I once observed a female crow merrily feeding a large cuckoo chick in its nest. This seemed bizarre in the early stage of bird watching in my hometown Jabalpur. But little bit of reading enlightened me about brood or social parasitism among the birds especially the cuckoos of sub family Cuculinae of family Cuculidae.
The crow seemed entirely convinced about the chick being its own and was feeding it with great devotion like any other mother would. What really I was witnessing is a parasitic behaviour where a species was using another for hatching and rearing its brood as a way of alternate breeding behaviour.
The process begins with the female cuckoo laying its egg in the host nest. She may remove the host nest and place her own or in certain instances to mitigate competition for food or this will not happen and the parasitic chick shares food with the host chicks. Sometimes the nest may be ejected by the growing parasite chick in order to reduce competition.
The egg may resemble the host eggs in order to reduce the chances of rejection. This is known as egg mimicry. I certain cases like hawk cuckoo and Indian cuckoo the bird mimics accipiters in appearance thus frightening the host from the nest whence she comes to lay an egg.
Babblers and corvids have been seen to be preferred hosts of the cub family cuculinae.
Not all cuckoos are parasitic as may the case with Coucal of sub family centropodinae and genus centropus which builds its own nest.
