Darwin’s finches are highly innovative. They show a very wide range of behaviours in the feeding context that are highly unusual for passerines such as breaking eggs of seas birds, habitual tool use or blood drinking. Here we define innovation as behaviours and food types that are unusual for passerines and have evolved after the arrival of the ancestor of Darwin’s finches on the Galapagos Archipelago. Recently we have observed the first behavioural innovation in this species group outside the foraging context: Individuals of four species of Darwin’s finches picked the leaves of the endemic tree species (Psidium galapageium) and rubbed them into their feathers. We hypothesised that this behaviour evolved to repel ectoparasites and tested the repellent quality of Psidium leaves against parasitic insects that have been shown to affect fitness of Darwin’s finches, namely mosquitoes and the recently introduced parasitic fly Philornis downs. The mosquitoes transmit recently introduced pathogens and the larvae of the fly suck blood from nestlings and incubating females. Thereby they reduce the breeding success of Darwin's finches drastically. We found experimental evidence that Psidium leaves repel mosquitoes and have a growth inhibiting effect on the blood-sucking larvae of the parasitic fly.

It is possible that this behaviour has evolved a long time ago and has gone unnoticed. However, since several research groups study the behaviour of Darwin’s finches at the same study site since 20 years, it is more plausible that the behaviour has developed recently or has increased in frequency during the last years in response to the selection pressure imposed by novel pathogens and parasites.