Does the environment constrain avian sound localization?

Abstract

A bird needs to keep track not only of social interactions of conspecifics but also of their changing locations in space by determining their directions and distances. Current knowledge of accuracy in the computation of sound source location by birds is still insufficient, partly because physiological mechanisms of few species are studied in well defined laboratory settings, while field studies are performed in a variety of species and complex environments. Velocity gradients and reverberating surfaces may conceivably induce inaccuracy in sound source location (mainly elevation) by distorting the directional cues. However, most birds possess an inherently directional pressure difference receiver, which enhances the directional cues (mainly azimuth), and a computational mechanism in their auditory pathways to suppress echoes of redirected sound.

Sound localization in a small passerine bird: discrimination of azimuth as a function of head orientation and sound frequency

“…These results are interesting because 'seet' calls are produced with relatively high sound frequencies (~7-8·kHz), leading to the suggestion that they might be difficult to localize (e.g. Larsen, 2004;Marler, 1955).…”

“…Calford and Piddington, 1988;Hyson et al, 1994). We do not yet fully understand how the interaural pathway functions in small birds (Klump, 2000;Larsen, 2004;Larsen et al, 1997), however female parasitoid flies can assess azimuth with a resolution of only 2° (Mason et al, 2001) and these flies appear to use mechanical coupling between the left and right tympani to achieve this high level of performance (e.g. Mason et al, 2001;Miles et al, 1995;Robert et al, 1996).…”

“…Humans, for example, use interaural time differences (ITD) when localizing relatively low sound frequencies (<~1.5·kHz) but use interaural level differences (ILD) or spectral cues when localizing relatively high sound frequencies (>~1.5·kHz; Moushegian and Jeffress, 1959;Rayleigh, 1907;Stevens and Newman, 1936). Small birds are thought to use these same sound-localization cues (Klump, 2000;Larsen, 2004) although the transition between using ITD and ILD is expected to occur at a higher sound frequency since phaselocking might be limited to below 3-4·kHz (Gleich and Narins, 1988) and because the towhee's relatively small head (1.8-2.0·cm) may generate useful ILDs only over relatively high sound frequencies. Towhees also use different cues when judging distance to stimuli produced with sound frequencies above and below ~3.5·kHz (Nelson, 2002).…”

Lateralization of acoustic signals by dichotically listening budgerigars (Melopsittacus undulatus)

“…The maximum ILD is about 10 dB for an 8 kHz tone presented slightly behind the animal (e.g., Klump, 2000), and is 5.6 dB for a 4 kHz tone presented off to the side (Larsen et al, 2006). However, a maximum difference of around 3 dB has been measured for frequencies below 4 kHz, the range where budgerigars hear best (Dooling et al, 2000;Park and Dooling, 1991;Larsen, 2004;Larsen et al, 2006). Park and Dooling (1991) computed the minimum ITDs for budgerigars based on their interaural distance and minimum audible angles, and found that acuity would be around 50 ls, at best (e.g., for 2 kHz stimuli).…”

“…Whether or not behavioral thresholds match these predictions remains to be seen. Small birds, like mammals, are thought to use both ITDs and ILDs to localize sounds (Klump, 2000;Larsen, 2004;Nelson and Suthers, 2004), though no lateralization tasks have been conducted to explore this possibility. It has been suggested that great tits (Parus major), birds with a similar interaural distance to the budgerigar, for example, might be resolving binaural time differences of about 18 ls to account for their free field acuity of 20 (Klump et al, 1986).…”