Believed to be at least third and fourth generation eagles since they were first observed to breed in the gorge adjacent the Witpoortjie waterfall during the 1930’s the present female and male is estimated to be approx. 25 and 15 years old respectively. The young male replaced the previous male during April 1998 when he mysteriously disappeared whilst the pair was incubating two-week-old eggs.
From their first recorded observation up until the early 1970’s the pairs’ greater home range may have been approx. 10-15km² which is deemed the norm in ideal circumstances. However, from mid-1970 to the present (2000), the environs have altered considerably with encroaching urbanization rapidly transforming the once pristine Rocky Highveld Grassland into roads, residential and commercial development. It was much later during mid-2003 that we established (through landowner communication) that the eagles occupied a home range of approx. 250km² which is indicative of abnormal conditions directly associated to habitat loss.
In 1982 the Witwatersrand National Botanical Garden (renamed WSNBG in March 2004) was founded but had been a popular venue for outings since the 1800’s (SANBI 1982). With the establishment of the WSNBG also brought increased visitors, traffic and noise pollution and by 1990 development was already on the doorstep and in view from the eagles’ nesting site. Fortunately, the approx. 300ha WSNBG creates a buffer zone between that and development but by 1991 its closest boundary to development was a mere 300m distant, which is extremely close to the nest and it was evident that this ‘human invasion’ caused the pair considerable stress and it became evident that they were in dire trouble.
In 1993, I became a member of the Black Eagle Monitoring Project that was later renamed the Black Eagle Project Roodekrans (BEPR) in 1997, a non-profit non-government Section 21 company. I was project coordinator from 1998 to 2000.
The 1990’s and early 2000’s was an extremely difficult period for the pair as development continued unabatedly resulting in the increased destruction of immediate hunting habitat resulting in reduced prey availability that escalated during their annual breeding season that usually commences in March – notably earlier than non-urban species in the wild. Prey stress (the inability to hunt wild fare) had become increasingly evident during their 1993 breeding season when free-range chicken became a regular item caught and landed on the nest. Often recorded was that the fact that neither eagle caught anything substantial despite having hunted all day only to return to the nest with a paltry feral pigeon or scavenged meat from an unknown source. On numerous occasions such was their inadequacy that it caused major conflicts of aggression towards another on and off the nest, in so much that a young 40-day-old eaglet was almost shoved off the nest by parental aggression. Irregular small prey items can never sustain two adult eagles and their young adequately and whereas other same species in the wild usually hunt one hyrax per day, this pair pushed it to the limit, feeding only once in two to three days, which is highly abnormal. The chicken-factor posed the biggest problem to overcome as not only will a chick growing up to juvenile and fledgling stages imprint upon such an item deeming it as “normal” but an irate farmer/land owner will certainly destroy it without hesitation.
The BEPR increased their focus upon the problem at hand and some members spent at least 90% of their free time observing the eagles’ every move especially during the 44-day incubation period and 3-month post hatching period. Of importance is to record the direction the eagle(s) departed into, how long they were away for, what mammal/bird was hunted, from which direction they returned, and had they fed on the prey prior to returning to the nesting gorge. This data inclusive of other equally important information is collected on formatted sheets for later analysis.
Artificial supplementation generally consisted of a mammal or large bird carcass that included the occasional hyrax, Helmeted Guineafowl or semi-domesticated rabbits that were purposely bred by rabbit farmers for slaughter and provided to selected butcheries. Very important was to source the pelt colour of each rabbit to closely resemble that of natural prey i.e. Jameson’s Rock Rabbit and Scrub Hare that the eagles also hunted from time to time. All the carcasses were weighed, tagged, placed in plastic bags and frozen. On Thursday mornings a carcass of approx. 1.8-2.8kg would be taken out to thaw readying it for supplementation usually on Saturday mornings from 07h00 onwards before the eagle(s) left to hunt. A predetermined “feeding site” was selected that was out of view of the nest, residential homes and visitors to the botanical garden. Each carcass in its plastic bag is placed inside a purposely used rucksack by the project coordinator. The exact same clothing was always worn by the individuals who supplemented ensuring that the eagles recognized us in the field. Walking to the ‘attraction site’ that is situated about 250m from the nesting gorge, we were then in view of the eagles’ regular roosting tree. Depending on the situation at the nest (incubation or brooding a chick), there would always be at least one eagle at their overnight roost whose attention was attracted by us. As soon as the thermals are favourable, the eagle (in most instances the male) would fly in our direction whilst we headed back to the drop-off site.
The male always feeds in a methodical way; the first 5-8 minutes was spent plucking tufts of hair off the mammal’s pelt at the chest and abdomen area, then proceeds feeding on the head pecking the eyes out, tongue and eating all facial/ears/head skin until the skull is bare and was then decapitated. Tearing open the chest area, it would eat the heart, liver, lungs and kidneys. Gutting the mammal, he consumes the intestines but discards the colon and the stomach. Some of the flesh and bones are also consumed and after approx. 20 minutes of feeding (dependent on carcass size and weight), finishes showing a healthy crop. Cleaning his beak on a rock or nearby tuft of grass, he relaxes a little albeit continuously aware of his surrounds and our presence, occasionally ruffles his feathers, and then takes a firm grasp on the carcass. When the slightest breeze is favourable, with one talon take the remains taking off with the least effort gaining height and heads into the direction of the nesting gorge.
Depending what the situation dictated at the nest, if the female were incubating her clutch of eggs, the male would leave the prey on a nearby rock where the female will then go to feed and the male would then resume incubation. Should the female brood a chick, the carcass would be landed on the nest for her to feed the youngster and thereafter herself. Should a growing juvenile of about 50 days-old be on the nest, both eagles would have fed on the carcass prior to the remains being taken to the nest for the youngster to finish off.
Over the years from 1993 onwards the method of one artificially supplemented carcass per week proved to be very successful although a single carcass was never adequate enough to sustain the adult pair and an offspring, and the eagles were yet obligated to self-hunt alternative prey. In the least, they were guaranteed of a substantial meal and extremely uplifting too was the fact that free-range chicken became a less required prerequisite.
Considering that under ideal circumstances a Black Eagle pair and one offspring are able to consume approx. 350-400 hyraxes per annum, which equates to 90% of their prey species. However, in the Roodekrans setting, hyraxes accounted for a low 20% during the 1989 breeding season with 59% of prey items consisting of birds (Branfield & Branfield, Lambrechts & Verdoorn 1990). A far cry from its ideal of forty years earlier when hyraxes were known to exist in great abundance along the entire Roodekrans ridge system.
Whilst on a 4-month sabbatical from the BEPR, in March 1997 I took it upon myself to explore the ridge system in search of natural prey availability, and also to understand the reasoning why there is such a shortfall within what is deemed to be ideal hunting habitat?
Commencing my walk from a point where the pristine ridge system abuts residential development 6km southeast of the nesting gorge, heading into a north-northwesterly direction that would take to the nesting site then traversing an additional 17km beyond to where the ridge system ended on the outskirts of the Krugersdorp Game Reserve, a total distance of 23km.
Because of the remoteness of ridges my whereabouts were pre-logged and relatives/landowners notified of my intended route in that should I not return, they would know where to send the search party. It may sound like drastic measures but everyone is fully aware that there are unexpected risks involved that requires to be taken into account.
A considerable amount of data was collected during the weekend walks and occasionally was assisted by interested parties which were welcomed as it ‘killed’ the monotony and isolation of being out there on my own.
Raptor Conservation Project (RCP)
Ill at ease with the long-term negative impact that is associated with continued artificial supplementation and in October 2000, I resigned from the BEPR and initiated our own project known as the RCP to determine what natural species prevailed and the abundance thereof within the eagles’ greater home range.