Gabriel Jamie writes:
Research recently published in the journal Evolution shows that the nestlings of brood-parasitic finches mimic the appearance, sound and movements of their host’s chicks. Working in the savannas of Zambia, Dr Gabriel Jamie and a team of international collaborators collected images, sounds and videos over four years to demonstrate this striking and highly specialised form of mimicry.
The study, funded by The Leverhulme Trust, focussed on a remarkable group of finches that occurs across much of Africa called the indigobirds and whydahs (genus Vidua).
This group of 19 species, like many cuckoos, forego their parental duties and instead lay their eggs in the nests of other bird species (“hosts”). The hosts then incubate the eggs and feed the young of a different species. The hosts are unusual among birds in having brightly coloured and distinctively patterned nestlings. Furthermore, the nestlings of each host species have their own unique appearance, begging calls and begging movements. Vidua finches are extremely specialised parasites, with each species mostly exploiting a single host species.
Dr Jamie and colleagues show that the nestlings of the parasitic finches have evolved to mimic the colours, patterns, sounds and movements of their host species’ nestlings. This ensures that they get fed by their foster parents. While previous work by Robert Payne and Jürgen Nicolai had suggested such visual mimicry may exist, it had never been quantitatively tested from a bird’s visual perspective.
Birds process colour and pattern differently to how humans do, so it is important to use models that approximate their visual systems. This paper therefore answers a long-standing question as to whether these parasitic finches really do mimic their respective host’s nestlings.
While the mimicry is astounding in its accuracy, the authors found minor but striking imperfections. These imperfections may exist because 1) there has not been enough time for more precise mimicry to evolve, 2) the current levels of mimicry are already good enough to fool host parents or 3) the imperfections are actually enhanced versions of the host’s own signals, forcing the host parent to feed the parasite chick even more than it would its own.
The mimicry is not only amazing in its own right but may also have important implications for how new species of parasitic finches evolve. Previous work has shown that speciation in Vidua finches is intimately connected to host switching, owing to a remarkable quirk of their natural history: young Vidua finches imprint on their host species, such that males grow up to imitate the song of their host and females grow up to be attracted to males who sing like the host she was raised by. Females also prefer to lay their eggs in the nest of the same host species as she was born in.
Compare the begging calls of parasite nestlings with their host species nestlings below (all recordings (c) Gabriel Jamie):
Host species begging calls
Vidua species begging calls
Therefore, if a female accidentally lays her egg in the nest of a new host species, she has the potential to start a new lineage of Vidua that now specialises on the new host and is separated from Vidua lineages specialising on traditional hosts. Recent work has suggested that this process may not be sufficient to prevent the collapse of potential new Vidua species through hybridisation (interbreeding between lineages). Instead, the mimetic adaptations to different hosts identified in this study may also be critical in the formation of new species.
Furthermore, this specialised mimicry is a compelling example of the role of learning in genetic evolution. The importance of behavioural flexibility in influencing the course of genetic evolution is increasingly recognised among biologists.
Vidua nestlings imprint on their hosts, altering their mating and host preferences based on early life experiences. These preferences strongly influence the host environment which their offspring grow up in, and therefore the evolutionary selection pressures they experience from foster parents. When maintained over multiple generations, these selection pressures are what generate the astounding host-specific mimetic adaptations that this paper reveals.
Have a go at our Cryptic Egg Hunt to see more examples of mimicry in action
Read the full paper in Evolution here: Multimodal mimicry of hosts in a radiation of parasitic finches
Find out more on African Cuckoos: research on brood parasites and other curious African birds