On Sun, 21 May 2000, Vella wrote:
> On "60 minutes" (channel 9) this evening was a segment on the Lake Ayre.
> As many are aware it is full of water after several periods of being
> bone dry, bringing about the breeding of thousands if not millions of
> Pelicans, Banded Stilts, Silver Gulls and many other water birds. It
> mentioned that Pelicans can detect ultrasonic signals from lightning
> which explains probably how they find this inland sea.
Ultrasound is above the normal human hearing range, say >20 kHz.
The absorption of sound by air increases with frequency. As a result
ultrasound propagates poorly and most ultrasonic sources can only be
detected at ranges of less than 100 metres, often much less.
Infrasound is below normal human hearing range, say < 20 Hz. Infrasound
propagates well. Its thought African Elephants can communicate at
distances as large as 10km using infrasound.
It is known some birds can perceive infrasound. Lab experiments have
suggest pigeons can detects sounds down to 0.05 hz. There are suggestion
that infrasound may provide important cues for migrating birds. A recent
paper [1] suggests the sonic boom of the Concorde may temporarily or
permanent disrupt the ability of homing pigeons to detect infrasound
and hence cause them to fail to "home".
There are number of environmental sources of infrasound including storms
and breaking waves. Its possible some, perhaps many, bird species can
detect these over distance of at least tens of kilometres.
Infrasound from storms can't directly lead birds to Lake Eyre as the
water that is filling Lake Eyre fell far from Lake Eyre (didn't it?).
Its conceivable that the presence of water in Lake Eyre could produce
infrasonic cues which birds could perceive at considerable distances -
but its far from obvious that this is even part of story.
Andrew Taylor
[1] Hagstrum, J. T. (2000). Infrasound and the avian navigational map.
The Journal of Experimental Biology 203, 1103-1111
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