Rearing other peoples' young; Brood-mixing in the shelduck Tadorna tadorna

The mixing of ducklings from different broods, in several species of waterfowl, involves care of offspring other than the parents' own. In the eider, Somateria mollissima, this behaviour appears to be of benefit, in terms of increased adult and duckling survival, to both donor and recipient parents, but in the shelduck, Tadorna tadorna, brood-mixing appears not to be advantageous. Dominant males tended to lose ducklings to more subordinate ones but there was no significant difference between donor and recipient parents in the number of their own offspring reared to fledging. The apparent lack of any advantage in brood-mixing suggests that the behaviour might be a consequence of high brood density and that in the shelduck brook defence is adapted to low density.