Shared care provides time-budgeting advantages for female eiders

Shared parental care resulting from brood amalgamation is often considered beneficial to parents. However, the benefit of grouping (a decline in individual vigilance despite an increase in collective vigilance, with a concomitant increase in individual feeding time) has not been evoked as a factor promoting shared care. Eider females, Somateria mollissima, are subject to substantial energetic costs during breeding, and sometimes share brood-rearing duties. We compared the activity budgets and feeding behaviour of lone tenders and multifemale tenders. We also measured the collective vigilance of multifemale tenders and examined whether the coordination of feeding activity among females changes with time. As expected, the proportion of time spent feeding increased, and the proportion of time spent vigilant decreased, as the number of females attending the brood increased. None the less, the collective vigilance of multifemale tenders was at least 20% higher than the vigilance of lone tenders. Furthermore, multifemale tending allowed females to feed more optimally, as dive duration increased with the number of females tending the brood. The level of parental investment by individual females after hatching of the eggs is therefore related to the number of tending females and, based on previous work, a female's body condition when the eggs hatch is also logically connected with the number of tending females. Whether females sharing brood care on a permanent basis coordinate their feeding activity, so as to increase the potential protection of ducklings against predation, remains equivocal. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.