Condition and coalition formation by brood-rearing common eider females

Partner choice is important in nature, and partnerships or coalitions within which reproduction is shared are the subject of growing interest. However, little attention has been given to questions of which individuals are suitable partners and why. Common eider (Somateria mollissima) females sometimes pool their broods and share brood-rearing duties, and body condition affects care decisions. We constructed a model in which females, based on their body condition and the structure of the joint brood, assess the fitness consequences of joining a coalition versus tending for young alone. We tested the model’s predictions by comparing data on the condition of females in enduring and transient coalitions. Our model showed that the range of acceptable brood arrays in a female coalition decreases with increasing condition of the female, so females tending alone should be in better condition than multifemale tenders. This prediction is in agreement with previous data. The model also predicts that females in good condition should join coalitions with females in poor condition and not with other females in good condition. This prediction was also supported by data: in enduring two-female coalitions, the positive correlation between the better female’s condition and the difference in condition between the two females was stronger than would be expected by random grouping of females. In contrast, in transient coalitions of females, this correlation did not differ from the correlation expected under random grouping. Model assumptions seem to fit with eider natural history, and the model may prove to be a useful way to study brood amalgamation behavior of waterfowl in general.