There is a silent killer lurking in the French Alps: ski lift cables. Over the last 60 years, the wires have accounted for almost 600 recorded landfowl deaths in the region. Black grouse (Lyrurus tetrix) account for ~70% of the grizzly toll, and that is probably the tip of the iceberg. Figuring out how well the birds see was an urgent priority for wildlife conservationists from the Parc national de la Vanoise, France, to allow them to design cable markers that stand out clearly for black grouse.
The conservationists asked independent French researcher, Simon Potier and Jean-Marc Lassance and Marjorie Liénard from University of Liège, Belgium, to investigate the bird's vision. The team publish their discovery that black grouse cannot see dark red and that their vision is quite poor in Journal of Experimental Biology. They also recommend that black and white markers will stand out best for the birds to help them avoid collisions with ski lift cables.
'Fortunately, we were able to collaborate with Frank Grosemans, an experienced breeder in Belgium', says Potier, explaining that Grosemans' understanding of the grouse was essential to keep the nervous birds calm as the team tested their eyesight.
First the researchers photographed the birds' retinas from all angles – in front, behind and both sides of the head – to map their view. Impressively, the black grouse have one of the widest known fields of view known (353deg.), with just a 7deg. blind spot at the back of their heads and a 28deg. region at the front with 3D vision. But how sharp is their vision, allowing them to pick out objects from a distance?
Measuring the depth of black grouse eyes, which are a good indicator of how sharply they can see, Potier revealed that the bird's vision is quite poor: similar to that of other landfowl, but only a fraction of that of the sharp-eyed wedge-tail eagle, which is the best by far.
And when the team calculated the minimum-sized object that flying black grouse could pick out from 32 m – which would give them enough time to evade the obstacle – it was just ~3 cm, which is 4–5 times larger than the smallest object humans can see over that distance. So, large warning markers are better for black grouse and the markers need to be placed no more than 16 m apart to give the birds time to react as they fly toward a cable.
Next, the team analysed the grouse's colour vision and were astonished to find that the birds can't see deep shades of red. 'This was unexpected, because, to the human eye, the combs above the eyes appear very red', says Potier. This means that dark red warning markers are less likely to stand out for the birds than people had thought.
However, black grouse likely see more shades of blue than us, possibly even into the UV.
And when the researchers checked how many shades of grey the birds can pick out – known as visual contrast – it was clear that they'd struggle to see dark ski lift cables against dark green trees or a white marker against the snow.
So, what is the best design for markers to make lethal ski lift cables stand out?
'In bright daylight, a marker of approximately 9×9cm composed of 3cm squares could be detectable at a distance sufficient to reduce collision risk', says Potier.
He adds that combining strongly contrasting black and white squares will help markers stand out more for black grouse in most lights and hopes that this knowledge will help conservationists keep the birds safe on ski slopes.
