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Black Harriers – Ecology and Fitness

The Black Harrier Circus maurus is among the rarest raptors in southern Africa. At an estimated 1 000 individuals it vies with Botha’s Lark Spizocorys fringillaris as South Africa’s least common endemic. With previous research indicating it shows no mtDNA genetic diversity, this 18-year research programme seeks to determine why the species is so rare and what conservation measures are required to safe-guard its future in the modern world.

This migrant and nomadic species spends the winter and spring in the Fynbos biome breeding after the winter rains. As the hot summer sun dries out its foraging grounds in the west, it wings its way across the arid Karoo to the green highveld grasslands of Lesotho, the Free State and the Eastern Cape. Satellite-tagging studies show this journey of 1 000 km is taken at speeds averaging 60 km/h and birds can be in the Drakensberg four days after leaving the Cape Fold mountains and Langebaan breeding areas. Unlike most other raptors, the Black Harrier shows low fidelity to its breeding grounds and may breed up to 500 km away from its previous years nest – a unique behaviour amongst Africa’s raptors.

Significant progress has been made over the last four years in understanding the reasons for Black Harriers’ scarcity, with the findings from Marie-Sophie Garcia-Heras’ PhD research. She conducted fieldwork during the 2012-2015 breeding seasons in two geographical regions: one coastal in the Western Cape Province, and the second in the hills of Nieuwoudtville in the Northern Cape. Historical data collected by Rob Simmons covering 13 years prior to that allowed a large sample of 490 nests to form the basis of this research.

The main questions to be answered were: where is it most productive? (so those areas can be conserved); what health factors may be limiting it ?(such as chemical pollutants) and do climatic factors play a role in its rarity ?(climate change is predicted to increase temperatures and decrease rainfall in its core breeding areas of the Western Cape). Marie-Sophie’s main findings are:

  • The importance of coastal regions (especially the fynbos biome) was highlighted because productivity in this habitat was almost 3-fold higher than in inland areas. This is because breeding starts earlier, lasts longer, nesting occurs at higher density, and food supplies are consistently higher.
  • Climatically cool weather conditions are more conducive to day-long hunting; birds breeding in the hotter interior provisioned fewer mice at midday in warmer temperatures than those at the coast in cooler conditions. This may have arisen from the reduced activity of the mice or the harriers themselves; this climate signature suggests that climate change in their breeding areas may have a negative impact on the birds in the long term.
  • Neither provincial nor private reserves offered protection against persistent chemical pollutants such as DDT and PCBs – some of the highest levels of PCBs were found in the Koeberg Nature Reserve and other reserves to the north;
  • Blood samples indicated that 79% and 84% of 114 nestlings and adults tested were contaminated with PCBs and DDT respectively. As banned chemicals in the Western Cape these results for DDT (in chicks) provide a conundrum – are these pesticides still used or is there an unknown pathway into the Black Harrier population?
  • For PCBs (used in electrical transformers) a possible pathway was uncovered – nests with higher densities of electrical transformers within a 5 km radius showed significantly higher levels of contamination. The transformers may be leaking PCB oils into the environment.
  • Sub-lethal effects of these pollutants were picked up in assays of stress hormones for chicks and adults indicating high Heterophyl to Lymphocyte white blood cell ratios. This suggests that the immune system iscompromised at high PCB levels.
  • This also had a knock-on effect in the carotenoid colouration of the cere and tarsi for chicks and adults. Individuals with high PCB levels showed paler, less rich yellow colouration, which is known to be an important signal of health and vigour in harriers.


  • Marie-Sophie graduated with a PhD in July 2017 and has had five papers published in high impact factor peer-reviewed journals.

Impact of the project

Marie-Sophie’s findings suggest that despite 50% of the Black Harrier population occurring in protected areas, the species is under pressure from the genetic to the population level and conservation action is critical if the species is to survive. These findings were recently presented to BirdLife South Africa and the Endangered Wildlife Trust, and an Action Plan is being implemented with both organisations to:

  • highlight to Eskom the need to replace leaking transformer boxes throughout the range of this species; (this will benefit other raptor species too)
  • focus conservation efforts in the most productive coastal breeding areas and over-summering areas in high occurrence areas of Lesotho and the highveld grasslands – these might be IBAs or new areas identified through Maxent modelling;
  • reduce fragmentation of pristine fynbos habitat as one of the chief limiting factors from previous work in this programme;
  • understand what is the source of the DDT contamination in the Western Cape and reduce its impact through legislation or better monitoring.

If any or all of these conservation initiatives can be achieved, then the Black Harrier’s bleak future could be turned around.

Key co-sponsors

National Research Foundation; Golden Fleece Merinos; BirdLife South Africa, Tygerberg Bird Club.

Research team

Dr Rob Simmons (FIAO, UCT)
Dr Beatriz Arroyo (CSIC, Spain)
Dr Francis Mougeot (CSIC, Spain)
Dr Raphael Mateo (CSIC, Spain)
Pablo E. Camarero (CSIC, Spain)
Dr Graham Avery (Iziko Musuems)
Dr Margaret Avery (Iziko Museums)
Marlie Martins (Birds & Bats Unlimited)

Student: Marie-Sophie Garcia-Heras (PhD, UCT)