Internal incubation key to cuckoo success

Cuckoos give their eggs a head start in life by incubating them internally before they are laid, scientists have learned.

The strategy ensures the eggs hatch before those of the host bird whose nest the cuckoo has invaded.

Once the cuckoo chicks emerge, they throw the unhatched eggs out of the nest and are adopted by their unwitting foster parents.

How the cuckoo always manages to win the incubation race has long been a mystery.

Now scientists believe they know its secret.

The bird incubates its eggs internally for up to 24 hours, allowing them to reach a later stage of development before they are laid.

Taking into account the cuckoo’s internal body temperature of 40°C, which allows for faster development, it means the eggs have a 31-hour head start over the host eggs.

Study leader Tim Birkhead, professor from the University of Sheffield, said: “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.”

The Sheffield scientists worked with colleagues from across Europe to examine the newly laid eggs of eight cuckoos. They discovered that each one was at a more advanced stage than was normal among small birds.

To test their theory, they simulated internal incubation in another bird species, the zebra finch.

Recently laid finch eggs were taken and artificially incubated for 24 hours at 40°C, the body temperature of both finches and cuckoos.

After this time, the eggs were at the same stage of development as cuckoo eggs are when they are laid.