Sharing Your Mate May Be a Good Idea
Abstract from the Journal of Animal Ecology
30th May 1996
Many Great Reed Warbler males have more than one wife - great for the males but what is in it for the females? Steffan Bensch, writing in the Journal of Animal Ecology, 65, 283-296 has proved that there is a cost to being a secondary female within the nesting relationship but that they gain from selecting a male with a good territory and good breeding potential.
His study covered 192 breeding attempts by 104 different females - of which 40% paired with already mated males. He followed the breeding attempts and assessed their clutch size, hatching and fledging success, nest survival, fledging mass and survival to breeding age and the female's mass when feeding and their survival rate. The only cost that was detected was in survival of the nestlings - the secondary females had higher losses presumably because the males were unable to devote as much time to them as their primary mate. He had a number of natural experiments where the primary nest failed and the secondary birds was promoted. In these cases their nestling survival was improved to that of primaries.
However the number of recruits per breeding attempt, carefully controlling for other factors, was very similar - 0.53 for primary females, 0.51 for secondary ones - so the ultimate result in real terms was very similar. In fact there may be a cost to the primary females as the chances of a nest being predated is increased by the presence of a secondary female. In a previous paper he has actually recorded the possibility of secondary females destroying the primary females nests!
Whatever the exact costs and benefits of polygyny, they are clearly finely balanced with the Great Reed Warbler with the costs for the secondary female being pretty well balanced by access to high quality territories - and males!