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.

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.

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

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 below) 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.

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.