Parasitism, Mimicry and Speciation in the Indigobirds and Whydahs of Africa
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BIOLOGY big little liars Parasitism, mimicry and speciation in the indigobirds and whydahs of Africa TEXT GabrieL A. JAMIE It’s the start of the rains in southern Zambia and a small, glossy black finch is singing incessantly from the top of an exposed tree. The song is mostly a hurried ramble of chatters and scratchy warbles, but occasionally the bird intersperses trills and tinkling notes that sound nearly identical to those of a lo- cal species of firefinch. This seemingly inconsequential observation hints at a fascinating evolutionary story unfolding in woodlands and savannas throughout Africa. he finch is an indigobird, a member of the family Viduidae that also includes the whydahs Tand Cuckoo Finch. As the latter’s name suggests, the family’s reproductive be- haviour shares similarities with that of many cuckoos. Rather than constructing a nest, incubating eggs and feeding young, they adopt a ‘brood-parasitic’ lifestyle, depositing their eggs in the nests of other birds and letting them do all the work. As strategies go, it’s a good one. Rear- ing offspring is a time-consuming and energy-intensive endeavour. While off- spring are young and unable to care for themselves, parents must return fre- quently to the nest site to bring food, carry out nest maintenance and defend their chicks from predators. This se- WARWICK TARBOTON verely curtails the adult’s mobility and parental care, they free up time and en- A male Long-tailed Paradise Whydah increases its own susceptibility to attack. ergy for producing more eggs. These (opposite) surveys his territory. As a nestling, What’s more, the adult literally has all eggs can then be spread across many this bird would have grown up in the nest of its eggs in one basket. If something goes host nests rather than being concentrat- a Green-winged Pytilia (above). wrong at any stage in the process, its re- ed in a single one. This distributes risk productive output is completely lost and and means that even if one of the nests tied to a single nest site and having com- its efforts wasted. gets preyed upon, the parasite’s entire re- mitted the parasitic act they are free to Brood parasites, in contrast, have productive output is not compromised. move as other pressures, such as food, no such constraints. By outsourcing The adult parasite’s movements are not moult and weather, dictate. > TERTIUS A. GOUS JANUARY/FEBRUARY 2021 BROOD PARASITES 35 An adult male Purple Indigobird (above). This that make the parasite’s job much harder. All indigobirds and whydahs are spe- bird would have been raised in the nest of its These include mobbing adult parasites cialised brood parasites on species in the specialist host, Jameson’s Firefinch (opposite). seen near the nest and removing odd- grassfinch family (Estrildidae). There looking eggs or chicks that could have are 10 indigobird species and nine why- WARWICK TARBOTON (2) However, despite the apparent benefits come from another species. dahs currently recognised. Indigobirds family or why it is so diverse yet species- THE ESTRILDIDAE ARE EXCEP- humans, allowing them to perceive ultra- of a parasitic lifestyle, less than one per Even if hosts lack these defences, not mostly parasitise firefinches and twin- specific. Parasitism by Vidua alone can- TIONAL IN THAT, UNLIKE THE violet light. Moreover, nestling begging cent of the world’s bird species exclu- all are suitable candidates for parasitism. spots, while the hosts of whydahs are not account for it, as many estrildid finch NESTLINGS OF MOST BIRDS displays involve multiple dimensions, in- sively adopt it. Globally, obligate brood Some have nest entrances that would be generally pytilias and waxbills. Unlike species in Asia and Australasia, beyond corporating not just visual but vocal and THAT ARE CRYPTICALLY parasitism is thought to have evolved too small for the adult parasite to enter most other avian brood parasite–host the range of Vidua, have such ornamen- postural components too. These, it turns independently seven times in birds, with or the fledgling parasite to leave, while relationships, in which the interacting tation too. PATTERNED, estriLDID YOUNG out, are as diverse and species-specific three separate origins among cuckoos others feed their young the wrong type parties are often much more distantly However, we do know that the col- boast THE Most ELaborate across the Estrildidae as their mouth and one each in the honeyguides, cow- of food. Taken together, this means that related, the Viduidae and Estrildidae ours and patterns of each estrildid spe- AND DIVERSE APPEARANCES OF markings. Does Vidua mimicry extend birds and parasitic finches. Even a some- the range of possible hosts occurring at a are sister families thought to share a cies are important in ensuring parents ANY BIRD FAMILY IN THE WORLD into these modalities too? what anomalous duck in South America given locality for a given parasite can be common ancestor around 10 to 15 mil- feed them adequately. When the mouth It was in this context that I, together has gone down this route. But why don’t quite small. The parasite may then itself lion years ago. markings of nestling estrildid finches by the foundational research of Jürgen with Claire Spottiswoode and other col- more bird species implement the strat- have to evolve specialised adaptations to The Estrildidae are exceptional in are altered slightly, they are fed less by Nicolai and Robert Payne. However, leagues, conducted research on the Vidua egy? One possibility is that while being a circumvent host defences. While these that, unlike the nestlings of most birds parents than those with ‘normal’, un- while these studies laid the foundation radiation. We set out to quantitatively test parasite avoids certain problems, it gen- adaptations make the parasite effective that are cryptically patterned, estrildid manipulated markings. This suggests for our current understanding of the whether Vidua nestlings truly do mimic erates a whole set of new ones. at exploiting one host, they can equally young boast the most elaborate and di- that, in order to convince host parents Vidua finch radiation, methodologi- the appearance, sounds and movements The challenges of being a parasite be- make it poorly suited to another. As verse appearances of any bird family in to feed them enough food, Vidua nest- cal limitations at the time meant that of their host nestlings. The first step, come apparent when one realises that parasites become specialised on a small the world. Some have an almost other- lings need to have similar mouth mark- the existence of this mimicry could not however, was to find a parasitised nest! hosts are not passive and willing par- number or even a single host, they be- worldly appearance, with luminous ings to those of their host. be tested in a systematic or quantitative This was easier said than done, but over ticipants, eager to have the role of foster come dependent on that species and are papillae lining the gape and complex manner, nor tested from a bird’s perspec- five years of field work at our study site parent foisted upon them. From an evo- vulnerable if it goes locally extinct. combinations of spots and bars adorning Nestling mimicry by Vidua tive. Subjective human assessments are near Choma, southern Zambia, together lutionary perspective, the host’s effort the palate. These patterns vary widely finches not necessarily good proxies for simi- with Collins Moya, Silky Hamama and in rearing the distantly related young of The Vidua finches between estrildid species but little within The idea that Vidua nestlings may larity as perceived by birds, since birds a team of skilled nest-finding assistants, another species is entirely wasted and This brings us back to our seemingly them, making them highly character- mimic the appearance, and particularly process colour and pattern differently we managed to find parasitic chicks of diverts investment away from its own innocuous indigobird singing ebul- istic of each. It is not yet understood the mouth markings, of their specific to humans. For instance, many birds three Vidua species: the Pin-tailed Why- offspring. Many hosts have therefore liently in the midday heat of southern what evolutionary pressures led to the estrildid host goes back to the work of have four colour-receptive cones in their dah, Broad-tailed Paradise Whydah and evolved a suite of defensive strategies Zambia. origin of nestling ornamentation in this Robert Neunzig and was later extended retina, as opposed to just three in most Purple Indigobird. > 36 AFRICAN BIRDLIFE JANUARY/FEBRUARY 2021 BROOD PARASITES 37 more precise mimicry to have evolved or because current levels of mimicry are already good enough to fool the host par- ents. Some imperfections might actually be enhanced versions of the host’s signal, stimulating it to feed the parasite chick even more than it would its own chicks, though this hypothesis remains untested at present. Vocal imitations of hosts by adult Vidua That some of the adult indigobird’s vo- calisations resemble those of a firefinch is not coincidental. It stems from the ca- pacity that many Vidua finches have to imprint on their hosts, as uncovered by a species. Male song and female host and The author’s set-up to photograph inside the series of detailed and painstaking experi- mate preferences develop differently de- mouths of nestling birds. ments by Robert Payne and colleagues. pending on the host species they are raised Specifically, male Vidua learn the song by. If a female accidentally lays her eggs in sanguinidorsalis and its association with of the host species in the nest and grow the nest of a new host species, her sons the Jos Plateau Indigobird Vidua maryae.