Coevolution is the process by which two or more species influence each other’s evolution. Brood-parasitic birds, the cheats of the bird world, give us an ideal opportunity to study coevolution in the wild. Coevolutionary “arms races” arise when hosts evolve defences such as rejecting parasitic eggs, which imposes natural selection for parasitic counter-adaptations such as mimicry of host eggs, and in turn for ever more sophisticated defences from hosts. Three long-term projects address different aspects of this fascinating model system for coevolution.

Claire Spottiswoode’s team works on a variety of brood-parasitic systems in Zambia, focusing mainly on three broad questions. First, how do interactions between species generate diversity among individuals? Specifically, how do biological arms races between hosts and parasites shape phenotypic diversity in both parties? For example, parasites diversify to mimic multiple hosts, and in response hosts sometimes diversify with defensive adaptations to foil mimicry, such as visual 'signatures' of identity. Second, how is specialisation to different coevolutionary partners maintained? The genetic basis of signature-forgery arms races is almost entirely unknown. In collaboration with Prof. Michael Sorenson (Boston University), we are using genomic approaches to ask how specialised adaptations to different host species (mimicry of host eggs) are maintained within a single parasitic species (e.g. Cuckoo Finches Anomalospiza imberbis and Greater Honeyguides Indicator indicator) in the absence of parasite speciation. We are also interested in the genetic basis of host defences, and whether convergent genetic mechanisms have evolved in their parasitic mimics. Third, what is the role of phenotypic plasticity (such as developmental differences and learning) in coevolution, and how might such plasticity facilitate exploitation of new host species in the absence of appropriate genetic adaptations? We are addressing this question for indigobirds, whydahs and honeyguides.

Fitz Research Associate Jessie Walton has been studying Brown-backed Honeybirds Prodotiscus regulus, which parasitise Karoo Prinias Prinia maculosa at a high rate in the Bot River area of the Western Cape. Brown-backed Honeybirds have blue eggs, highly unusual in piciform birds, broadly mimic the blue eggs of their hosts. Moreover, up to three honeybird chicks are raised in the same host nest, despite killing host young with their bill hooks. How honeybirds escape being killed by their nestmates remains an intriguing mystery.

Robert Thomson’s team works in Finland, where their research focuses on how host pairs of Common Redstarts Phoenicurus phoenicurus can decrease the chance of a Common Cuckoo Cuculus canorus parasitising their nest. Hosts that are able to avoid parasites decrease the fitness costs of parasitism; the earlier that avoidance occurs during the breeding cycle, the lower the cost. Therefore, host adaptations before egg laying should be especially beneficial. The Finnish project investigates the redstart’s frontline defences (nest site choice, habitat selection, nest building decisions) and the cuckoo’s counter-adaptations, including prospecting and laying strategies, which have received little attention to date. Redstarts are the only regular cuckoo host that breed in cavities, which makes it difficult for female cuckoos to lay eggs and for their newly-hatched chicks to evict host eggs/chicks. This project also studies whether cuckoo females use behaviour and physiology to enhance the manipulative signals that cuckoo nestlings use to extract as much care as possible from their foster parents.

Activities in 2023

  • We carried out two seasons of fieldwork in Zambia: rainy season fieldwork in January–March (Tanmay Dixit), and dry season fieldwork in September–November (Jess Lund, Mairenn Attwood, Maggie Mwale and Joel Radue), working on Cuckoo finches, honeyguides, African cuckoos Cuculus gularis and their hosts, and the thermal ecology of ground-nesting birds, together with our local field team led by Collins Moya, Silky Hamama and Onest Siakwasia.
  • Gabriel Jamie (Fitz Research Associate) carried out a successful season of wide-ranging fieldwork in Zambia as part of his Leverhulme Early Career Research Fellowship together with Professor Per Alström (Uppsala University) and Professor Urban Olsson (Gothenburg University). Gabriel is exploring the evolution of polymorphisms across the Cisticolidae family which exhibit some of the most complex and diverse eggs of any group of birds in the world. This includes sampling species and populations that have rarely, if ever, been sequenced before.
  • Maggie Mwale (CB MSc) and Joel Radue (BSc Hons) joined the Choma field team for their research projects on different aspects of the thermal ecology of ground-nesting birds in Zambia, and how this may trade-off against egg camouflage against predators. Their projects were co-supervised by Claire Spottiswoode and Shannon Conradie at the Fitz, and Nicholas Horrocks at the University of Cambridge.
  • Robert Thomson visited the field site in Finland during June and July to continue the long-term monitoring of cuckoo parasitism of the redstart population. Extremely high nest predation rates of redstart nests (~90% of nests) caused havoc during the season.

Highlights

  • Angela Moreras was awarded her PhD during 2023 for her thesis entitled “Frontline strategies in the arms race between hosts and their brood parasites: the case of the common redstart.” Angela was also awarded the 2023 Faculty of Science Purcell Memorial Prize. The prize is awarded annually for the best MSc or PhD thesis in a zoological study area at UCT.
  • Mairenn Attwood, together with colleagues from the Fitz and Choma field team, published a paper “Aggressive hosts are undeterred by a cuckoo’s hawk mimicry, but probably make good foster parents”, in Proceedings of the Royal Society B. The paper investigated the costs and benefits to the African cuckoo of specializing on a highly aggressive host species, the Fork-tailed Drongo Dicrurus adsimilis, using field experiments in Zambia. It showed that aggression can exacerbate the trade-off a parasite faces in choosing which host species to parasitise: drongo aggression undermined the effectiveness of hawk mimicry, but drongo nest survival was high relative to other potential host species with similar nesting ecology, suggesting that successful parasites secure high-quality care for their offspring.
  • Jess Lund, also together with colleagues from the Fitz and Choma field team, published a paper in Proceedings of the Royal Society B: “When perfection isn’t enough: host egg signatures are an effective defence against high-fidelity African cuckoo mimicry”. The paper shows that African cuckoos mimic all egg types of their fork-tailed drongo hosts, and do so with near-perfect accuracy. However, the high degree of variability in appearance of drongo eggs – creating individually-distinctive egg ‘signatures’ – means that drongos have the upper hand in the arms race against their parasite. Jess’s paper was featured on some prominent media including The Guardian and New Scientist.
  • Gabriel Jamie gave the Hemborg Lecture at Uppsala University in Sweden, an annual lecture honouring the memory of Dr Christer Hemborg, a former postdoc at the Fitz. Gabriel’s lecture was entitled “The evolution of nestling ornamentation”.
  • Tanmay Dixit (Fitz Research Associate) published a paper in Biology Letters entitled “Combined measures of mimetic fidelity explain imperfect mimicry in a brood parasite-host system”. This study was part of a collaboration between our group and Prof. L. Mahadevan and his lab at Harvard University. Together we combined mathematical tools and field experiments in Zambia to quantify a key difference – “squiggle” markings – between the eggs of hosts (tawny-flanked prinias) and parasites (cuckoo finches). We showed that suboptimal behaviour on the part of prinias allows cuckoo finches to get by with an imperfect copy of prinia eggs.
  • Tanmay and collaborators published a paper in Evolution entitled "Repeatable randomness, invariant properties, and the design of biological signatures of identity”. The study uses methods from applied maths and computer science to predict the properties of egg signatures that hosts (tawny-flanked prinias) should use when detecting parasitic (cuckoo finch) eggs in their nests, and test this experimentally against the reality in the field in Zambia. The paper also introduces a new method for “unwrapping” the patterns on a 3D egg onto a 2D plane for analysis.
  • Tanmay, together with Fitz and Choma colleagues and others including several computer scientists, published a paper in Nature Ecology and Evolution showing how rapid coevolution of both tawny-flanked prinia eggs and their cuckoo finch mimics means that mimicry doesn’t get any better over time.
  • Tanmay published a paper in Evolution entitled “A synthesis of coevolution across levels of biological organization”. In it, he argues that we must extend the definition of coevolution, beyond reciprocal adaptive evolution of different species, to encompass all examples of reciprocal adaptive evolution, including those between traits within a species.
  • Tanmay passed his PhD at the University of Cambridge with no corrections – a very rare achievement. His thesis entitled “Signatures and forgeries: optimality in a coevolutionary arms race” was based on his work in the Zambia brood parasite team. Tanmay then began his Junior Research Fellowship at University of Cambridge, continuing his work in Zambia, and remains a Research Associate at the Fitz.
  • Jess Lund was awarded an R. C. Lewontin Graduate Research Excellence Grant from the Society for the Study of Evolution which will enable her to expand her investigations into the fascinating double lives of honeyguides as parasites and mutualists.
  • The Zambia brood parasite team continued their diverse outreach and collaborative activities internationally, including presenting talks to schools in both Zambia and the UK. Tanmay Dixit was again a co-presenter of the “Evolution Crash Course” online course for students in the Global South.

Key co-supporters
Biotechnology and Biological Sciences Research Council; The Leverhulme Trust; Natural Environment Research Council; Societas pro Fauna et Flora Fennica; Finnish Cultural Foundation; LUOVA Doctoral Programme in Wildlife Biology at the University of Helsinki.

Research team 2023
Prof. Claire Spottiswoode (FIAO, UCT / U. Cambridge)
A/Prof. Robert Thomson (FIAO, UCT)
Dr Gabriel Jamie (U. Cambridge/FIAO, UCT)
Prof. Michael Sorenson (Boston University)
Prof. L. Mahadevan (Harvard University)
Dr Nicholas Horrocks (University of Cambridge)
Dr Christopher Town (University of Cambridge)
A/Prof. Rose Thorogood (U. Helsinki, Finland)
Dr Jere Tolvanen (U. Oulu, Finland)
Jessie Walton (FIAO, UCT)

Students: Teresa Abaurrea (PhD, U. Helsinki); Mairenn Attwood (PhD, Cambridge); Tanmay Dixit (PhD, Cambridge); Jess Lund (PhD, Cambridge); Angela Moreras (PhD, UCT); Cameron Blair (MSc, UCT); Maggie Mwale (CB MSc, UCT); Joel Radue (BSc Hons, UCT)

Research assistants: 
Zambia: Silky Hamama, Collins Moya, Onest Siakwasia, Sylvester Munkonko, Sanigo Mwanza, Oscar Siakwasia, Iness Liteta, Milton Simanunki, Aron Muntanga and many others.