Thanks to Andrew and others for correcting my mistake of referring to the
ultrasounds of thunder when I should have been referring to infrasound.
Mike's suggestion of pelicans making periodic exploratory flights to inland
areas is also very plausible and is probably the most likely explanation for
knowing where and when to breed. However, I wouldn't write off some of the
possible physiological responses to ambient environmental conditions, e.g.
changes in atmospheric pressure associated with heavy rains. After all, many
bird species use the Earth's magnetic field to orientate in the right
direction when on migration, so why can't some species be sensitive to
atmospheric pressure changes or the smell of floodwaters?
Carl quite rightly mentioned that the filling up of inland wetlands (e.g.
Lake Eyre) are usually as a result of river flows resulting from heavy
rainfall in distant locations (e.g. the Channel Country). If waterbirds use
physical environmental cues from those heavy rainfall events to move inland,
maybe they have learned to orientate themselves in a direction that will
take them to those breeding areas that fill up with water, rather than
directly towards the rainstorms.
The other issue is the urgency at which the pelicans (and other
waterbirds)need to begin nesting when they reach these inland floodwaters.
It's my understanding that many female pelicans lay fertilized eggs within a
day of their arrival, suggesting that they have mated with a male before
arriving at the site. This further suggests that they either had prior
knowledge of the suitability of the inland floodwaters as a breeding site
before they arrived there, or they just took their chances and mated before
arriving at the site.
Stephen Ambrose
Ryde, NSW
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