Scientists at the University of Sheffield say female cuckoos "internally incubate" their eggs for 24 hours before actually laying them in a host victim's nest, giving the cuckoo egg a head start over the host's eggs, a university release said.
This allows the cuckoo chick to hatch before its nest mates, evict them and monopolize the food brought by the foster parents.
"The idea of internal incubation in birds has until now been considered impossible because it was assumed that female birds could not hold onto a fully formed egg," Tim Birkhead of the university's Department of Animal and Plant Sciences said.
"In fact, the idea of internal incubation by cuckoos was suggested as early as 1800, but then ignored.
"Our results show that internal incubation gives cuckoo chicks that crucial head start in life, allowing them to dispose of their nest mates -- a superb adaptation to being a brood parasite."
The study was published in the journal Proceedings of the Royal Society of London.