckling transfer is associated with physical and cognition-related characteristics of donors and adoptees, without necessarily implying an adaptive strategy for either. Multiple tending females per brood prevented assignment of adopted ducklings to a unique recipient; nonetheless, previous studies suggest recipient females 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.