Saturday, 20 February 2016

Eagle combat

Spanish Imperial Eagle and Griffon Vulture (Tom Wallis)
As winter closes it is eagle time and the pair of Spanish Imperial Eagles at the Portilla viewpoint in the Monfragüe National Park are at their most flamboyant - and the Griffon Vultures, now incubating on the nests on the ancient quartzite outcrop must think they have neighbours from hell! Sometimes it must be triggered by a perceived and instinctive sense of threat. As a vulture glides close to the vicinity of the eagle's nest site, a blunt barking warning call is heard, followed almost always by the appearance of the eagle to mob the larger vulture. But sometimes, to our eyes there seems no provocation and as we watch the Spanish Imperial Eagles and Griffon Vultures, rhythmically and gently spiralling together in the uplift, it becomes an almost therapeutically relazing sight. Suddenly this is broken by some invisible stimulus which leads the eagle into a diving mobbing attack onto a hapless vulture. My companions that morning were Tom and Mary Wallis, and Tom took the remarkable photo above of the Spanish Imperial Eagle appearing to be holding onto the back of the Griffon Vulture with its talons, whilst the vulture peers over its shoulder in shock. The vulture is pushed sideways by the force of the eagle's landing and they then lock talons and catherine-wheel downwards, parting just above the tree-tops. These assaults are not one-offs, indeed during our watch there we witnessed several, which must be partly the explanation for the decidedly ragged appearance of the flight feathers of some of the vultures present. The most dramatic was a dive-bomb by the eagle on one vulture with such force that both plunged together like a leaden bag, dropping from the sky to disappear into the trees. The eagle reappeared quickly but it was a couple of minutes before the Griffon Vulture emerged, looking somewhat unsteady.  This release of aggression in unleashed many time a day, every day upon the vulture neighbours, which one can only admire for their stoicism.

Spanish Imperial Eagle grabbing the secondary feathers of Griffon Vulture (Tom Wallis)
In the space of an hour, the Spanish Imperial Eagle not only entertained us with their squabbles with the vultures. They were busy too in preparation for their own breeding season, with several visits to the tree hosting their nest to bring in branches that had been collected in the vicinity. The bird would spend several minutes organising this addition to the structure, even if on one occasion, the branch was so big it got snagged in the canopy of the tree and there followed a less-than dignified yanking to free it up. Twice during our watch, the larger female flew to a prominant outcrop and awaited the approach of the male, whose arrival in the sky above was advertised by his barking, followed by a stoop to land right beside her.  After a bit of shuffling, he then mounted her for in avian timescales, a quite prolonged mating, whilst she flattened herself against what inevitably became known as the copulation rock.

Spanish Imperial Eagle pair on the "copulation rock" (Tom Wallis)
Mating Spanish Imperial Eagles (Martin Kelsey)
The resumption of the breeding activities of this pair of Spanish Imperial Eagles has coincided with the publication of the results of the 2014 bird of prey census for the Monfragüe National Park and its surrounding buffer zone, Monfragüe Biosphere Reserve. Further information and analysis is given in Spanish in the blog by Javier Prieta. The headline news from this was the evidence that the population of Black Vultures now stands at 354 breeding pairs (307 within the area of the National park itself), making it the the most important area for the conservation of the species (in terms of breeding density) in the world. The Spanish Imperial Eagle's resilient neighbours, the Griffon Vultures, were shown to have 600 breeding pairs (with a further 56 outside). However this was overshadowed by the worrying figures of the large resident eagles. The Golden Eagle population has fallen to four pairs in the park (with another four in the buffer zone, Bonelli's Eagles likewise have four pairs in the park, whilst the Spanish Imperial Eagle now stands at five pairs inside the park and five in the surrounding zone. Within these figures are data which allude to the problem that these eagles face. All three species are showing very poor breeding success. Only three of the Spanish Imperial Eagle pairs bred successully, raising a total of four young. The success story of these eagles in the extraordinary recovery since the 1960s now seems severely compromised by low reproductive success.

The biggest threat to these species is the collapse of one of their key food sources, the rabbit, whose population in Spain has crashed, mainly because of RHD (Rabbit Haemorrhagic Disease) which first appeared in Spain in 1988. The conservation status of rabbit is now classed as Vulnerable. Rabbits are keystone species in the Mediterranean ecosystems, with two emblematic species, the Spanish Imperial Eagle and Iberian Lynx being highly specialised as rabbit predators.

I watched the combat now between the Spanish Imperial Eagle and the Griffon Vulture with a different light. No longer was the vulture the weaker victim, it was the eagle that bore fragility, vulnerable not to the outcome of aerial battles, but to the impact of the workings of an invisible virus.

 

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