Most South Africans may have never noticed one of the largest forest raptors breeding in the city of Cape Town – the black sparrowhawk . However a new research project from the Percy FitzPatrick Institute of African Ornithology has hopefully changed this, and is shedding light on interesting aspects of the ecology of this species.
The black sparrowhawk (Accipiter melanoleucus) occurs in two different plumage colours: it has either light or dark feathers on the front. Colour polymorphism is one of the mysteries in Darwin’s idea of the “survival of the fittest”. Many animals, especially birds of prey, occur in different skin, fur or plumage colours, but an unanswered question what is the advantage of being of one colour or another? Natural selection ought to favour the survival of one the colour morphs. Researchers from the Percy FitzPatrick Institute of African Ornithology have been working on the phenomenon of colour polymorphism for many years, their long-term project focuses on the black sparrowhawk, a raptor species that recently colonised the Cape Peninsula and is breeding in forest plantations. Their study published in Journal of Animal Ecology investigates if parents of different morphs might affect the fitness of their offspring differently depending on local conditions, and whether this might explain why both plumage types are maintained within the population.
The results suggest that offspring survival and recruitment into the breeding population did indeed differ between the morphs and on the local weather conditions experienced during the breeding season. Young were more likely to enter the breeding population if they had a dark morph father but only if they were reared at the start of the breeding season, by the end of the season this advantaged had reversed and offspring from light morphs fathers were now favoured. These relationships are likely to be due to difference in the weather condition between the early and late breeding seasons, and the hunting success (i.e., prey provisioning) of the fathers.
Dark morph males are able to hunt more successfully in rainier and cloudier (less light) conditions, which are the conditions experience earlier in the breeding season. In contrast light morph males may be more successful later on, when weather conditions become brighter and drier. These differences in hunting success and light levels are thought to be a result of the different morphs having better camouflage under these different light conditions, which dark morphs being more concealed in darker conditions.
Raptors have traditional roles when raising their young, the female spends more of her time on the nest, keeping the chicks warm and defending them from predators. During this time, the male is responsible for providing the food for both the female and the chicks. It is therefore not surprising that it is the father’s plumage colouration, rather than the mother’s colouration, that appears crucial in the subsequent success of the offspring, depending on the light conditions in the environment.
The scientists from the University of Cape Town have been monitoring all the active black sparrowhawk nests around Cape Town for over 15 years, but they have also relied on the general public to report new nests or resightings of individually colour ringed birds, which was crucial for the analysis exploring survival rates. More information about Black sparrowhawks of southern Africa is available at the website and Facebook page.
Percy FitzPatrick Institute of African Ornithology