How is it that the singing of the cuckoo (keel) signifies the onset of spring ?

The cuckoo (or keel) is famous for its melodious song. Did you know that it is the male cuckoo which sings beautifully and not the female?
The male and female cuckoos look very different in colour. The male is black and shiny while the female is ash coloured with white spots. The cuckoos mate during the spring season. During this time the male cuckoo sings to attract the female bird. Once their mating is over the male stops singing. The cuckoo mates during the spring season and they sing only during the mating period. This is why we associate the singing of the cuckoo with spring.
The cuckoos unlike most other birds do not build nests to hatch their eggs. Instead they lay their eggs in a crow's nest. The crow does not suspect any thing as the eggs looks similar to its own. Once the cuckoo lays its eggs, it flies to the dense forests as it prefers large dark trees. We only hear its singing in spring when it comes out to the open fields and grounds for mating.