Certain species of birds, like the brown-headed cowbird or the European great spotted cuckoo, lay eggs in other birds’ nest. They hope that those birds will raise their young for them. These kinds of birds are known as brood parasites. They’ve used this strategy for a long time, but we’re still learning about how they manage to succeed.
When a bird discovers a parasite egg in its nest, it either pushes it out or just raises that egg, too. The way a bird reacts to a parasite egg is dependent on other ecological factors, such as how populous the parasite species is.
The nesting bird’s reaction is an important part of the mafia hypothesis, which states that birds are more likely to raise parasite eggs if they fear retaliation from parasite birds.
The mafia hypothesis is expressed in this behavior: a cowbird lays an egg in another bird’s nest, and if that egg gets destroyed, they subsequently attack the nest and destroy the other eggs, forcing the birds to lay more eggs.
Once this happens, the host birds don’t kill further parasite eggs out of fear that their brood will be destroyed again.
A mathematical model developed by scientists from the Max Planck Institute for Evolutionary Biology found that this is the most common factor in determining how host birds react to parasite eggs.
If they have no reason to fear retaliation, say because there aren’t that many cowbirds around, they go ahead and push out that egg.
Other scientists question the hypothesis, though, because of a practice called farming, in which a parasite bird destroys a nest of eggs so that the hose has to lay a new clutch, and then sneaks one of its eggs in there.
Farming seems to require that the host bird not notice or not care about this new egg, though, and that doesn’t look very likely.