By Ella Davies
Earth News reporter
Birds fly into power lines because they have "blind spots" in their field of vision, according to new research.
Vision experts found that cranes, bustards and storks were unable to see obstacles straight ahead when they tilted their heads downwards in flight.
Birds often look down during flight to find fellow birds as well as nesting and feeding areas, say the researchers.
The new evidence suggests that the problem cannot be prevented by altering the appearance of power lines.
Millions of birds are thought to be killed by hitting power lines globally each year.
Clear statistics are difficult to obtain because birds' remains are often scavenged before the cause of death can be recorded.
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.
Scientists from the Centre for Ornithology at the University of Birmingham, UK, studied three particularly affected species: kori bustards (Aerdeotis kori) blue cranes (Anthropoides paradisea) and white storks (Ciconia ciconia).
Their findings, published in the journal Biological Conservation, identify that these species share significant blind areas to the front of their heads.
The blue crane's eyesight is better suited to eating than flying.
Although the heavy bustard differs greatly in general body shape from the delicate crane and stork, the birds share a foraging technique - visually guiding their bill to take food items.
This technique requires excellent vision at the end of the bill, resulting in a narrow field of vision and wide "blind spots".
"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."
Many species of bird have been observed looking down during flight, possibly to locate fellow birds and suitable foraging and nesting sites.
Narrow binocular fields combined with birds' tendencies to look down effectively means certain species cannot see power lines until it is too late.
"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.
This new evidence could help to inform both ornithologists and power distribution companies seeking to avoid crashes.
"Simply putting devices on power lines to make them more conspicuous will not work in many situations... ways need to be found of decoying birds away from power wires at sites where collision rates are high," says Professor Martin.
Of the African birds studied, the blue crane is the most vulnerable according to the IUCN red list.
Conservationists point to power line incidents as a key factor in the blue cranes' population decline.