Why Birds Crash
Our human eyes may have blinkered us to the way other species see the world, and understanding how birds see could help to reduce the number of fatal collisions with manmade objects such as wind turbines, power cables and even buildings. Now, writing in the journal Ibis, Graham Martin argues that to cut down on deaths, we need to see the world from a bird's perspective.
Sadly, collision with manmade objects seems to account for the largest unintended human cause of avian fatalities worldwide, and many bird species are prone to collisions with structures that appear very conspicuous to us. For example, behavioural observations have shown that White-tailed Sea Eagles in Norway show no tendency to avoid wind turbine blades, simply treating them as if they were invisible.
We know that birds have very good vision - with some species able to pick out fast moving prey from incredible distances, and some species are incredibly dynamic on the wing, able to change direction and speed with apparent ease. So why should objects so obvious to us pose such a problem to birds?
Although some research has looked at collisions from the perspective of flight behaviour & manoeuvrability, very little has been published on the visual and perceptive aspects of this problem. Assuming that the visibility of an object at a distance is the problem, most collision-reducing measures in the last 30 years have involved marking an object with flags or reflective balls; however the probability of collision still remains high.
It's clear that birds don't see the world in the same way as humans - there are distinct differences in eyeball anatomy, location on the head and how the signals are processed in the bird brain. Martin argues that in order to devise effective strategies, we must develop a sensory framework based around a bird's perception of the world.
We are used to the idea of being able to see directly in front of us - usually in the direction we are travelling, but many birds are adapted for coverage alongside, above and behind the head - this may give a wider field of view, but reduces the region of binocular vision in the direction of travel. This lateral vision may play an important role in the detection of predators and foraging, but comes at the expense of being able to see where they're going! Even the Peregrine Falcon, whose vision is good enough to enable them to pick other birds out of the sky, relies mainly on lateral vision - viewing prey from the side and only switching to front-facing, binocular vision at close range.
So what can be done to reduce collisions? There's no catch-all answer, as what works for one species may be invisible to another, but there are some general principles that might help. Large, high contrast markers that employ movement should be deployed around an obstacle, both on the ground nearby as well as directly in front; redirecting flight paths may be more effective than hazard markers, but above all, understanding more about the specific species at risk, and how they see the world, could help to reduce these accidents.