African and Diederik Cuckoos

Introduction

African Cuckoo ejecting

An African Cuckoo hatchling ejects the egg of a Fork-tailed Drongo.

Diederik Cuckoos Chrysococcyx caprius are small glossy cuckoos related to the bronze-cuckoos of Australasia, which have been beautifully studied by Naomi Langmore and her team at Australian National University. Diederik Cuckoos parasitise a large number of different host species (mostly weavers, bishopbirds, sparrows and buntings) and, like Common Cuckoos, they have distinct host-specific races which mimic the eggs of each of their hosts. Like African Cuckoos and Cuckoo Finches, they have to contend with a high degree of variation in egg appearance between different females of the same host species, and in turn have themselves evolved corresponding variation among females of the same cuckoo host-race. We haven’t studied Diederik Cuckoos in the field so far, but have tested various evolutionary questions on them with data previously obtained from our study area by Major John Colebrook-Robjent.

For example, we found that Diederik Cuckoo females parasitising thicker-shelled host species lay thicker-shelled eggs themselves (see publication below). This makes sense if the function of thickened eggshells in parasites is to discourage hosts from rejecting potentially parasitic eggs, lest they damage their own eggs in the process. 

Collins lowering cuckoo

An African Cuckoo chick being lowered from a drongo nest in a Mubula (Parinari curatellifolia) tree by Collins Moya.

We have studied African Cuckoos Cuculus gularis and their interactions with their host, the Fork-tailed Drongo Dicrurus adsimilis, in the field since 2009. African Cuckoos look extremely similar (and are closely related) to the Common Cuckoo Cuculus canorus of the northern hemisphere, which has been beautifully studied for many years by scientists in Europe and Asia. Like Common Cuckoos and indeed all other cuckoo species in the subfamily Cuculinae, African Cuckoo chicks eject any host eggs or chicks from the nest a few days after hatching. 

Unlike Common Cuckoos, though, African Cuckoos parasitise just one host species, so they have not evolved host-specific races that mimic the eggs of different host species: they only need to mimic drongos.

However, this is not so straightforward: drongos are formidably well-defended hosts. In addition to defending their nests highly aggressively (we often find cuckoo feathers stuck to or underneath drongo nests), Fork-tailed Drongos are extremely picky parents. Not only do they show variation in egg appearance among different females which makes it hard for a cuckoo to produce a perfect mimic (just like Cuckoo Finch hosts), but they are also extremely good at spotting a foreign egg in their nest. Consequently, African Cuckoo eggs have to be incredibly good drongo mimics in order to be accepted, and we’re often unsure ourselves whether an egg is a cuckoo’s or a drongo’s until it has hatched.

In 2012, the BBC Natural History Unit came to Choma to film our drongos and cuckoos for a forthcoming TV series. 

We also found that host species of the Diederik Cuckoo defend themselves against cuckoo parasitism by evolving unpredictable visual signatures on their eggs (left): host species of many brood-parasitic birds have evolved 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 Diedierik Cuckoos (as well as of Cuckoo Finches) 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 and the data were all obtained from Major John Colebrook-Robjent’s wonderful egg collection. (Read the full paper on the journal website [Open Access])

For more information about Diederik Cuckoos please also see the website of David Lahti at Queens College, New York, who has done wonderful work on their interactions with their weaver hosts.

Cuckoo research in the news

Article about internal incubation by cuckoos and honeyguides: BBC News website.

Our Cuckoo publications

  • Caves, E.M., Stevens, M. & Spottiswoode, C.N. 2017 Does coevolution with a shared parasite drive hosts to partition their defences among species? Proceedings of the Royal Society of London B 284: 20170272 Read on journal website [Open Access]

  • 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. Read on journal website [Open Access]

  • Birkhead, T.R., Hemmings, N., Spottiswoode, C.N., Mikulica, O, Moskát, C., Bán, M. & Schulze-Hagen, K. 2011 Internal incubation and early hatching in brood parasitic birds. Proceedings of the Royal Society of London B 278, 1019-1024 Read on journal website [Open Access]

  • Spottiswoode, C.N. 2010 The evolution of host-specific variation in cuckoo eggshell strength. Journal of Evolutionary Biology 23: 1792-1799. Read on journal website [Open Access]

News

New paper published on Weber’s Law and mimicry

Our paper ‘Why and how to apply Weber’s Law to coevolution and mimicry’ has been published in the journal Evolution. This perspectives paper, written by Tanmay Dixit, Eleanor Caves, Claire Spottiswoode, and Nicholas Horrocks, argues that Weber’s Law of proportional processing can lead to otherwise counterintuitive predictions about the evolutionary trajectories of mimicry systems.  Weber’s Law states that when the magnitude of a stimulus is large, it is more difficult to discriminate a change or difference from that stimulus. In other words, relative differences are more salient than absolute differences. We show that Weber’s Law could have implications for mimicry: when stimulus magnitudes are high, it should be more difficult to discriminate a model from a mimic. This leads to testable predictions about evolutionary trajectories of models and mimics. We also present a framework for testing Weber’s Law and its implications for coevolution. 

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New paper on evolution of egg signatures

Our paper “Hosts elevate either within-clutch consistency or between-clutch distinctiveness of egg phenotypes in defence against brood parasites” has just been published in the journal Proceedings of the Royal Society of London B. In this study, led by Eleanor Caves, we asked how host eggs evolve adaptations that allow them better to discriminate their own eggs from parasitic eggs. Theoretically, hosts can generate their own individually-distinctive egg ‘signatures’ by laying eggs that appear similar to one another (consistency) but look very different from other individuals’ eggs (distinctiveness). In this new study, we show that host species of two African brood parasites deploy either consistency or distinctiveness, but not both, as defences, and achieve distinctiveness by combining egg colours and patterns in unpredictable combinations.

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Mairenn Attwood awarded Cambridge teaching prize

Congratulations to Mairenn Attwood for being awarded the Janet Moore Prize for teaching in the Department of Zoology at the University of Cambridge, for her outstanding tutorial supervision of final-year undergraduate students, who praised her breadth and depth of knowledge, enthusiasm, and friendliness.  Mairenn follows in the footsteps of Tanmay Dixit who was awarded the Janet Moore Prize in 2020. Well done both for inspiring the next generation of behavioural ecologists!

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