On Fri, Jun 24, 2005 at 05:50:52AM +1000, Ricki Coughlan wrote:
> Given that birds hear sounds up to 200 cycles per second, versus the
> roughly 20 cycles per second of humans,
You might have confused upper and lower limits. It varies between species
but the upper threshold for many birds tested is 8-9khz. The upper limit
for human hearing is significantly higher at ~20khz (20000 cycles/second).
Lower frequency thresholds are less clear cut but for most birds tested,
sensitivity is definitely declining by 200hz. 20hz is often nominated
as the lower limit of human hearing. A few birds are known to perceive
much lower frequencies than humans (infrasound).
In most respects humans have better audio perception than most birds
tested. As their head only has room for a much smaller ear, the basic
physics are against birds. Although if you handle a dead Tyto owl,
you can see how their external ear attempts to overcome this.
Particular bird species are known to exceed human perception
is some aspects. For example, sound localization is known to be
significantly better in Barn Owls than humans.
Birds may generally have better temporal perception than humans although
I've only seen results for a couple of species and the differences are
not huge (a factor of 2). This might indicate that bird hearing is
optimized for temporal discrimination at the expense of other aspects.
So fine differences in song may be perceptible to a bird but not a human.
This may well allow a bird to recognize conspecific individuals where
a human can't. Species recognition is another matter and I'd like to
see a number of examples to convince me this was a significant issue.
> it is a fact that variances in syringeal morphology may not produce song
> which is distinctive enough to the human ear for accurate diagnosis
I'd be very surprised if variation in syringeal morphology produced song
differences perceptible to birds and not humans, and would like to see
the data. Syringeal morphology determines the gross characters of the
song such as fundamental frequency. Fine temporal structure presumably
stems from neural control.
Non-functional syringeal variation might be apparent on dissection but
this seems unlikely to provide a characters for separating crypto-species,
at least in groups with well-developed song.
Without multiple examples of species which can be readily separated by
syringeal morphology and not by diagnosable characters, Watson's claims
are pretty speculative.
> There may be many species out there speaking entirely different
> languages and without further study of vocalisations, ...
I agree but vocalizations are a diagnosable character and heavily used by
birdwatchers so you are not supporting Watson's thesis.
> The numbers of specialists in various areas who
> will relish the difficulties in diagnosing various hard to split
> groups of species accounts for a very small number of birdwatchers.
Perhaps, but these are very people Watson says are hampering bird
taxonomy, the "senior ornithologists with extensive experience" on
checklist committees.
Andrew
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