The Black Eagle Project was established in 2011 in collaboration with the Animal Demography Unit at the University of Cape Town, and over the last two years the Cape Leopard Trust has been privileged to share its journey. Set up by PhD student Megan Murgatroyd, future research will continue to focus on the effects of land use on the diet and hunting habits of the Black eagle (Aquila verreauxii), with the field studies concentrated in the Cederberg Mountains and the Sandveld.
The outcomes of the project will enable a comprehensive assessment of the effects land use has had on the Black eagle and its primary prey, the rock hyrax (Procavia capensis). We look forward to following Megan's new discoveries and developments.
Distribution of the Black eagle
Black eagles occur through most parts of sub-Saharan Africa, as far north as Israel, where mountainous habitat correlate with a high density of their main prey, rock hyrax Procavia capensis. In southern Africa their most notable absence is from most of Botswana and eastern Namibia where there are no suitable mountains due to the Kalahari Desert. Perturbation of habitats through human influence has resulted in declining hyrax populations. This is considered a major contributing factor to eagle pairs being lost from former breeding areas.
Nests are normally built on rocky cliff ledges or pillars in hilly and mountainous regions. This preference means that the nesting sites themselves are naturally robust against human encroachment. Breeding activity commences in winter with nest building as early February. However, timing is highly variable according to region.
Egg laying in the Western Cape usually peaks in June. Generally two eggs, but sometime only one egg is laid. Incubation will last between 44 and 46 days. If both eggs hatch successfully 'cainism' will occur, whereby the first chick to hatch will out do its younger sibling for food and cause physical injury until death. The remaining chick will then take around 90-98 days to fledge and remain with its parents for a further 3 - 4 months. Once the young eagle is capable of feeding itself it will be forced out of the territory by its parents.
Black eagles in the Cederberg
In the Cederberg Mountains it is still a common sight to see a pair flying in a unique pendulum formation over the cliffs and territory that they defend. However, in other parts of the country their long-term survival might be threatened by radical change in land-use. In the Western Cape at present these eagles remain prevalent and breeding in the Sandveld. Yet this area is now classed as the second most highly threatened ecosystem in South Africa and at least 50% of the land has been converted for agriculture (C.A.P.E. 2008; Low 2004).
The Black Eagle Project takes as ecosystems approach. As a top predator this eagle is vulnerable to environmental disturbances at lower trophic levels. As an umbrella species, successful conservation of the Verreaux's eagle is dependent on the entire ecosystem remaining healthy. At the same time, the conservation of this top predator helps enable the ecosystem to remain healthy by preventing trophic cascades, the process whereby the loss of a predator enables a prey species to increase in abundance in such a way that the stability of the ecosystem is disturbed.
The primary aims of the project are to:
- Assess the breeding success of Black eagles within the two research areas.
- Assess the diet and habitat use of the Black eagle in both areas.
- Use the data obtained to create awareness and interest in this species.
- Engage and inspire landowners and stewards to direct conservation efforts to preserve the environment which these eagles rely upon.
Alongside extensive hours in the field, major research tools will include:
- GPS technology, which will examine the land use of Black eagles using high resolution tracking to enable an accurate assessment of the size of home ranges and habitat preferences or avoidance in the study areas.
- Remote cameras, which will be installed at nests to make a comparison of chick diet and food delivery rates. Prey caught in the field is both rarely seen and difficult to positively identify. This analysis will provide further data on the eagle prey base and explore the possibility of diet diversification in times of food scarcity in different habitats. The cameras will give additional insight into chick fate which is seldom recorded.
In additions to the research tools listed above, it will be important to investigate hyrax populations in both areas identifying the effects of land use on this prey base. It is documented in other parts of the country where the Black eagle's range, that hyrax form 70 - 98% of their diet. Therefore hiking surveys will record distribution and size of hyrax colonies.
Although classified as by the IUCN (2011) as a 'least concern' species, Black eagles have been threatened by habitat degradation and land use change. In various locations in South Africa declines in breeding pairs has occurred. For example on Table Mountain, Cape Town, actively breeding pairs have gone from seven down to just one in the last 20 years.