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Altruism
Altruism is behaviour that benefits others at a cost to one's self. Song birds display altruistic behaviour as they feel obliged to sit on eggs that are present in their nest. Cuckoos have exploited the song birds' good parenting and lay their eggs in the nests. Once the cuckoos have hatched they eject the song bird eggs and babies from the nest.
However, Lotem et al (1995) had noticed that song birds have picked up upon this parasitism and have learnt to recognise their species markings on the eggs so that when they spot a different species egg in their nest, they eject the egg before the cuckoo can do any damage. But even the cuckoos have got smart and noticed that their offspring weren't surviving and started to make their eggs look more like song birds eggs..
Richard Dawkins described it as an evolutionary arms race n his book “the selfish gene”. He noticed that for cuckoo's there was selection in favour of eggs that look like song birds; as these are the ones that make it into the next generation and pass on their egg shell design genes. 
Also there was selection for song birds with sharp eyesight, as it was these birds that spotted the cuckoo egg wasn't theirs. So if their eggs then survive, then so do the sharp eyesight genes.
What is also unfortunate for the song birds is that if it is their first brood and the cuckoo egg does hatch, when they have a second brood, they are most likely to kill their babies as the parents are expecting baby cuckoos.
