I continue to be surprised by the amount of discussion that this topic
has generated. Perhaps a conference is on order - because there are
clear disparities on "the state of knowledge" and the understanding in
the various factions in the avian audiology community. I have
received many "offline" responses in addition to the listserve
discussion.
In these I find that there are many who are firmly entrenched in the
evidence that "birds" do not hear much above 6 kHz except for Tyto
alba. There are also many who believe that there is probably some
correlation between vocalization frequencies and frequency
perception. This second position runs afoul (pun intended) of the
first when the vocalizations exceed the "known" auditory bandwidth
confirmed by laboratory controlled measurements and testing.
In my opinion some of the "measure and test" paradigms are
occasionally framed by incomplete assumptions. Of course my personal
interest in all of this is resides in my attempts to understand the
mysteries of nature. Others may be more concerned with the
repeatability of our laboratory procedures and the importance of
understanding and building an unambiguous model of the subjects of our
inquiry.
>From my perspective, if we observe animal behaviors that indicate
facilities or capabilities outside of what is indicated by our test
results, it is consistent with our jobs as scientists to not dismiss
the behavior but rather to question the testing assumptions.
To wit: It seems that most of the audiograms that indicate that
"birds" can not hear sounds above 6 kHz are trained behavioral
audiograms that often use sinusoidal signals. I offer some of the
following questions that might be used to test our assumptions:
Are the subjects autonomic or sympathetic nervous systems stimulated
by frequencies outside of the frequencies that invoke a voluntary
response?
Are the subjects less responsive to simple sinusoids than to other
types of signals?
Do the subjects respond to signals differently in habitat and among
conspecifics or predators than in the lab?
And perhaps the etiological question: What would the purpose of these
"extra-audiological" signals and/or hearing capabilities?
As you may recall, this inquiry was opened up by a question about
differences in vocalizations of the bird predator Nyctalus while
hunting birds and hunting insects. This setting is much different than
what is typically produced in the lab.
It was by dint of this line of questioning that the American shad
(fish), which was thought to have a similar limited hearing range (200
Hz - 6 kHz, determined by behavioral audiograms) were later found to
be able to hear up to 80 kHz. This was inadvertently discovered when
ichthyologists who were looking for a way to scare schooling shad away
from power plant cooling intakes decided to use signals that mimicked
the hunting vocalizations of their dolphin predators. The signals
worked - but did not agree with the previous orthodoxy. Fortunately
the ichthyologists had the imagination to not assume the previous
measurements and tests were irrefutable.
Regards,
Michael Stocker
-----Original Message-----
From:
On Behalf Of Harald Yurk
Sent: Monday, March 31, 2008 10:16 AM
To:
Subject: Re: Songbird audiograms
Hi Everyone,
If what Martin says is correct, it would mean that producing higher
frequencies in a bird that is deaf for those frequencies would not require
increased energy intake, i.e., food. Otherwise natural selection could have
favoured those individuals who did not produce those higher frequencies,
regardless of whether males hear them or not. Unless of course females are
able to hear these frequencies and can use them as a proxy for prowess in
males with regard to territory acquisition and maintenance. Are there any
studies on the ability of female birds to hear higher frequencies in the
respective species?
Harald
on 3/28/08 6:37 AM, Martin Braun at wrote:
> Michael Stocker wrote:
>
> "We could infer that they do use these sounds for something which
> involves their being able to hear them ...."
>
> This may not always be true. Imagine the following scenario:
>
> A bird is deaf above 6 kHz. If this bird has sound components around
> 10-12 kHz in its calls, it cannot hear these components and it is thus
> unable to influence its voice organs to reduce or even eliminate them.
>
> Martin
>
> ---------------------------------------------------------------------
> Martin Braun
> Neuroscience of Music
> S-671 95 Klässbol
> Sweden
> web site: http://w1.570.telia.com/~u57011259/index.htm
>
>
>
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