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."