Understanding colour polymorphism in birds
Many species show a variety of different phenotypes. How such 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. 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 influence the performance and fitness of individuals through direct effects of pigment production (e.g. camouflage or thermoregulation) and indirect effects (e.g. 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 adult morph. The morph distribution across South Africa follows a cline associated with rainfall seasonality and the intensity of solar radiation, with a far higher proportion of dark morphs in the Western Cape than the rest of the country. Our research has shown that provisioning rates differ depending on ambient light levels, with dark morphs providing more food to chicks in duller light conditions than light morphs, possibly due to improved background crypsis. Thus, when the two morphs breed together in a ‘mixed’ pair, they might complement each other by expanding the conditions (daylight hour, prey or habitat types) in which parents can forage successfully.
The idea that pairs consisting of the two morphs complement each other is termed the complementarity hypothesis. Support for this hypothesis comes from prey provisioning behaviour. Nest camera data shows that mixed-morph pairs provide food more consistently to the nest than like-morph pairs. This indicates that the two morphs forage under different ambient light levels and are expanding their foraging niche as a pair. This research, which forms part of Carina Nebel’s PhD thesis, was published this year in Behavioral Ecology.
Another aspect of Carina’s PhD has been to extend our understanding of this system through population modelling. Working with Dylan Childs from the University of Sheffield, through a grant from The Royal Society, Carina has developed an Individual-Based Model (IBM) to better understand the different components of the system, and to explore whether the complementarity hypothesis could operate to maintain the stable polymorphism seen in our study population.
Additional research has focused on the genetics behind plumage polymorphism. In many vertebrates discrete colour polymorphisms have been linked to the MC1R gene, which is involved in the regulation of melanin production. However, we have established that this is not the case for Black Sparrowhawks, and several other potential candidate genes also have been excluded. The differences in melanin production between the two morphs appears to be associated with different regulation of key genes involved in melanin production. Interestingly, variation in phaeomelanic plumage in juveniles appears to be regulated by expression differences in the same genes. We are now testing for differences in telomere dynamics between the two adult morphs to determine if there are any pleiotropic effects of morph. We also quantified haemosporidian blood parasite prevalence and intensity from blood samples using qPCR, to supplement previous data based on microscopy screening of blood slides.
Activities in 2020
- The COVID-19 pandemic made it difficult to conduct fieldwork in 2020, but we managed to ring 30 Black Sparrowhawk nestlings from 15 nests. Ringing nestlings is an integral part of the long-term monitoring and research of this population.
- PhD student Carina Nebel visited the University of Sheffield in January, funded by a Royal Society International Exchange grant, to construct an Individual-Based Model with Dylan Childs. This model will allow us to identify the driving mechanisms of colour polymorphism maintenance in the Black Sparrowhawk.
- Carina finalised her thesis. One manuscript, exploring whether immune function differs between nestlings of mixed- and like-morph pairs is currently under review, and another, focussing on morph inheritance, is ready to be submitted.
- Post-doc Chima Nwaogu analysed the influence of weather conditions on timing of egg-laying and breeding success on Black Sparrowhawk morphs. His results show that interactions between sex, timing of breeding and rainfall may underlie the maintenance of colour polymorphism and the increasing frequency of light morph Black Sparrowhawks in Cape Town.
- Carina Nebel published her second PhD cahpter in Behavioural Ecology in which she showed that Black Sparrowhawk parents breeding as a mixed-morph pair provide food more consistently to their nestlings than parents breeding as a like-morph pair. This supports the complementarity hypothesis, which psits that parents of different mrophs can expand their temporal foraging niche, and could explain the higher reproductive success of mixed-morph pairs and higher fitness of their offspring.
- Former Honours student Samantha McCarren published her findings on the blood parasite Haemoproteus nisi infections in Black Sparrowhawks in the Journal of Ornithology. She related infection intensity to breeding performance and fitness, and found that blood parasites are not assoicated with selective pressure in the species.
Impact of the project
This project will add to our understanding of 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.
DSi-NRF CoE grant, UCT Visiting Scholars Fund, Royal Society International Exchange Grant.
Research team 2019
Dr Arjun Amar (FIAO, UCT)
Dr Chima Nwaogu (FIAO, UCT)
Dr Petra Sumasgutner (FIAO, UCT and U.Vienna)
Dr Dylan Childs (University of Sheffield)
Dr Arne Hegemann (MEEL, Lund University)
Dr Gareth Tate (EWT)
Dr Jacqui Bishop (Biological Sciences, UCT)
Dr Chevonne Reynolds (Wits)
Dr Chris Briggs (Hamilton College, USA)
Students: Carina Nebel (PhD, UCT); Edmund Rodseth (PhD, MCB, UCT).
Research Assistants: Rebecca Muller, Burghen Siebert
Volunteers: Ann Koeslag, Marlene Hofmeyr, Paddy Walker, Margaret MacIver, Antje and Bernard Madden.