Indigobirds and Whydahs

See also: an article about our research in The Leverhulme Trust‘s newsletter

Introduction: a case study in mimicry, learning and speciation 

Purple Indigobird chick

Village Indigobirds are raised by Red-billed Firefinches. This male imitates the vocalisations of its host when attracting a mate.

The savannahs and woodlands of Africa provide the setting for a remarkable evolutionary story. Announcing its presence, a glossy black finch sings incessantly from an exposed branch at the top of a tree. Interspersed amongst its chattering and scratchy warbles are a few simpler elements which pioneering ornithologists noticed sounded near-identical to the vocalisations of a local species of firefinch.

The singing finch is a species of indigobird, a member of the brood-parasitic genus Vidua that also includes the spectacularly-plumaged whydahs, and is the evolutionary sister group to the brood-parasitic Cuckoo Finch. An understanding of the remarkable breeding system of Vidua finches is needed to explain the mimetic elements in the songs of adult male indigobirds. The remarkable story of the Vidua finches was brought to light thanks to the pioneering work of several ornithologists – particularly Robert PayneJurgen Nicolai and Michael Sorenson. All of the discoveries summarised below are not our own, but the result of work by these scientists or their labs.

Like many cuckoo species, Vidua finches are all brood parasites – instead of constructing their own nests and raising young themselves the females lay their eggs in the nests of other birds. Most Vidua are host-specialists, each exploiting a single species of grassfinch (family Estrildidae) as a host to raise their young. Indigobirds parasitise firefinches and twinspots whilst whydahs parasitise waxbills and pytilias.

These remarkable birds are the focus of Gabriel Jamie‘s PhD research, funded by The Leverhulme Trust.

Learning and the origin of new species

Their brood-parasitic lifestyle means Vidua finches are raised in close association with their respective host. Amazingly, the male Vidua imprints on his host’s vocalisations. When the males mature and start to display they incorporate elements of their host’s vocalisations into their own repertoires. As a result, male Vidua broadcast the identity of the host they were raised by both to female Vidua and to birders! This presents a unique opportunity amongst brood parasitic birds whereby host-usage can be ascertained with relatively little effort – only sound recording rather than DNA analysis is required.

In addition to Vidua males mimicking the songs of their hosts, Vidua females also imprint on their foster parents. A female Vidua acquires a mate preference for male Vidua that sing like the host she was raised by. The result is that male and female Vidua raised by the same host tend to mate with one another. Additionally a female grows up to parasitise the same host species as she was raised by. If a female lays in the nest of a new host species she has the potential to initiate a new lineage of Vidua which specialises on that new host and is reproductively isolated from populations of Vidua associated with different hosts – namely a new Vidua species! This is the process thought to be responsible for the recent and rapid diversification in Vidua and to have generated the 19 species (10 indigobirds, 9 whydahs) recognised today.

New host races of indigobird are regularly found and our knowledge of their distributions is patchy. In fact, Robert Payne was led to the discovery a new species of firefinch to science by hearing an indigobird in Nigeria which mimicked the calls of an unknown host!

Mimetic mouths

Purple Indigobird chick

The ornamented mouth of a Purple Indigobird chick, which is an exact copy of that of its host, the Jameson’s Firefinch.

Not only do Vidua show behavioural imitation of their hosts through song, they also provide a remarkable example of morphological mimicry. Members of the grassfinch family Estrildidae, the hosts of Vidua finches, have extraordinary mouthpart markings as nestlings (see photo). The mouths of different estrildid species are adorned with an amazing array of different of colours and patterns. The degree of elaboration seen in estrildid chicks is unique amongst birds with perhaps only nestling rails (family Rallidae) coming close.

What is even more impressive is that the mouthparts of nestling Vidua have converged on those of the specific estrildid finch that they parasitise. The degree of matching is exquisite and it is not possible to tell which nestling is the host and which the parasite by looking at the mouth markings alone.

Unlike many cuckoos and all honeyguide species, Vidua nestlings are often raised alongside the host. As such they must compete with host nestlings for access to the food provided by the foster parents. It is thought that having mimetic mouths allows Vidua nestlings to better solicit food from their host parents. A study in 2005 by Justin Schuetz found that when the mouthpart markings of one estrildid species – the Common Waxbill Estrilda astrild – was manipulated (by marking over a white spot with black ink) the nestling received less food and did not grow as well as sham manipulated nestlings (spots painted with see-through ink).

Vidua Finch Natural History Gallery

News

Evolutionary Biology Crash Course

Tanmay Dixit was a member of a team organising and lecturing in the inaugural Evolutionary Biology Crash Course. This course, aimed at undergraduate or early-postgraduate students, teaches evolutionary principles to students who have had limited opportunities to be exposed to evolutionary ideas. The course is funded by the Equal Opportunities Initiative Fund of the European Society of Evolutionary Biology (ESEB). Tanmay presented lectures on behavioural ecology and evolution, focussing on kin selection, coevolution, and parasitism. Over 700 students, with the vast majority from the global South, attended the course, which was a resounding success!

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New paper on visual complexity & mimicry

Our paper “Visual complexity of egg patterns predicts egg rejection according to Weber’s Law” has just been published in the journal Proceedings of the Royal Society B. This research was led by Tanmay Dixit, and carried out together with Andrei Apostol, Kuan-Chi Chen, Tony Fulford, Chris Town and Claire Spottiswoode, in a collaboration between biologists and computer scientists. We used machine learning to compute a biologically-relevant measure of egg pattern complexity, and combined this with field experiments in Zambia to investigate how complexity evolves in an arms race between host egg signatures (by tawny-flanked prinias) and parasitic egg forgeries (by cuckoo finches).

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Fieldwork and teaching at APLORI, Nigeria

Dr Gabriel Jamie is continuing his fieldwork on the evolution of polymorphisms in cisticolas and prinias in Nigeria, where he is also a teaching fellow at the AP Leventis Ornithological Research Institute (APLORI). The image shows the 2022 APLORI MSc class during the Global Birding Big Day on 14 May. The team recorded 135 species while walking around the nature reserve surrounding the institute.

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New paper on the genetics of cuckoo finch egg mimicry

Our paper “Genetic architecture facilitates then constrains adaptation in a host-parasite coevolutionary arms race” has been published in the Proceedings of the National Academy of Sciences of the USA. In it, we address the long-standing puzzle of how exquisite mimicry of the eggs of several different host species can evolve within a single species of brood-parasitic bird. We show that in cuckoo finches in Zambia, egg mimicry of different host egg phenotypes is maternally inherited, which allows mothers to transmit host-specific adaptations to their daughters irrespective of which host species the father was raised by. This study was a team effort from colleagues at the University of Cambridge and University of Cape Town (Claire Spottiswoode, Wenfei Tong, Gabriel Jamie), at Boston University (Katherine Stryjewski, Jeff DaCosta, Evan Kuras and Michael Sorenson) and in the Choma community in Zambia (Ailsa Green, Silky Hamama, Ian Taylor and Collins Moya).

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