Photo: Wikipedia, CC
Not Adapted to This New Threat
For most of their existence as a species, birds haven't evolved in an environment where something like a power line could be in their flight trajectory. Because of this, they mostly look down while flying to find other birds, nesting areas, and places where they can feed. If there's a tree or a mountain in front the bird and it is looking down, it's going to see the trunk or the lower portion of the mountain; with power lines, unless it's right at the pylon, there's nothing under the lines to give warning.
Photo: Flickr, CC
Millions of birds are thought to be killed by hitting power lines globally each year. [...]
"Once we saw the wisdom of looking at the problem through birds' eyes rather than human eyes, it all made sense," says Professor Graham Martin.
"These birds can see straight ahead in flight but they only need to pitch their heads forward by a small amount and they will be blind in the direction of travel." [...]
"Not all birds will be blind ahead when they look down but we can certainly suggest that this will apply in all crane and bustard species and probably in larger raptors including eagles and vultures," Professor Martin tells the BBC. [...]
Despite efforts to make power lines more visible through the use of reflective markers and high-visibility tags, certain species still have high mortality rates from power line crashes. (source)
Photo: Flickr, CC
So some birds are less affected than others, but it's a small consolation because it's a very hard problem to solve, and while we figure it out, million more birds - some of them endangered - will die unnecessary deaths.
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