Most birds lay eggs in their own nests. But brood parasites, such as cuckoos and cowbirds, lay their eggs in the nests of other bird species. A bird could defend against this behavior by rejecting any eggs it didn’t lay, but some let the foreigner stick around.
There is a cost to letting that egg stay in the nest. Sometimes the foreign chick will kick out the other nestlings, killing them. This often happens with cuckoos. Even if that doesn’t happen, there is a burden on the parents from having to feed another chick. And that burden can be even greater if the foreign nestling is bigger than the other chicks.
To figure out why only some birds reject interloper eggs, Iliana Medina and Naomi Langmore of the Australian National University in Canberra gathered data on egg rejection for 198 species of birds. They considered where the birds lived, which species — if any — parasitized them and other factors. The pair then created a phylogenetic tree of all those birds so that they could see if the relatedness of species played a role in whether a bird would reject a foreign egg in its nest.
Despite the complexity of the parasite-host relationship, Medina and Langmore found several trends within their data. Species in higher latitudes, for instance, were more likely to be rejecters. In places that experience seasons and colder months, there may be fewer opportunities for re-nesting if all a bird’s chicks are lost to a parasite. So it makes more sense to get rid of the intruder as soon as possible.
Birds that are unsuitable hosts for nearby parasitic species — such as when there is a diet mismatch between the species — are less likely to reject a foreign egg, the analysis revealed. In this case, the intruder is probably going to die, so the host bird has little need to get rid of its egg. In contrast, there is a greater cost for host bird species parasitized by larger birds, and so these species have a higher likelihood of egg rejection.
The team also found that bird species that are closely related are likely to reject foreign eggs — whether or not they are typically subject to brood parasitism. This suggests “that non-current, suitable hosts may have been hosts in the past,” the researchers write. The egg rejection behavior, therefore, may stick around for many generations after the threat has gone, the researchers report July 7 in Biology Letters.
The defensive behavior may actually succeed so well it drives cuckoos, cowbirds and other parasitic species to prey on other birds. Egg rejection rates of 100 percent were found in about half of non-current hosts but less than a quarter of current host species, the researchers note. “Hosts evolve effective egg rejection and retain it, leading to host switch by the parasite,” they write.
So why don’t all birds reject the foreigners in their nests? Sometimes, the parasites’ eggs look enough like the host’s that it can be difficult to spot the intruder. And there can be a risk that a host will kill own of its own. So some species, Science News reported last month, have evolved unpredictable egg patterns that make it more difficult for a cuckoo or other bird to slip an egg successfully into the nest.