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 melanoleucus 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 offspring 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 extended the research project from correlative studies to conducting field experiments and behavioural tests under laboratory conditions to understand the mechanisms behind the observed fitness advantages for breeding with a partner of the opposite morph.
Activities in 2018
- In 2018, we performed an immune challenge on 40 Black Sparrowhawk chicks from 21 nests to understand the strength of the innate immune function in relation to different colour morphs of the parents.
- PhD student Carina Nebel conducted a behavioural experiment at the field station in Bainskloof, where she tested the reaction time of pigeons towards an approaching hawk of different plumage morphs.
- Adrien Pajot from Bordeaux Sciences Agro, France, worked in the research group during three months and analysed video footage and screened blood slides for avian malaria infection rates.
- Samantha Kirves completed her BSc Hons project investigating the impact of blood-parasite infection on fitness parameters in the polymorphic Black Sparrowhawk.
- Arjun Amar collaborated with Dr Chevonne Reynolds (Wits) and Dr Chris Briggs (USA) on a project to explore whether the morphs of Swainson’s Hawks vary clinally across their North America breeding range. The paper was accepted for publication in the Biological Journal of the Linnaean Society.
- Arjun Amar presented research on the use of MORPHIC to explore patterns of polymorphism at the British Ornithological Union’s conference in Nottingham, UK, the 4th Learn About Birds (LAB) conference in Langebaan, Western Cape, and at the Annual Meeting of the Raptor Research Foundation (RRF) in Skukuza, RSA.
- Carina Nebel presented research from her experiments at the Annual Meeting of the Raptor Research Foundation (RRF) in Skukuza, RSA, where she was awarded the William C. Andersen Memorial Award of the Raptor Research Foundation for the best student presentation.
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 that is now experimentally tested.
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: Margaret MacIver, Antje and Bernard Madden
Students: Carina Nebel (PhD, UCT); Edmund Rodseth (MSc, MCB, UCT); Samantha Kirves (BSc Hons, UCT)
Research Assistants: Dr Shane McPherson, Kyle Walker