The vast majority of the 10,000+ living species of birds are passerines, and the vast majority of those have a similar system of breeding: Mom and dad bird make a nest and share parental responsibilities roughly equally, if not identically. There are variations on that theme, of course. Even the non-passerines often follow this pattern. So, when we find a pattern that is different it is reasonable to try to explain this in adaptive terms; what features of this variant pattern provide enhanced fitness, and what ecological or social factors underly it?
There are two patterns that are fairly extreme that fall into this category: brood parasitism and helper-at-the-nest strategy. In the former, a female lays her fertilized egg in the nest of another species, in the hopes that her offspring will be raised by the unwitting hosts. In the latter, three or more adult individuals contribute to the raising of offspring. In the case of brood parasitism, made famous by the many species of Cuckoo as well as cow birds and some fiches, among others, one might expect the host birds to evolve anti-parasitism strategies. In the case of helpers-at-the-nest one might expect that this strategy arose because of certain ecological or social conditions. It turns out that the two strategies may be related. Brood parasites might parasites helper-at-the-nest species because the latter are so darn good at raising offspring under certain conditions. Or, helping-at-the-nest might be a good strategy to avoid parasitism.
A recent paper in Science, Brood Parasitism and the Evolution of Cooperative Breeding in Birds by Feeney, Medina, Somveille, et al, looks into this interesting possible relationship. ...
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