An interesting study has found that Cuckoos keep their eggs inside their bodies for a day longer than most other birds this allows their chicks to hatch much earlier than other birds young.
Many species of Cuckoo (incuding the one we see in the UK) lay their eggs in other birds nest. The young Cuckoo hatches first and then "throws out" all the other eggs and is given all the food brought from the unsuspecting foster parents.
The Cuckoo strategy of delayed laying gives it an edge.
Researchers don't know why cuckoos have evolved a longer egg retention time, it's possible the trait may have predisposed them to parasitism.