Re: What is evolutionary advantages of bird brood parasitism?

For the brood parasite mother the advantage is that no energy is expended 
to raise the offspring. Also, one individual's offspring are spread 
across many parents, thereby increasing the chances that two offspring 
will survive to reproduce. As for limits to opportunities to parasitize 
many parasitic species get around this by not being species-specific in the 
nests they use. Individual birds are generally limited to the nests of 
specific host species but this does not necessarily hold for the parasitic 
species. Also, it appears that brood parasites tend to pick on smaller 
birds as hosts. This would allow higher population densities for the hosts 
as opposed to the parasites. Thus the brood parasites tend not to be 
limited by nest availability (natural selection has reinforced behaviors 
that limit host specialization). More generally, the process of laying eggs 
in a nest that is not one's own is actually quite common among birds. When 
individuals of the same species are involved this is referred to as "egg 
dumping" rather than brood parasitism. Genetic tests of offspring and their 
brood parents has indicated that many avian females do this. These genetic 
tests also indicate that paternity is not as clear cut as once thought. 
Even among birds where both parents raise the offspring one can find 
fathers caring for clutches that aren't their own.