AfricanCuckoos
Adaptation, mimicry and co-evolution in Africa's avian cheats: cuckoo finches, honeyguides, indigobirds & cuckoos

 

 

CUCKOO FINCHES

Introduction | Natural History Gallery | Past Research | In the News | Publications

Introduction

A Cuckoo Finch chick about to fledge from the nest of a Red-faced Cisticola.

The Cuckoo Finch Anomalospiza imberbis is a unique and little-known species of parasitic finch. Although completely unrelated to cuckoos, the Cuckoo Finch is also a brood parasite and lays its eggs in the nests of other bird species that act as host parents. It is a member of the same family (Viduidae) as another group of African brood parasites, the indigobirds and whydahs, and is the only species in its genus (appropriately meaning 'strange finch'), which Michael Sorenson and Robert Payne discovered split off from the indigobirds and whydahs about 20 million years ago.

Unlike the indigobirds and whydahs, which have since split up into numerous species each specialising on a single host species, the Cuckoo Finch has remained one species comprising several host-specific races. For instance, the Cuckoo Finches we study in Zambia lay their eggs in the nests of the Tawny-flanked Prinia Prinia subflava and at least three Cisticola species.

What is interesting about this situation is that each female Cuckoo Finch only lays a particular type of egg mimicking a particular host species, and will specialise only on that species, so that within a small area, neighbouring Cuckoo Finches may look and sound the same, but have hatched from different host nests and lay eggs that mimic that host species. For example, the eggs in the header image above are all Cuckoo Finch eggs from an area of less than a thousand hectares on our study site in Zambia, and come from the nests of three different host species.

A three day old Cuckoo Finch chick (left) with a Tawny-flanked Prinia hatchling (right) which is about to die of starvation.

This diversity of mimicry has evolved because hosts fight back, since they pay a high price if they are tricked by a Cuckoo Finch. The young finch usually hatches a day or two in advance of the host chicks, which typically only survive a day or two before starving to death, while the Cuckoo Finch obtain's the lion's share of the food the host parents bring to the nest.

Host species therefore defend themselves against parasitism by rejecting foreign eggs from their nests, and this selection pressure has resulted in the evolution of superb egg forgeries by Cuckoo Finches. Not only do Cuckoo Finches closely mimic the colour and pattern of host eggs, but as we have seen, different host-races of Cuckoo Finches lay different eggs that mimic those of the specific host species they specialise upon. For example, the photo below shows Cuckoo Finch eggs (right column) mimicking the eggs of two different host species (left column): Red-faced Cisticolas in the first two rows, and Tawny-flanked Prinias in the last four rows.

Cuckoo Finch eggs (right column) mimic their eggs of their hosts (left column). The top two rows of eggs are from Red-faced Cisticola nests and the bottom four rows are from Tawny-flanked Prinia nests. Tawny-flanked Prinias lay a range of egg colours and patterns to foil Cuckoo Finch mimicry.

You will at once notice another striking thing about this photo: host eggs of the same species vary greatly in appearance, too. This is because in response to such Cuckoo Finch mimicry, many host species have evolved a further defence: a diversity of spots and squiggles on their eggs that act like signatures that are hard for the Cuckoo Finch to forge. A host female will lay very similar eggs within her lifetime, but every female's eggs are slightly different. Crucially, this means that a female Cuckoo Finch isn't assured of fooling any female of its specialist host species. As you can see in the right column, Cuckoo Finches have evolved different egg colours and patterns that are approximate forgeries of different individual host 'signatures', but the problem lies in matching them up correctly. What we see in the field is that Cuckoo Finches gamble with their eggs, laying their eggs haphazardly in the nests of the appropriate host species and relying on chance matches to succeed. They commonly get it wrong (for example, laying a blue egg in the nest of a white-egged prinia female) and their egg is then soon speared on the end of the prinia's beak and carried away [read more].

Instead of evolving high variation in appearance among the eggs of different females, other host species (such as the Red-faced Cisticola Cisticola erythrops) have instead become extremely good at spotting tiny discrepancies in egg appearance, and we've found that these two strategies seem to be equally successful at getting rid of a parasite [read more].

Our Cuckoo Finch research focuses on two general questions: exactly how cuckoo finches dupe their hosts into accepting these foreign eggs, and on the genetic mechanisms that allow egg mimicry of different hosts to be evolutionarily maintained. This is a puzzle because we would expect interbreeding among males and females raised by different hosts to cause such specialised adaptations to break down. For more information, see the Research page.

Cuckoo Finch Natural History Gallery

Here is a summary of the life of a Cuckoo Finch:

Hatch from the egg a day or two before any foster siblings, and beg furiously from your foster parents. Any foster siblings that hatch later will probably shrivel up and die as you outcompete them for their parents' food, leaving you to monopolise the nest.
You're actually more likely to hatch alongside a sibling or half-sibling, since your mother probably laid two eggs in the same host nest. This is not a problem: you'll thrive alongside each other. This Cuckoo Finch chick at left has hatched just before its sibling at right. Note the absence of any fine lines (scribbles) on the Cuckoo Finch egg, unlike on Tawny-flanked Prinia eggs. Although these scribbles should be perfect signatures of identifty that reliably inform prinia parents which eggs are their own and which are a parasite's, puzzlingly, they often still accept unscribbed eggs.
Very rarely, any foster siblings you might have may survive alongside you. They have two black spots on their tongues, such as on the prinia chick at left, unlike your unmarked pinkish mouth at right.
Fledge from the nest when you're roughly double the weight of your foster parents, if they're Tawny-flanked Prinias.
In a few months, moult into bright yellow plumage if you're a male, or dull brown plumage if you're a female, such that you look very much like a female bishopbird. This may not be a coincidence.
If you're an adult female, lay your eggs in exactly the same host species as your mother did. Hope that at least some of your eggs will match the host clutch more like nest number 13 on the left, than nest number 15 on the right (in each case, the egg at far right is that of a Cuckoo Finch, and the others are those of the Tawny-flanked Prinia host). Your egg will therefore have a much better chance of being accepted by the host parents (the whole lot will probably still get eaten by a snake, though).

 

Summaries of some of our Cuckoo Finch research to date

On the evolution of egg signatures as host defences:
Caves, E.M., Stevens, M., Iversen, E. & Spottiswoode, C.N. 2015 Hosts of brood parasites have evolved egg signatures with elevated information content. Proceedings of the Royal Society of London B 282: 20150598.

Host species of brood-parasitic birds can evolve features such as spots, squiggles and colours on their eggs that act like ‘signatures' that are difficult for parasites to forge, helping hosts to detect and reject imposter eggs. In this paper, we show that hosts of cuckoo finches (as well as hosts of diederik cuckoos) in Zambia have optimised this defence by arranging signature traits in unpredictable combinations. Thus, egg signatures are individually distinctive and hard for parasites to mimic, helping hosts distinguish parasitic eggs from their own. The paper arose from MPhil research by Eleanor Caves (co-authored by Martin Stevens, Edwin Iversen and Claire Spottiswoode) and the data were all obtained from Major John Colebrook-Robjent's wonderful egg collection.

See also: News articles about this research
Read the full paper on the journal website [Open Access]

 

On the evolution of aggressive mimicry by adult Cuckoo Finches:
Feeney, W.E., Troscianko, J., Langmore, N.E. & Spottiswoode, C.N. 2015 Evidence for aggressive mimicry in an adult brood parasitic bird, and generalised defences in its host. Proceedings of the Royal Society of London B 282: 2015079.

This paper shows experimentally that adult female cuckoo finches (at left in photo) in Zambia have evolved to resemble harmless and abundant bishop-birds (right), which should help them to slip past being attacked by host parents while they try to lay their egg. However, hosts are not fooled by this attempted deception, and defend themselves against parasitic cuckoo finches and harmless bishop-birds alike. To our knowledge this is the first time that such “wolf-in-sheep’s-clothing” plumage mimicry has been experimentally shown to exist in any adult bird.

See also: News articles about this research, and this nice blog article from Faansie Peacock who very kindly provided illustrations for the paper
Read the full paper in Proceedings of the Royal Society of London B [Open Access].

 

On laying multiple eggs to better to trick hosts:
Stevens, M., Troscianko, J. & Spottiswoode, C.N. 2013 Repeated targeting of the same hosts by a brood parasite compromises host egg rejection. Nature Communications 4: 2475.

Cuckoo Finch eggs mimic those of their hosts, and the same Cuckoo Finch female commonly lays two or more eggs in the same host nest. In this paper we show that this increases her chances of tricking hosts into accepting the parasitic eggs as their own. By increasing the proportion of foreign eggs in the clutch, Cuckoo Finches appear to be able to take advantage of uncertainty in the sensory and cognitive processes that hosts use to distinguish foreign eggs from their own.

See also: News articles about this research
Read the full paper in Nature Communications [Open Access].

 

On rapid changes in egg appearance over time:
Spottiswoode, C.N. & Stevens, M. 2012 Host-parasite arms races and rapid changes in bird egg appearance. American Naturalist 179: 633-648.

Cuckoo Finch eggs beautifully mimic the eggs of their various prinia and cisticola host species, to increase their chances of being accepted by choosy host parents. In turn, many host species have evolved astonishing variation in the colour, spots and squiggles of eggs laid by different individual females, to render their eggs harder to mimic by the parasite. In this paper we show that the Cuckoo Finch’s mimicry—and its commonent host's defenses against it — appear to have caused the appearance of both species’ eggs to change over time, in a race between the host evolving new signatures and the parasite new forgeries. During the course of our fieldwork in Zambia, we noticed that Cuckoo Finch and Tawny-flanked Prinia eggs we were seeing in the wild differed markedly from those in a museum collection made by John Colebrook-Robjent at our study site. For example, 30 years ago, Cuckoo Finches predominantly laid red eggs, but now mainly blue ones, and prinias in turn now more commonly lay olive-colored eggs, perhaps to escape egg mimicry by their pursuing parasite. This makes sense if the dominant colours and patterns of prinia eggs serves as a good defence only until Cuckoo Finch mimicry catches up. At that point, natural selection should begin favour prinias that lay eggs of novel appearance which make the Cuckoo Finch eggs stand out, such that the two interacting species are locked in a perpetual race around the colour and pattern spectrum.

See also: News articles about this research
Read the full paper in PDF format

 

On different strategies for host defence:
Spottiswoode, C.N. & Stevens, M. 2011 How to evade a coevolving brood parasite: egg discrimination versus egg variability as host defences. Proceedings of the Royal Society of London B 278: 3566-3573.

Cuckoo Finches parasitise various species of prinia and cisticola, whose drab appearance betrays the remarkably colourful arms races played out within their nests. Hosts are highly discriminating about what they will risk incubating, and this selection pressure has resulted in the evolution of superb egg forgeries by Cuckoo Finches, which closely mimic the colour and pattern of host eggs. But as the Cuckoo Finch has become more proficient at tricking its hosts with better mimicry, hosts seem to have evolved increasingly sophisticated ways to fight back. This paper shows that their arms race has escalated in strikingly different ways in different species. One strategy is for every host female to lay a different type of egg, such that egg colour and pattern vary greatly among nests. This is the strategy taken by the Tawny-flanked Prinia , resulting in the evolution of an amazing diversity of prinia egg colours and patterns. These variations seem to act like the complicated markings on a banknote: complex colours and patterns act to make host eggs more difficult to forge by the parasite, just as watermarks act to make banknotes more difficult to forge by counterfeiters. Since the female Cuckoo Finch always lays the same type of egg throughout her lifetime, she cannot change the appearance of her egg to match those of different host individuals. Thus her chances of laying a matching egg are exasperatingly small, and hosts have the Cuckoo Finch on the run. Other Cuckoo Finch hosts appear to use an alternative strategy: Red-faced Cisticolas lay only moderately variable eggs but are instead extremely discriminating in deciding whether an egg is their one of their own. Thanks to their excellent discrimination, these hosts can spot even a sophisticated mimic. The experiments showed that these different strategies are equally successful as defences against the cuckoo finch. Moreover, one species that has done a bit of both – the Rattling Cisticola – appears to have beaten the cuckoo finch with this dual strategy, since it is no longer parasitised at our study site.

See also: News articles about this research
Read the full paper on the journal website [Open Access]

 

On egg rejection and egg signatures by Tawny-flanked Prinias:
Spottiswoode, C.N. & Stevens, M. 2010 Visual modeling shows that avian host parents use multiple visual cues in rejecting parasitic eggs. Proceedings of the National Academy of Sciences of the USA 107: 8672-8676.

This paper investigates natural selection underlying the evolution of extreme levels of signature-like variation in egg appearance (colour, spots and squiggles) among females of the Cuckoo Finch's commonent host in Zambia, the Tawny-flanked Prinia. A prinia female lays the same type of egg within her lifetime, but every female's eggs are slightly diferent. Such variation among individuals can be an effective defence against a brood parasite that mimics host eggs, because it renders it harder for the parasite to match any one host clutch well enough to evade detection. In this paper, we asked how prinias distinguish a potentially parasitic egg from their own, using a combination of field experiments at host nests in the wild in Zambia, and newly-developed sensory models of bird vision to quantify egg appearance through a birds' eye rather than the human eye. This allowed us to disentangle natural selection on different aspects of egg appearance from a bird's rather than a human's (comparatively limited) visual perspective. We found that prinias are very good at rejecting foreign eggs, and that they use several different aspects of egg appearance to spot parasitic egg: not only colour, but also different aspects of pattern such as the size and distribution of markings. Mysteriously, however, they do not seem to use the scribbles that uniquely occur only on the prinias’ eggs. We found that the specific traits that prinias used to distinguish foreign eggs were exactly those that differ most between host eggs and real parasitic eggs. This suggests that natural selection is currently acting to make Cuckoo Finch eggs better mimics of their hosts’, and also that Prinias use the most reliable information available in deciding whether there is a potentially parasitic egg in their nest.

See also: News articles about this research
Read the full paper on the journal website [Open Access]

Cuckoo Finch research in the news

Articles about escalation of host egg signatures in defence against Cuckoo Finches:
Q&A on Royal Society Publishing Blog | Science News

Articles about aggressive mimicry by adult Cuckoo Finches:
Nature | The Conversation | EurekAlert | IFLScience | ScienceDaily | BirdWatch magazine

Articles about repeated laying by Cuckoo Finch females enhancing host acceptance: The New York Times | BBC Nature News website | Wired | Christian Science Monitor | Smithsonian Magazine blog | Pentagon Post | LA Times (and others)

Articles about rapid changes in Cuckoo Finch and host egg appearance over time: New Scientist | Forskning.no | ScienceNews magzine

Articles about two different strategies for host defence against Cuckoo Finch egg mimicry: The New York Times | USA Today | The Times | BBC Radio Wales | BBC Radio Scotland | NRC Handelsblad | De Volkskrant | Science et Vie | WDR5 | Il Venerdi di Repubblica

Articles about exactly how prinias spot Cuckoo Finch eggs in their nests: BBC Earthnews website | Wired

Our Cuckoo Finch publications