Cuckoos' shameless egg-laying tactics revealed
Great spotted cuckoos lay their eggs while magpie mothers are still sitting on their nests, a study has shown.
Cuckoos are known for deceiving other birds into raising their young by depositing their eggs into host nests.
It had been thought female cuckoos waited until host birds left the nest before secretively laying their eggs.
But researchers were surprised when they recorded female cuckoos forcing magpies out of the way as they sat incubating their eggs.
The scientists, from Spain and Belgium wrote: "Great spotted cuckoo females entered the nest more frequently when the magpie female was incubating than when the nest was empty.
"This implies that great spotted cuckoo female behaviour is not secretive at all."
Details of their findings are published in the journal Ethology.
The scientists, who work at the Universidad de Granada, Spain, planted miniature cameras in 29 magpie (Pica pica) nests in the Hoya de Guadix plain.
They had expected to observe great spotted cuckoos (Clamator glandarius) using secretive behaviour around the nests, such as working in pairs to distract the magpies into leaving their clutches unattended.
Instead, they discovered female cuckoos endured attacks from incubating magpies and forced them out of their nests so they could speedily lay their own eggs in the clutch.
This egg-laying tactic appeared to work every time. And after the cuckoo had left, the magpie incubated the imposter egg along with its own.
"In spite of the violent attacks by magpies, they never succeeded in preventing the cuckoo female from laying her egg," the researchers observed.
The video recordings could offer new insight into the "arms race" between "brood parasite" birds and "host" birds - where species adapt and counter-adapt to try to succeed over each other.
The researchers suggest the magpies may have evolved the defensive behaviour of remaining sitting on their nests in response to cuckoos' attempts to distract them.
"Cuckoos then are forced to adopt the only possible solution: to lay their eggs facing the risk of being attacked by the incubating magpie," they wrote in the study.
The distinctively-patterned great spotted cuckoo is found in southern and eastern Europe and northern, southern and eastern parts of Africa. Magpies, crows and starlings are the main "hosts" targeted by the birds.
Female great spotted cuckoos may lay up to 13 eggs in a single host's nest.