Understanding colour polymorphism in birds | FitzPatrick Institute of African Ornithology

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 was published in Behavioral Ecology, and forms part of Carina Nebel’s PhD thesis which was awarded in 2021.

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 Newton 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. Using a multi-generational pedigree, we have recently established that the most likely inheritance mechanism in this system is one where the dark allele is dominant. 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.

We have also continued to use web-sourced photographs to better understand the distribution of colour polymorphisms across different species’ ranges. In collaboration with Dr Chris Briggs at Colgate University, USA, and Dr Chevonne Reynolds at Wits University, we have collected data on the morph distribution of Red-tailed Hawks and Ferruginous Hawks across their North American range, these data will be combined with previously analysed data on Swainson’s Hawks to examine for consistency in the association between morphs distribution and climatic variables.

Activities in 2021

In 2021, we ringed 50 Black Sparrowhawk nestlings from 24 nests. Ringing nestlings is an integral part of the long-term monitoring and research of this population. Unfortunately, due to cuts in the CoE funding, 2021 will be the last year of nest monitoring on this project.
PhD student Carina Nebel was awarded her PhD for her thesis entitled ‘A mechanistic approach to understanding the colour polymorphism in Black Sparrowhawks (Accipiter melanoleucus)’.
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.

Highlights

Carina Nebel published two of her PhD chapters. The first paper, published in Scientific Reports showed that the offspring of mixed morphs did not appear to invest more resources into their immune function, and this, therefore, was unlikely to be the mechanism behind higher survival rates of these offspring. The second paper, published in the Journal of Zoology, revealed that our multigenerational pedigrees are incompatible with a one-locus two-allele system, where the light allele is dominant (as previously suspected), but are consistent with a scenario where the dark allele is dominant instead.
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 associated with selective pressure in the species.

Impact of the project

This project adds to our understanding of the maintenance of genetic diversity in populations. It provides the first empirical evidence for the light level hypothesis for the maintenance of colour polymorphism in birds that is now experimentally tested.

Key co-supporters

DSI-NRF CoE grant, UCT Visiting Scholars Fund; Royal Society International Exchange Grant.

Research team 2021
A/Prof. Arjun Amar (FIAO, UCT)
Dr Chima Nwaogu (FIAO, UCT)
Dr Rob Ingle (MCB, UCT)
Dr Petra Sumasgutner (FIAO, UCT/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, Kyle Walker

Volunteers: Margaret MacIver, Jacques Giliomee, Marlene Hofmeyr, Gillian Cowan, Megan Jackson, Robert Daley, Ruben Jenkins-Bate, Ann Koeslag, Michael Lamprecht and Cameron Blair.

In 2021, Juvenile Black Sparrowhawks were monitored after fledging and a photographic inventory was created for 38 of the 50 ringed juveniles. These photographs will be useful for future analyses of colour polymorphism starting at the juvenile stage (Photos: Kyle Walker)..

A dark morph black sparrowhawk. This female was ringed in 2013 as a nestling and recruited to the breeding population in 2017. Through its unique colour ring combination, we can identify an individual's age and place of birth and follow its breeding success (Photo: Carina Nebel).

A4-week old Black Sparrowhawk chick during a ringing procedure. Body morphometric measurements and feather and blood samples are taken to allow us to draw conclusions about colour polymorphism maintenance and the influence of urban stressors. A unique colour ring combination will make this individual identifiable for the rest of its life (Photo: Mayur Prag).

Black Sparrowhawks in the City of Cape Town are raising three chicks in the Constantia Greenbelt in view of many residents (Nest camera footage).

Nest camera in Fernwood – the female (dark morph) provisions food to her offspring.

Two juvenile Black Sparrowhawks in their typical rufous plumage. In all Accipiter hawks, the female (above) is significantly larger than the male (below) (Photos: Ian Junor).

Dark and light morph Black Sparrowhawks. Pairs that consist of contrasting morphs were found to produce more young than pairs consisting of the same morphs (Photos: Ann Koeslag).

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