New research from the Max Planck Institute for Evolutionary Biology reveals that some species of parasitic bird - which lay their eggs in the nests of other bird species - will bring on a heavy retaliation if the host bird abandons the parasite eggs.

The researchers propose that the relationship is akin to a mafia group leaving a business alone so long as it pays its dues.

"We tested and confirmed the mafia hypothesis, which was controversial among scientists," lead study author Maria Abou Chakra said in a statement. The mafia hypothesis suggested that parasitic birds use aggressive behavior such as destroying native eggs in a host nest if the parasite egg is abandoned to extort the hosts, forcing them to cooperate. "They give the hosts no choice. If they wish to avoid retaliation, they need to keep the foreign egg."

Abou Chakra and her colleagues contend that the host birds are capable of learning that if a parasite approaches the host nest in an act of aggression more than once, then it's best to cooperate.

"For the hosts, the best thing is to remove the foreign egg from their own nests. But if they encounter a retaliatory parasite that destroys their nest, it's best to adapt and accept the parasitic egg," said Abou Chakra.

To test this hypothesis, the researchers developed mathematical models. It appeared that a parasite bird laid no more than one egg in a host bird's nest, which that bird may either accept or reject. A more complex model offers more than one egg in the host nest, each of which could be kept or disposed of by the host bird.

"Biologists prefer the complex model, because it is closer to reality. But we came to the same conclusion with both models: the dynamics of the interaction between host and parasite is cyclical," Abou Chakra said.

The researchers found that an equilibrium between the host and parasite birds is never established. Instead, as the host birds abandon parasite eggs more frequently, the parasite birds become more aggressive with the host birds until eventually the host birds accept the parasite eggs. Eventually the host birds will again become bold enough to abandon a parasite egg, thus restarting the cycle all over again.

Interestingly, some species of parasite birds have eggs that clearly stand out against the host's native eggs.

The researchers theorize that this may be beneficial for the parasite birds so that they can lay eggs in the nests of a variety of host bird species and exert their mafia-like presence on a number of species.

In the future, the researchers plan to study which situations the parasite birds find it beneficial to specialize and in which scenarios the aggressive mafia-like behavior pays off.

Abou Chakra and her colleagues published their work in the journal Scientific Reports.