What you can’t see can kill you. Researchers investigating why some birds are especially prone to hitting power lines have discovered that they literally can’t see where they are flying. That means typical anti-collision efforts, such as hanging warning markers on transmission lines, won’t help fowl that fly blind.
The world’s 65 million kilometers of power lines are a well-established threat to birds. In Europe, for instance, researchers estimated that 25% of adult white storks (Ciconia ciconia) and 6% of adults died annually from power line collisions and electrocutions during one 16-year study. In South Africa’s Overberg region, such collisions kill an estimated 12% of blue cranes (Anthropoides paradise) and 30% of Denham’s bustards (Neotis Denhami) annually. Some scientists fear that the Ludwig’s bustard (Neotis ludwigii) could become extinct due to cable strikes.
Researchers studying collision causes have typically focused on factors such as a bird’s maneuverability and flying speed. But G. R. Martin of the University of Birmingham in the United Kingdom and J.M. Shaw of the University of Cape Town in South Africa speculated that what some birds see might be more important. In part, that was because even after more than 30 years of marking lines with giant reflective balls, flapping flags and highly-visible wire coils, the death toll continued to mount for some species. In particular, they suspected that birds that rely heavily on their eyes to guide their bills to food might be particularly susceptible, because they often have unusually narrow visual fields and big blind spots.
To test the idea, they strapped three strike-vulnerable species — kori bustards (Ardeotis kori), blue cranes (Anthropoides paradise) and white storks — into a specially-designed device that allowed the researchers to measure eye movements. All three species had similar, horizontally narrow and vertically long visual fields. But they had different blind spots. If a bustard or crane glanced downward in a way that pitched the head just 25 or 35 degrees, it couldn’t see what was ahead or above. The storks became blind to oncoming objects with head movements of 55 degrees, the researchers report in the current issue of Biological Conservation.
“That flying birds can render themselves blind in the direction of travel has not been previously recognized,” the authors note. It also may explain why many birds are killed by thin insulating wires strung above thicker electrical wires – the birds can’t look up as they try to navigate over the thicker, more visible lines. To protect blind-flying species, Martin and Shaw suggest trying strategies that “distract birds away from the obstacles, or encourage them to land nearby,” perhaps by using decoys or building artificial roosts. “An effective all-purpose marking device is probably not realistic,” they conclude, “if some birds do not see the obstacle at all.” – David Malakoff
Source: Martin, G., & Shaw, J. (2010). Bird collisions with power lines: Failing to see the way ahead? Biological Conservation DOI: 10.1016/j.biocon.2010.07.014
Image © Ivonne Wierink | Dreamstime.com