Black Eagle nesting density in the Southwestern Valley of the Erongo Mountains

by Katrin Denker

Introduction

The Erongo Mountains in western Namibia rise as a massive bulk several hundred metres, at times more than thousand metres, over a gap in the Namibian Escarpment which, in the form of an inclined plain, gradually drops towards the Atlantic Ocean (Blümel et al, 1979). With a diameter of about 40km the mountains cover an area of approximately 120 000ha.

Black Eagles (Aquila verreauxii) are very common in the Erongo Mountains. There is an abundance of suitable nesting sites, and the habitat is ideal for Rock Hyraxes (Procavia capensis), which are probably the most important prey animal of Black Eagles in the area.

The Black Eagles seem to rely on a healthy population of Rock Hyraxes. Following the remarkably good rains of 2005/06, the population of Rock Hyraxes in the study area increased dramatically and Black Eagles were seen very frequently. In 2010 an epidemic broke out amongst the Rock Hyraxes and they nearly vanished from the area. For several years, one hardly ever saw a Rock Hyrax and Black Eagles too were seen less often.

During the last few years Rock Hyraxes have become more numerous again and Black Eagles are often seen.

During (and before) the period when Black Eagles and Rock Hyraxes were so numerous, several nesting sites were discovered, some of which are still in use now. Some others, not known then, have since been found.

During the 2024 breeding season my father and I visited all known Black Eagle nesting sites, as well as some areas where Black Eagle nests were suspected, in order to determine the mean inter-nest distance and the nesting density.

Study area

The Southwestern Valley of the Erongo Mountains.

The Southwestern Valley of the Erongo Mountains with its surrounding mountain slopes, 110 km² (11 000ha) in size, was covered by this study. The valley leads out of the main basin of the former volcano towards the Southwest and lies between the basalt mountains of the former crater rim in the North and the granite, surrounding the Erongo Mountains, in the South. The floor of the valley lies at a height of 1100m above sea-level and the surrounding mountains reach up to a height of just above 2000m above sea level. The mountains were once under the Earth’s surface and were only exposed by erosion over millions of years.

The basalt mountains have massive cliffs and the materials that broke out of these, cover the steep slopes leading up to the cliffs with loose rubble. The slopes are mostly vegetated with thorn bushes. At the foot of the cliffs, the vegetation is denser, there are few thornbushes here and grasses grow thickly. Different tree species (especially figs and shepherd trees) and some grasses and shrubs grow out of the cliffs themselves. Rock Hyraxes are found at the bottom of the cliffs as well as Jameson’s Red Rock Hares (Pronolagus randensis caucinus) and Klipspringers (Oreotragus oreotragus).

The basalt area.

The granite area is in part comprised of sheer granite, in part of mountains made up largely of boulders, and has many cliffs, although these are not as high as those of the basalt mountains. Islands of bush and grasses are scattered over the granite. Bushes and trees grow amongst the boulders. The edge of the granite is bordered by thickets in most places. The granite thus provides an ideal habitat for Rock Hyraxes, which find plenty of food and cover here. Klipspringers are also very numerous in the granite areas.

The valley itself is mostly covered by thornbushes, but there are also some open areas. Damara Dik-diks (Madoqua kirkii damarensis), Cape Hares (Lepus capensis) and Francolins are likely prey species of Black Eagles that are common in this area.

The granite area.

Methods

Every nest was visited on foot at least once to determine whether it is occupied. The nests were then all plotted on a 1:50 000 topographic map of the area. The distance between the plotted nests was then measured and the distance between the actual nests calculated.

To obtain the average distance between the nests, the distance to the three closest nests to each site was used.

Results

Seven occupied nests were recorded. The mean inter-nest distance was found to be 5.26 km, ranging from 1.95 km to 8.00 km.

The study area extends over about 110 km2 (11 000ha). The breeding density is therefore one pair per 15.7km2.

These results, compared with the information on breeding density or mean inter-nest distance of Black Eagles that we were able to find, make the Black Eagle population of the Erongo Mountains quite remarkable.

The highest breeding density known for Black Eagles is that in the Matopo Hills in Zimbabwe where the mean inter-nest distance was found to be 2.3km (ranging from 1.3km to 4.5km) and Black Eagles breed at a density of 1 pair/10.3km2 (Steyn, 1982 and Gargett in Boshoff and Palmer, 1988). In the Nuweveld in South Africa Boshoff and Palmer (1988) determined a mean inter-nest distance of 4.25km (ranging from 3.0km to 7.5km). The mean inter-nest distance in the Waterberg (South Africa) is 13.3km (with a range of 5.0km to 21.0km), that in the Magaliesberg (South Africa) 9.5km (range: 3.0km to 19.5km) and that in the Soutpansberg (South Africa) 6.8km (ranging from 2.0km to 14.5km) (Tarboton & Allan in Boshoff and Palmer, 1988).

The only data so far known for Namibia is a distance of 25km to 30km between nests at the Waterberg (Brown & Cooper and Simmons in Simmons et al., 2015).

The female incubating at the Trotzkopf nest.

Nests

During the breeding season 2024 we recorded seven occupied nests which are described below. The nests are situated on both sides of the valley, along two roughly parallel lines (See Figure 1).

Figure 1: Rough map of the study area and position of nests.

There is one more site in the granite area, which was probably occupied, but which we could not confirm and on the basalt side there is another large area which is very well suited for eagles to breed in, but during the limited time we had available when visiting this area, we did not find any nests there.

Site 1, Lӧwenkopf

Built on the steep cliff of the area’s highest mountain, this nest is remote and entirely inaccessible to anyone without wings. It can only be watched from a distance from the mountains below which makes it difficult to make detailed observations or to see the chicks. The nest is built at the base of the stem of a small tree, growing horizontally from the cliff, possibly for a more secure hold.

One of the adults sitting on the nest on 10 July 2024, seemingly bending down to feed a chick.

The eagles were seen at this nest several times on 10 and 11 July, sitting on the nest or on perches close by or circling in front of- or above the cliff on which the nest is situated. On one occasion they landed on a tree at the top of the mountain, probably a Shepherd’s Tree (Boscia albitrunca), possibly to break off twigs with which to line the nest. One of them then, after circling in front of the cliff, above the nest, dropped to below the nest, swooped up and landed on the nest. The other one followed. On the following day, one of the eagles, continuously came to the nest, left, and came back again. Once it seemed to fly away from the nest with something in its talons, circled for a short time and then came back to the nest still carrying the object. Once, when standing on the nest, it seemed to bend forward as if it was feeding a chick.

The three nesting sites closest to this nest were 3.75km, 5.55km and 6.75km away. The average distance to the closest nests was thus 5.35km. The nest itself is situated at a height of about 2000m above sea level.

Site 2, Kronenberg

This nest is situated in a kind of “window” in a relatively small cliff. It looks out along the valley, towards the desert, facing West. It is thus left in the shade for most of the morning and the eagles only seem to become active relatively late in the day. The nest was first seen to be occupied on 24 May, when an eagle seemed to be incubating, and was visited again in July when the eagles were seen sitting on or by the nest on several occasions. This nest too can only be watched from relatively far away, making observations difficult.

One of the adults perched below the Kronenberg eyrie.

The closest nests are 3.75km, 4.45km and 8.00km away from the Kronenbergnest, 5.40km on average.

Site 3, Trotzkopf

The Trotzkopf has a “nose” protruding from the rest of the cliff. If one climbs up there, one is quite close to- and a little above the nest, and although the sides of the nest are too high to see eggs or small chicks, it is very well situated to make observations. The slope leading up to the cliff, however, is steep and covered with rubble that broke out of the cliff over the years, making the nest difficult to reach.

We visited it on 20 June. On the way up we saw a Black Eagle fly away from the nest. It circled above us as we came closer and when we sat down at a spot from where we could see the nest. The eagle occasionally looked at the nest but did not alight there. After circling for some time, it flew down to the foot of the cliff, landed on a Shepherd’s Tree growing there, and after hopping about on the top of the tree for some moments broke off a spray of green and flew away, the twig in its beak. After some more circling, it landed on the nest, dropped the twig into the nesting mould, and then, carefully placing its large feet, settled down to brood. After a while it started circling again and we left so as not to disturb it too much.

The female alighting at the Trotzkopf nest.

This nest is 4.50km, 4.45km and 6.75km (on average 5.23km) away from the closest neighbouring sites.

Site 4, Horn

This nest is placed on a remote granite cliff facing North so that it gets full sun for the entire day. This may be an advantage on cold days, but one cannot help pitying the poor juvenile towards the end of the breeding season, when the days get warmer, and it sits without the least bit of shade and unable to leave. It saw us when we came to watch the nest for a while (although one cannot come very close) and flattened itself onto the nest, next to the cliff. After a while it sat up again, preened, and then turned its back towards the sun, leaned its shoulders against the granite and pulled its head into the shade of the body, only occasionally looking out at the side.

The chick on 24 July at approximately 7 weeks.

The fully feathered eaglet on 12 August.

A pair of African Hawk Eagles hunted Grey Louries in a thorn tree below, a Rock Kestrel glided along the cliff and past the nest, but we saw nothing of the adult eagles. The chick occasionally looked up at the sky as if it was expecting them now, but it was a while before they came. We were already on our way back then. One of the adults brought an unidentifiable prey item to the nest. After circling above for a while, they again disappeared. On 21 September we again visited this area and saw the fully fledged juvenile soaring in front of the cliff and then alighting on the rocks just next to the nest.

This nest is only 1.95km from the nearest neighbouring nest and 5.55km and 6.45km from the two next closest ones (on average 4.65km).

Site 5, Jonathans Berg

This nest is situated on a granite wall overlooking the valley, just beneath a Rock Splitting Fig, and half concealed by a large bush. There are no other high vantage points close by so that one cannot look into the nest or make observations from nearby. There is fresh whitewash however and the adults could be seen sitting on perches next to the nest for long periods.

An adult bird sitting on the Rock-splitting fig above the nest.

This site is the one nearest to other nesting sites 4.32km on average and 1.95km, 4.55km and 6.45km from the nearest neighbouring nests.

Site 6, Rondekop

We visited this nest in mid-July. It is situated on a granite mountain with a steep cliff towards the East and built on a small ledge on this cliff, away from any vegetation. There is a kind of granite roof some distance above the nest, under which small trees grow out of the cliff, on which the eagles like to sit in the shade. One can see the nest quite well from the slope of an opposite mountain, but it is not possible to look into the nest and one cannot get close enough to make detailed observations.

An adult bird (arrowed, top) sitting on a bush above the nest (arrowed, bottom).

We saw one of the eagles, sitting on the tree under the roof, from far away when we started walking towards the nest early in the morning. The path followed a dry riverbed, led through bushes, over some boulders and then, after crossing the fresh drag marks where a leopard had pulled its kill across the ground, up the slope opposite the nesting site. The Eagle was still sitting where we had first seen it in the morning, when we reached a suitable point for watching the nest and it remained there for a little longer. After some time, it started soaring and a little later the second eagle appeared. It may have come from the nest, it came from that direction, but we were following the flight of the first eagle, and not watching the nest, when the second eagle appeared. After circling above the cliff for a while, the eagles disappeared behind the mountain and did not return while we were there. We did not remain for very long however, as we were on our way to a valley some way on, where we thought another nesting site might be found.

One of the eagles sitting under the granite roof.

The three nests nearest to the one at the Rondekop are 4.55km, 6.45km and 6.85km away and the average distance to these nests is 5.95km.

Site 7, Zacken

None of these nests are easy to reach, but the way to the Zacken always seems to be particularly tyring. To even reach a spot from where the nest can be properly seen, one needs to cross the valley, behind which the cliffs lie, and which is covered by an awful maze of thorn bushes, and then climb up the steep, rubble-strewn slope leading up to the cliffs. However, the cliffs are impressive, and as they face South, blocking out the sun for most of the winter, and seem to get more rain than the rest of the area, they create a different, remote world, with small flowers, dense grasses and bushes that are rare below. And the cliffs themselves do not only provide breeding sites to eagles but are also home to a pair of Peregrine Falcons.

We only visited this site in the middle of August. There are four nests in the cliffs, one further away (which did not seem to be in use) and three situated close together. One of these three seemed to have fallen down and one looked as if it had a new layer of nesting material.

One of the eagles after depositing a branch at the place where one nest had fallen down. The nest which seemed to have a new layer of nesting material is visible further down.

In the late morning two Black Eagles came flying towards this site. One of them had a branch in its talons and landed at the place from where the one nest had fallen down. Both eagles again disappeared after some time and we only saw them, sitting in the cliff, some way below the nesting site, later.

There is one record where Black Eagles rebuilt their nest late in the breeding season after their first nest had fallen down (Steyn, 1982). We thought that the eagles at the Zacken might also be rebuilding their nest after a breeding failure, but when we again visited this site on 10 October they were not seen at the nest and it did not seem as if any nesting material had been added.

This site is 4.50km, 6.45km and 6.85km from the nearest nests and therefore 5.93km from the neighbouring nests on average.

References

Blümel, W.D., Emmermann, R. and Hüser, K. (1979). Der Erongo. Verlag der S.W.A Wissenschaftlichen Gesellschaft: Windhoek.

Boshoff, A.F. and Palmer, N.G. (1988). Black eagle nest spacing and estimated territory size in a Karoo habitat. S.Afr. J. Wildl. Res. 18(2). [Online]. Available at: Black eagle nest spacing and estimated territory size in a Karoo habitat: Short communication (journals.co.za)

Simmons, R.E., Brown, C.J. and Kemper, J. (2015). Birds to watch in Namibia: red, rare and endemic species. Ministry of Environment and Tourism and Namibia Nature Foundation: Windhoek.

South West Africa 1:50 000 Sheet, 2115DA Ombu. First Edition.

South West Africa 1:50 000 Sheet, 2115DC Usakos (North). First Edition.

Steyn, P. (1982). Birds of Prey of Southern Africa. David Philip: Cape Town.

All seven eyries documented in the Southwestern Valley (arrowed)