(“Mockingbird Music” by Dawn Henning)
Shiny cowbirds regularly visit mockingbird nests and attack and puncture any eggs they find there — damaged eggs are later removed by the mockingbird host. During these visits cowbirds will often lay their own eggs in the nest for mockingbirds to hatch and bring up alongside their own chicks.
Whilst mockingbirds will mob an attacking cowbird, once an alien egg has been laid in their nest they will usually accept it — even though it looks very different from their own eggs.
… ‘It might be expected that this high rate of parasitism would encourage a host to evolve more effective anti-parasite defences,’ said Ros Gloag from Oxford University’s Department of Zoology, an author of the report. ‘In fact, the opposite is probably true: the higher the intensity of parasitism, the higher the frequency of puncturing attacks and the greater the importance of diluting this risk. Thus, hosts benefit more by not rejecting parasite eggs when there are more parasites around.’