Female eider ducks are well known to team up and share the work of rearing ducklings, but it now appears that they also negotiate not only how much effort each puts into the partnership, but also profit-sharing. An international group of scientists used a long-running study of the eider population in a Finnish archipelago to test predictions about how each hen seeks to maximize her benefits from the partnership without making it so unattractive that other hens withdraw their participation.
As hens arrive at the rearing-area with their ducklings, a period of intense socializing ensues. The hens then sort themselves into cliques – pairs, trios, or quartets – with each hen in a group assuming a distinct role.
“Waterfowl have a reputation as being none-too-bright, but we think they are careful, sophisticated bargainers,” says team leader Markus Öst (University of Helsinki). “The socializing during the period prior to group formation is devoted to the searching for and negotiating with a suitable partner.”
As a group, each hen’s ducklings are kept warm, led to food, and fiercely defended against predatory gulls – all tasks for which central positions in the brood are the best and safest. Though the ducklings appear identical to human observers, hens can clearly recognize them and carefully manage their ducklings’ locations in the joint brood, apparently according to an agreement worked out with the other hens.
Behavioral ecologists have long been interested in so-called ‘co-operative breeders,’ but ducks have never before been considered in this category. Appearing in the January issue of The American Naturalist, this study expands the range of animal groups considered co-operative breeders and also suggests that the behavioral strategies involved may be more complex than previously thought.