When Graham Taylor and Adrian Thomas from the University of Oxford were asked to consult for a BBC programme fitting minute cameras to wild animals,they knew they were in for a treat. Taylor recalls that when he saw the eagle footage, he was struck by a flap of covert feathers from the lower surface of the eagle's wings that flipped out as the bird came in to land; `the feathers looked strikingly like the leading edge flaps used on high performance aeroplanes', he says. According to Taylor hydraulically driven flaps prevent these aircraft from stalling as they come in to land or perform steep manoeuvres; `they extend the wing's usual range of performance', he explains. Could the eagle's covert feathers perform the same function? Having teamed up with Anna Carruthers, Taylor and Thomas decided to film the wings of a free flying bird to find how the covert feathers function in free flight(p. 4136).
But finding the right bird to study proved tricky. Taylor explains that most falconers train their birds for short flights. Fortunately Thomas knew the World Champion Danish paraglider pilot Louise Crandal who had a steppe eagle, called Cossack, trained to soar next to her as she glided. Better still, the eagle was trained to fly wearing a camera. The team travelled to Cossack's Danish home in spring 2006, strapped a tiny wireless spy camera to the eagle and let him glide in sea cliff updrafts. Taylor remembers that `it was bitter, the sea was frozen', but it was well worth the discomfort. The team could clearly see the wing's leading edge feathers flip out for less than a second when Cossack encountered the cliff top updraft, perched and, most surprisingly, also at the end of every wing beat.
Realising that it took less than 60 ms to deploy the leading edge flap, the team switched to tripod-mounted high-speed cameras to capture the movement in fine detail. The precise high resolution films of the wings' activity allowed the team to analyse the flap's deployment quantitatively. They saw that the under wing covert feathers flip out passively; instead of being moved by erector muscles at the base, the feathers were moved by the air `like a Mexican wave moving along the wing until all were deployed to form a continuous flap-like structure', says Taylor.
Looking for other wing structures deployed during manoeuvres, the team noticed that the alula also swept out from the wing as Cossack perched. Taylor explains that the alula is a group of feathers at the front of the wing which are attached to a moveable joint (the remnants of the bird's thumb). The feathers were thought to be actively deployed during manoeuvres, but when the team scrutinised Cossack's landing sequence they realised that instead of moving from the base, the feathers were initially lifted by the airflow. Like the covert feathers, the alula was initially moved passively. The team speculate that sensors in the alula detect the feather's passive movement before triggering the alula joint to move, only deploying the alula when the wing is in danger of stalling.