As a child I was captivated by accounts of Gerald Durrell as he traversed the world to increase his collection of wild animals. Living in South Africa his African excursions resonated with me. Tales of local residents arriving at his camp or accommodation with bundles of trapped animals which were bought depending on their health and condition.
After six years of conducting extensive bird surveys on both proposed and completed wind farms in South Africa, Amanda and I have striven to collect precise and comprehensive avifaunal data, particularly of the priority species.
The process of avifaunal surveys on wind farms function to identify the species present, an estimate of population size and behavioral traits of priority species. The data is collated, analysed and a proposal of wind turbine array that will mitigate fatal collisions is presented to developers
In our arsenal of monitoring tools we combine digital equipment like Cybertracker, QGIS, and GPS to digitize the behaviour, location and flight paths so that collision mitigation procedures can be planned from solid objective data.
One of the species that has become a favourite and a concern is the Verreaux’s Eagle. As nobles of the mountain peaks of the greater escarpment, they are maestros of updraft slope and ridge soaring with the precision of an acrobat.
Their flight is defined by two factors, namely the prevailing wind and the topography of the landscape. As a pair, they will fly from their roost and ridge soar along a cliff. Their flights can be distinguished into three loose categories, namely hunting, aerial display and ‘leisure’.
For aerial display and leisure both male and female will ascend from a cliff face and gain height on an updraft. Depending on wind strength display swoops can be performed between 100m and 1000m above ground level and can extend over valleys and plateaus.
Leisure flights, also influenced by wind strength, are conducted between 75m and 800m above ground level, more often following a ‘corridor’ of updraft defined by ridges and cliffs. While these flights are mostly flown on the contour updraft, they are not restricted to them as a pair will glide across valleys and between peninsulas.
True soaring is rarely performed and we conclude that this is due to the probability that in strong wind regions with undulating topography thermal updrafts are less developed compared to a flat landscape in light wind conditions.
The synchronized performance as a pair skims a cliff hunting for Hyrax or small mammals on a slope is spell binding. It is as a pair fly in formation along a cliff face that the size distinction between sexes is blatantly apparent, the female seemingly double the size of the male. This definitely translates to greater lift by the female with her easily lifting prey that a male is unable to. Again, topographical updraft plays a substantial role in the pair being able to lift prey to be carried back to their roost site or nest.
This week we were fortunate to witness a male Verreaux’s Eagle successfully kill a Hyrax. It was early morning in calm conditions halfway down a rocky 40m slope. He ate for an hour consuming half the prey before attempting to fly off. The carcass proved too heavy for the male and in the calm conditions he was unable to take off.
With a labored low flight he relocated to a different section of the slope. For the next 40 minutes he faced down the slope testing the developing light SE wind.
Then something amazing occurred. He relinquished the carcass, took off and spiraled upwards to 30m above the cliff. It was at this time that we witnessed the female arrive from the SE at about 80m above ground level. She descended to 60m while the male rose up to the same height. They spiraled together for thirty seconds before she descended to the carcass, closely followed by the male. After he presented her with the carcass she fed lightly (her crop was full indicating that she had eaten already). She then lifted the carcass and relocated to a higher position up the slope.
They rested for a few minutes before they took off together, the female carrying the carcass, spiraled to gain height to 100m when they then took off on a straight course for 5.2km towards the area we suspected the presence of their nest.
There are a few amazing things witnessed during this encounter, namely the female finding the male nearly two hours after he made the kill. Secondly that the male waited for the female. Also that their departure with the carcass coincided with the onset of a light wind to assist with their flight.
So while Mr Durrell’s collection methods may not have permitted him witnessing such an astounding interaction, we are left contemplating how to determine what made it possible.
Such are the advances of technology that studies now can approach a comprehensive understanding of the essential nuances of the natural world and animal behaviour without a scalpel or cage.