Biology Reference
In-Depth Information
Q If birds have eagle eyes, why do they crash into windows, power lines, and guy
wires?
A
Window glass is not only clear; it's reflective. Sky and trees are mirrored in windows,
and since there was no such thing as glass in the natural world for the millions of years
that birds have been evolving, few wild birds have yet evolved any ability to notice it.
Window glass may have been used in Italy nearly 3,000 years ago, but it wasn't com-
mon in England until the seventeenth century. Huge picture windows have become wide-
spreadonlyinrecentdecades,muchtoorecentlyforbirdstohavedevelopedmechanisms
to avoid them. Conservative estimates put the number of birds killed by glass every year
in the United States from 100 million to as many as one billion.
Unlike branches and other natural structures, power lines and guy wires are straight
and relatively thin, so they apparently appear two-dimensional, making it difficult for
birds to gauge their distance from one until they're crashing into it. There is little data in
North America, but extrapolating from data in Europe, as many as 174 million birds may
be killed in North America each year by high-tension line collisions. Nocturnal migrants
are attracted to the vicinity of communications towers and their guy wires by the lights
necessary to warn planes of their presence. Again little data are available, but the num-
bers of birds killed at communications towers and their guy wires is estimated to be from
5 million to 50 million every year.
Inplaceswherepowerlinekillsareknowntobehigh,little,inexpensiveobjectscalled
flight diverters can be placed on the lines, significantly reducing the kill. Some of these
are as simple as wire coiled into a loose cone shape; others are more complicated, swiv-
eling or rotating in 3 to 5 mph winds. They're expensive to place on existing power lines
but much less soto put onnew lines as they're being installed. Because they are three-di-
mensional and in some cases make noticeable movements, birds detect them and gauge
their distance from them much better than they gauge their distance from wires without
them. Encourage your local power company to use them on new wires.
BIRD ON A WIRE
Why is it birds can sit
on electrical wires and not get zapped? To get zapped, they
need to close the circuit so the current actually flows through their body. If you were
standing on a metal ladder and touched a bird on a power line, you'd both get zapped,
because the ladder touching the ground would close the circuit. A squirrel can safely run
along a power line, but when it reaches the end, if it makes contact with the transformer
while it's still touching the wire, it will be electrocuted. When large birds perch on or
very close to transformers and power lines, they often get electrocuted. As a matter of
fact, in some areas electrocution is the main cause of mortality for Harris's Hawks.
