Friday, 18 December 2015

Vulture nuptials

Adult Griffon Vulture (Martin Kelsey)
The Griffon Vultures were busy. As we stood before the vertical strata of quartzites at the Portilla del Tiétar in the magical Monfragüe National Park,  two dominant impressions started to pull on our senses. First the purposeful movement of Griffon Vultures, which was a striking contrast to their loafing behaviour as we arrived. Then the very border of the rockface was marked by hunched figures, perched vultures which, reptile-like, appeared to need to draw on the winter morning sun's insipid warmth. Slowly some spread their wings and tilted carefully to maximise their exposure to this energy. A few then rose, seemingly without effort, to rise in the fluid currents developing in the air, a medium of gradients and forces invisible to human perception. But most when taking off headed in a level, flapping flight to the hillside. This directed our attention there, andwe could see numbers of vultures dotted on the grassy slopes, hopping towards the wisps of retama bushes, whilst others had settled in the sturdy encina oaks.  There they tugged and grappled and minutes later, with the same detemined flight path in reverse, they approached the cliff with foliage-covered branches in their bills. Their legs dropped down, the flight braked and with amazing accuracy, perfected a landing precisely  on a small ledge where an untidy platform of twigs lay and their mate stood to guard.

The second sensual impression was an assault of sound, a wheezing, baying and moaning which echoed across the small gorge. Unlike the barrage of sound typical of seabird colonies, this could be pinpointed to specific locations. There one beheld the spectacle of vigorous copulation between these massive birds. The male perched on top of the female, their tails swivelled to ensure the meeting of cloacas, This was not the split-second flurry typical of most birds. It was a heaving noisy union which lasted impressively close to a minute.

Mid-December and the breeding season of the Griffon Vultures was already underway. Indeed as we scanned the gorgeous architecture of these ancient rocks, we found at least one pair of birds where whilst one stood the other was sitting tight on the nest, apparently already incubating their single egg. My friend, Ángel Sanchez, who is a senior conservationist in the government of Extremadura, as well as being an excellent ecologist and naturalist, has described the fascinating ecological differences that drive the different breeding cycles of our two largest carrion feeders: the Griffon and Black Vultures. Griffon Vultures feed on the carcases of large herbivores, be they wild (deer or boar) or livestock such as sheep or cattle. For these species the mortality rates are highest in late summer to early winter. Black Vultures, on the other hand, nesting in trees and colonial in only a very loose way, and tending to be more solitary by nature, evolved to feed on smaller prey, especially rabbits. On a visit this week to the Lacañada Hide where animal remains had been laid out to lure-in vultures, it was the Griffons which poured in like a frenzied scrum to disintegrate a whole carcase in minutes, whilst the Black Vultures moved in to feed on the scattered remains and morsels that the Griffon Vultures had disregarded.
Black and Griffon Vultures (Martin Kelsey)
Rabbit mortality is higher in the spring, thus Black Vulture breeding activity occurs much later than that of the their cousins. Human activity is changing this relationship as the vulture species benefit from both the increased number of livestock grazing on the vast tracks of Extrenadura's pseudosteppes and dehesas and the intensification of large hunting estates, where it is estimated that during autumn and winter a thousand tonnes of animal remains from deer and boar are consumed by vultures, more than a third of their annual requirement. This, Ángel says, is underwriting the needs of the Griffon Vultures at the onset of their breeding and substituting for the diminishing rabbit populations for the Black Vultures. Hence the remarkable increase in the population sizes of both species, especially dramatic in the case of Black Vultures which in forty years have increased from about 90 to over 900 pairs in Extremadura, with concentrations in the Monfragüe region and the Sierra de San Pedro.

No comments: