Tuesday, 22 July 2014

As the vulture glides

Griffon Vulture (Raymond de Smet)
Calm, long summer days, not a cloud in sight, nor the brush of a breeze. Against the vastness of the blue dome there is just one movement that catches my eye. Approaching from the north, the object moves on an unwavering course, a flight path on fixed bearing, as it were. There is no sign of propulsion. The glide lasts across my entire view of the sky. Its wings, broad but tapering slightly towards their tips, show no motion. Their shape are sufficient to identify this as a Griffon Vulture, and as this individual disappears from view, two others are overhead, again as if drawn by invisible threads. On such a day, vultures are finding thermals rising from the plains. Up currents of air, triggered by slight temperature gradients are somehow located by soaring birds: the vultures, eagles, storks. It is by the presence of such birds, rising in spiral fashion (described often as a "kettle") that we detect these otherwise invisible pumps of air, acting like vast elevators for these birds, an easy lift to take them a thousand metres or more. Not for the first time it challenges my earth-bound perception of air and sky, birds feel the atmosphere so differently to us, as a fluid medium, beyond the gust of winds to flowing currents. Reaching the top floor of the thermal lift, the birds then do the next extraordinary thing... they step off and enter the glide with that dynamic mathematical relationship between gravity and lift, a controlled descent, stretched for kilometres until the next thermal is found. The bird passing overhead may have originated that morning from colonies over 50 kilometres away and will have made that journey by this wave-like pathway of vertical rises and gradual descents - quite possibly the only time it flapped it wings would have been the initial take-off from the cliff, an energy-lite expedition.

Griffon Vultures are resident here, but that bare statement obscures the fluid nature of the population in Extremadura, much as we cannot see either the atmospheric infrastructure that provides the means for these movements. Sightings of wing-tagged birds indicate that amongst the Griffon Vultures there are birds from colonies from right across the Iberian peninsular. Griffon Vultures take five years to reach adulthood and between two and four years of age they show considerable disperal behaviour, with movements especially noted in winter and spring (there is a tendency for northern birds to head southwards in the autumn). These journeys will have been taken with minimal exertion, deploying the thermal and glide technique to save energy, allowing some of these vultures, along with other gliding species, to make the crossing thus over the Straits of Gibraltar as well.
Griffon Vulture fledgling (Martin Kelsey)
Recently I stood in the Monfragüe National Park, the source no doubt of many of the vultures that glide on their forays over our house, and watched what might have been the first flight of a juvenile Griffon Vulture - a heavy, unsteady, deep flapping took it across the river to the rock below where I stood. The close up photograph above shows its whitish, close-cropped down on head and neck, yet to be stained by foraging on livestock carcases. It stood, I thought, slightly perplexed - sensing achievement but uncertain of what do to next. After a few minutes, its mind made-up, it returned to the security and familiarity of its nesting cliff.  In a few short weeks, this bird too will start to roam, on glides that start a mile or more above ground-level, with a vision evolved to be so acute that it will search the skies for other vultures, processing this information and, now a master of its wings, tilting and flexing them to take it precisely, confidently to its chosen destination.

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