Understanding colour polymorphism in birds
Many species show a variety of different phenotypes. How this diversity is maintained is one of the main questions in evolutionary biology. Colour polymorphism represents an ideal system to explore these issues because the different phenotypes represent their genotypes. Researchers have long been fascinated by colour polymorphism because the occurrence of two or more phenotypes in the same population runs counter to the notion that selective pressure should favour the optimal form for an environment, and any lesser quality individuals should be eliminated through natural selection. Colour polymorphism occurs in around 3.5% of bird species, but is more common in raptors and particularly within Accipiter hawks where about 25% of species are polymorphic.
Colour morphs are known to influence the performance and fitness of individuals through both direct effects of pigment production (e.g. camouflage or thermoregulation) and indirect effects (e.g. correlated physiological and behavioural traits). Hence, it is unlikely that different morphs are distributed randomly in the environment. Our focus species is the Black Sparrowhawk Accipiter melaonleucus which occurs as either a dark or a light morph. The morph distribution of this species across South Africa follows a cline which is associated with winter rainfall patterns and the intensity of solar radiation. Thus, within the Western Cape we have a far higher proportion of dark morphs than the rest of the country. Our research has shown that the foraging success of the two morphs differs depending on ambient light levels, with dark morphs foraging more successfully in duller light conditions than the light morph, possibly due to improved background crypsis. Thus, in theory the two morphs when breeding together in a ‘mixed’ pair might be able to complement each other by expanding the conditions (daylight hour, prey or habitat types) in which parents can successfully forage and provision their offspring. Indeed, pairs consisting of mixed-morph parents produce more young than parents consisting of the same morphs, and their offspring show higher survival rates. The idea that pairs consisting of the two different morphs may complement each other has been termed the complementarity hypothesis. However, this fails to explain the predominance of dark morphs, because the light morph gene is dominant.
We have developed a simple method to describe the morph distribution of birds and other organisms using photographic images sourced from the web. This method has been developed as a Web Application called MORPHIC which is free to use and is currently being used in research exploring multiple questions across the globe.
Activities in 2017
- PhD student Carina Nebel presented her proposal in October 2017. Her thesis will explore the complementarity hypothesis in more detail focusing on the likely mechanisms involved.
- Edmund Rodseth, PhD candidate in Molecular and Cell Biology (MCB, UCT), co-supervised with Dr Rob Ingle (MCB), continued to make good progress on his study to identify the genetic pathway for colour polymorphism in Black Sparrowhawks.
- In collaboration with Dr Christopher Briggs (Hamilton College, United States) and Dr Chevonne Reynolds (University of Witwatersrand) we are using MORPHIC to understand the spatial structuring of Swainson's Hawk Buteo swainsoni morphs across North America.
- Another collaboration with Dr Chiara Morosinotto (University of Turku), Prof. Jon Brommer and Dr Patrik Karell (University of Helsinki) is using MORPHIC to describe the spatial distribution of Tawny Owl Strix aluco morphs across Europe in relation to climate, habitat, anthropogenic impact and diet.
- Several members of the research group presented their research at conferences; the British Ecological Society Annual Meeting, Ghent, Belgium, the European Ornithologists’ Union, Turku, Finland and the Bird of Prey Programme of the Endangered Wildlife Trust, Witsand, Northern Cape.
- Gareth Tate published a paper in Scientific Reports using information on GPS tracked male Black Sparrowhawks to explore whether the two morphs differ in the amount of time spent foraging in relation to habitat type and light levels.
- Gareth Tate’s PhD thesis was awarded the Purcell Memorial Prize for best thesis in Biological Sciences at UCT in 2017.
- Former BSc Hons student, Gabriella Leighton’s paper in Methods in Ecology and Evolution introducing MOPRHIC, the web app using Google Images to describe spatial patterns of phenotypic variation, was awarded the British Ecological Society’s Robert May Prize for the best paper by an Early Career Researcher. Gabriella was invited to present the research and receive the award at the BES conference in Belgium.
Impact of the project
This project will add to our understanding and theory about the maintenance of genetic diversity in populations. It provided the first empirical evidence for the light level hypothesis for the maintenance of colour polymorphism in birds. Our demonstration that Google Images can provide robust unbiased information on the spatial distribution of visible phenotypes, and the development of the MORPHIC web app, has opened up a whole new approach for research in this field.
DST-NRF CoE grant; Claude Leon Foundation.
Dr Arjun Amar (FIAO, UCT)
Dr Petra Sumasgutner (FIAO, UCT)
Dr Gareth Tate (EWT)
Dr Jacqui Bishop (Biological Sciences, UCT)
Dr Rob Ingle (MCB, UCT)
Volunteers: Ann Koeslag, Margaret MacIver, Antje and Bernard Madden
Students: Carina Nebel (PhD, UCT); Edmund Rodseth (MSc, MCB, UCT)
Research Assistants: Dr Shane McPherson, Beckie Garbett, Jessleena Suri