Cuckoo finches have a sly way of pawning off parenting.

Researchers from the University of Exeter have found that African cuckoo finches hide their eggs in nests of African tawny-flanked prinia who land up hatching and raising the young as their own, according to a new study published in the journal Nature Communications.

"Our work shows that the cuckoo finch has evolved another novel strategy of attack, whereby it defeats both sensory and cognitive components of host rejection behavior," lead author Dr. Martin Stevens from the University of Exeter said in a statement.

The cuckoo finches get away with fooling the host parents by laying multiple eggs in a single nest. That combined with eggs that mimic their hosts, makes it even harder for the African tawny-flanked prinia to distinguish between theirs and the impostors.

"Our work shows that by laying multiple eggs in each host nest, the cuckoo finch has evolved a novel strategy, in addition to egg mimicry, to defeat host defenses and increase its reproductive success,” Steven said. “Laying several eggs in a host nest causes confusion in host defenses, and when combined with effective mimicry, they can outwit the hosts and help more of their young to be reared.”

Scientists came to the conclusion after observing the two birds in Zambia. They swapped prinia eggs in approximately 50 nests. The team took note of how many times the foreign eggs were removed from the nests and how similar the intruder eggs looked compared to the hosts’. Scientists found the prinia were more likely to reject the foreign eggs. In 31 of 48 trials the prinia rejected at least one foreign egg, but in 17 they accepted all the eggs even though there were intruders in the batch.

Researchers held two theories on how the prinia rejected the foreign eggs. One was based on the eggs’ appearance with the rest of the batch. Another focused on how the birds have a learned “internal template” of what their eggs should look like. Neither theory was completely true. While more research has to be done, scientists agreed that the visual differences between the eggs had to be profound in order for the prinia to realize the eggs didn’t belong to them.

"In general there are many species of brood parasite that we don't know much about and it will be exciting in the future to see what strategies they have to successfully parasitise hosts," Stevens told the BBC. "It would be great to know whether other parasites have a similar strategy to the cuckoo finch, and whether there is any way hosts can fight back."