Frequency sensitivity in Northern saw-whet owls (Aegolius acadicus)

Abstract

Northern saw-whet owls (Aegolius acadicus) are known for their unique asymmetrical ear structure and ability to localize prey acoustically, yet few attempts have been made to explore the auditory capabilities of this species. In this study, we evoked auditory brainstem responses (ABRs) with tonebursts to assess three main hypotheses regarding the evolution of auditory sensitivity: sender-receiver matching, ecological constraints, and phylogenetic/morphological constraints. We found that ABR amplitude increased with increasing stimulus level, which is consistent with results in other avian species. ABR amplitudes, latencies, and thresholds indicate that the hearing range of Northern saw-whet owls extends from 0.7 to 8.6 kHz, with an extended frequency range of best sensitivity between 1.6 and 7.1 kHz. Sensitivity fell off rapidly above and below these frequencies. The average audiogram was structurally similar to those found in other species of owls, suggesting that phylogeny or morphology may be constraining the frequency range of auditory sensitivity. However, ABR thresholds were 10-25 dB lower than those of Eastern screech-owls (Megascops asio), with thresholds below 0 dB SPL in some individuals. The lowest thresholds were at frequencies not found in the vocalizations of Northern saw-whet owls, suggesting ecological constraints rather than conspecific vocalizations are driving absolute sensitivity.

Field-based hearing measurements of two seabird species

“…Hearing data from the puffin were quite clear and provided an auditory curve that was not unlike AEP hearing thresholds in other birds, where the AEP thresholds generally are of the same shape but often 20-30 dB above hearing thresholds obtained by psychophysical methods (Beatini et al, 2018;Brittan-Powell et al, 2002, 2005Crowell et al, 2015;Henry and Lucas, 2010;Lohr et al, 2013;Maxwell et al, 2016). Lowest measured thresholds were found at 1-2 kHz with a gradual increase in AEP thresholds at lower frequencies and a steeper increase at higher frequencies (Fig.…”

“…Hearing was tested using AEP methods similar to prior AEP studies in birds (Beatini et al, 2018;Brittan-Powell et al, 2002, 2005Crowell et al, 2015;Henry and Lucas, 2010;Lohr et al, 2013;Maxwell et al, 2016). However, for the field study, the AEP equipment was outfitted in a rugged case.…”

Amphibious hearing in a diving bird, the great cormorant (Phalacrocorax carbo sinensis)

“…Two differential electrodes were placed right behind the left ear meatus (the active electrode) and on the vertex of the head (the inverting electrode), respectively, with reference to the third ground electrode placed in the nape of the neck. Note that the placement of the active and inverting electrode here was opposite to that of Brittan-Powell et al (2002, Crowell et al (2015) and Beatini et al (2018). Therefore, the ABR signals recorded here are inverted relative to the ones in those studies.…”

“…Terrestrial birds have excellent vision (Martin and Osorio, 2008). Most day-active birds of prey, for instance, primarily rely on vision to find their prey, whereas night-time hunters, such as owls, have acute hearing and vision abilities adapted for finding and catching prey in the dark (Payne, 1971;Kettler et al, 2016;Beatini et al, 2018). When entering the water, however, the eyes of terrestrially adapted birds lose the refractive power of their cornea (Katzir and Howland, 2003).…”

“…Also, it is not always trivial to compare physiological and psychophysical thresholds for birds (Brittan-Powell et al, 2002. Physiological techniques allow for several frequencies being tested carefully during a limited amount of time both in air and underwater, and therefore also a larger number of individual birds to be tested than when using psychophysical techniques (Beatini et al, 2018;Mooney et al, 2019a,b). However, the lower thresholds obtained by psychophysical techniques mean that these remain the 'gold-standard' for audiogram determination.…”