Brood parasites like cuckoos and cowbirds famously and sneakily lay their eggs into the nests of other birds. And while some host birds reject the foreign eggs, not all species do. Researchers investigating why some birds don’t evict alien eggs reveal that it depends on how steep the costs are to the hosts. The findings were published in Biology Letters this week. 

Having to take care of freeloading babies that aren’t their own is generally a costly thing for unsuspecting bird parents. Some oft-victimized birds have evolved sophisticated signatures to authenticate their eggs and fight back against forgeries. But the cost of parasitism varies. Newly hatched common cuckoo (Cuculus canorus) chicks push out the host’s own eggs and nestlings. Meanwhile, Molothrus cowbirds are non-evicting parasites, and their chicks might grow up alongside the host chicks more or less uneventfully. 

Australian National University’s Iliana Medina and colleagues compared the results of 198 studies on egg rejection rates to see what variables explain the evolution of this defense mechanism. Across all species, there were higher rejection rates at higher latitudes: Compared to the tropics, these areas have shorter breeding seasons, which cuts down on their opportunities to re-nest following parasitism. This pressure likely led to stronger selection for egg rejection. 

Meanwhile, rejection rates were the lowest with unsuitable hosts – birds who don’t share the same diet and nesting habits as the parasite. If the intruder is going to die anyway, Science News explains, the host bird has little need to get rid of the egg. That also means that if the host and parasite species are closely related, the host birds will have a higher need to get rid those foreign eggs – since the two likely share similar traits.

Additionally, whether or not an egg is rejected can also be predicted by the relative size of the parasite and if it can be raised with the host chicks. Small birds rearing larger parasites suffer higher provisioning costs than those raising parasites that are about the same size as their own chicks. For the hosts of evicting parasites like those pushy cuckoos, relative parasite size was the only major predictor of egg rejection. 

Finally, egg rejection rates were as high in non-current hosts as in current hosts. And clutch size and nest type (say, an enclosed nest with poor visibility) didn’t seem to influence whether or not the imposter egg was rejected.