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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Harald Yurk Sent: Monday, March 31, 2008 10:16 AM To: [email protected] 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 [EMAIL PROTECTED] 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 > > > > ----- Original Message ----- > From: "Michael Stocker" <[EMAIL PROTECTED]> > To: "'Martin Braun'" <[EMAIL PROTECTED]>; > <[email protected]> > Sent: Wednesday, March 26, 2008 4:55 PM > Subject: RE: Songbird audiograms > > > Perhaps not, but it would be biologically expensive to have all of > that energy dedicated to high frequency vocalizations and not be able to hear it. > We could infer that they do use these sounds for something which > involves their being able to hear them - but at the moment it may not > be indisputably proven. > > Michael Stocker > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Martin > Braun > Sent: Tuesday, March 25, 2008 12:14 PM > To: [email protected] > Subject: Re: Songbird audiograms > > Dear friends, > > it is not possible to infer from the sound spectrum of the calls of a > species anything about the hearing range of this species. > > Martin > ************************************************************************** Harald Yurk, Ph.D. Research Associate Cetacean Research Laboratory Vancouver Aquarium Marine Science Centre PO Box 3232 Vancouver, BC, Canada V6B 3X8
