Brood parasites are definitely the bullies of the avian world. They lay their eggs in the nests of other birds, sometimes destroying the host’s own eggs or just waiting for their nestlings to do the dirty work after they hatch. They then outcompete any surviving host nestlings for food, while the poor host parents are worked to the bone to feed the monstrous nest invader.
In spite of the steep costs of nest parasitism, most avian host species do not have effective mechanisms for detecting and removing brood parasites from their nests. So, why don’t mama birds notice they have a GIANT intruder in their nest and carry out some infanticide of their own? One hypothesis is that the cost of a mother bird making a mistake and pushing the wrong baby out (i.e. her own) outweighs the benefit of developing such a behavior.