2010-11-02 13:31:59
By Ella Davies
Earth News reporter
A dead bustard lies near power lines (c) Jessica Shaw
Birds collide with power lines because they have "blind spots" in their field
of vision, according to new research.
Vision experts found that cranes, bustards and storks are unable to see
obstacles straight ahead when they tilt 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 collisions cannot be prevented by altering the
appearance of power lines.
Millions of birds are thought to be killed by colliding with 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 collisions.
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.
A blue crane (c) Graham Martin
The blue crane's eyesight is better suited to eating then 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.
SOURCES
Visit the journal Biological Conservation to learn more about birds' vision
"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.
A white stork in flight (c) Manfred Heyde
White storks fly great distances to overwinter in Africa
Watch fantastic aerial feats from the BBC archive
"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 collisions.
"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 collisions as a key factor in the blue
cranes' population decline.