Don’t be taken in by their chirpy charm – cuckoos are the masters of playing dirty, and use mob-like tactics to force other birds to obey their wishes. Like human mafias, these avian bullies “lean” on their victims by hitting them where it hurts and going after their families in order to get their way. The effectiveness of this behavior and its impact on the relationships between bird species has now been analyzed as part of a new study, revealing how different birds adopt different strategies to deal with this strong-arming.
Like several other avian species, the European great spotted cuckoo lays its eggs in the nests of other birds so that it doesn’t have to bother itself with raising its own young. While some of these host birds will unconditionally accept the cuckoo’s eggs, others are less accommodating and eject these eggs from the nest. When this occurs, the cuckoo gets nasty and murders the host bird’s entire brood, in order to teach it a lesson.
Henceforth, the cuckoo’s eggs are seen as an “offer you can’t refuse,” and are accepted unconditionally by the host. This type of interaction is known as the mafia hypothesis, and goes some way to explaining why many birds end up allowing eggs from other birds into their nest.
However, a rival theory states that these parasitic birds kill their hosts’ broods not to intimidate them, but to force them to lay more eggs, thereby providing an opportunity to smuggle their own young into this new brood – a strategy known as “farming.”
To examine how these two types of interactions affect populations, researchers from the Max Planck Institute for Evolutionary Biology developed a mathematical model to predict the consequences of both strategies, publishing their findings in the journal Royal Society Open Science.
Birds like the brown-headed cowbird use dirty tactics to smuggle their eggs into other birds' nests. Galawebdesign via Wikimedia Commons
According to this model, if the majority of parasitic birds are “farmers,” then it makes sense for host birds not to unconditionally accept their eggs, as this will not have any bearing on whether or not their brood is killed. As such, conditional acceptors – meaning those that only allow foreign eggs into their nest after experiencing brutal retaliation for initially refusing – will be at an advantage, as they will exert less energy raising the young of other birds, and can therefore have more offspring of their own.
This, in turn, leads to an increase in the number of conditional acceptors within a population of birds, and a decrease in the number of unconditional acceptors. Once this happens, however, mafia-type birds gain the advantage over farmers, as only they can successfully coerce these conditional acceptors to raise their young, via crooked means.
Mafia parasites therefore increase in number while farmers decrease, which subsequently causes the number of conditional acceptors to drop while unconditional acceptors thrive, as their reluctance to refuse the cuckoos’ eggs means their broods are not slaughtered.
In this way, populations of farmer and mafia parasites, as well as conditional and unconditional hosts, are forever to-ing and fro-ing. Commenting on this finding, study co-author Arne Traulsen explained in a statement that “there is no optimal behavior in such host-parasite relations. Neither party can outsmart the other on a permanent basis.”
In other words, as the likes of Al Capone, John Dillinger, and Pablo Escobar prove, gangsters ultimately never win. Still, you may want to keep a watchful eye on your cuckoo clock.