High-frequency hearing in a hummingbird

Do nocturnal birds use acoustic and visual cues to avoid predation by bats?

Anti-predation strategies are critical to animal survival and are fundamental to deciphering predator-prey interactions. As an important defense strategy, sensory predator detection (such as through acoustic and visual cues) enables animals to assess predation risk and execute predator-avoidance behavior; however, there are limited studies on the anti-predation behavior of nocturnal animals. The prey of bats provides an excellent representative system for examining the anti-predation behavior of nocturnal animals. Here, we broadcasted different types of echolocation calls of the bird-eating bat Ia io to two wild passerine birds, namely, Zosterops japonicus and Sinosuthora webbiana, that are preyed upon by I. io, and presented the birds with individual bats under different light intensities. The results showed that both bird species were able to perceive the low-frequency audible portion of the bats' echolocation calls; however, they did not exhibit escape responses to the acoustic stimuli. In the dark and under moonlit conditions, both bird species were unable to respond to active bats at close range and the birds only exhibited evasive flight behavior when bats approached or touched them. These results suggest that nocturnal passerine birds may not be able to use acoustic or visual cues to detect bats and adopt evasive maneuvers to avoid predation. This work suggests that bat predation pressure may not elicit primary predator-avoidance responses in nocturnal passerine birds. The results provide new insights into the anti-predation behavior of nocturnal animals.

Hearing in helmeted guineafowl (Numida meleagris): audiogram from 2 Hz to 10 kHz and localization acuity for brief noise bursts

Behavioral hearing thresholds and noise localization acuity were determined using a conditioned avoidance/suppression procedure for three Helmeted guineafowl (Numida meleagris). The guineafowl responded to frequencies as low as 2 Hz at 82.5 dB SPL, and as high as 8 kHz at 84.5 dB SPL. At a level of 60 dB SPL, their hearing range spanned 8.12 octaves (24.6 Hz-6.86 kHz). Like most birds, they do not hear sounds above 8 kHz. However, the guineafowl demonstrated good low-frequency hearing (frequencies below 32 Hz), showing thresholds that are more sensitive than both the peafowl and pigeon, both of which hear infrasound. It thus appears that infrasound perception may be more common than previously thought and may have implications for species that inhabit areas with wind energy facilities. The guineafowls' minimum audible angle for a 100-ms broadband noise burst was 13.8 °, at the median for birds and near the mean for mammals. Unlike in mammals, the small sample of bird species and limited representation of lifestyles do not yet allow for meaningful interpretations of the selective pressures or mechanisms that underlie their abilities to locate sound sources.

A parametric blueprint for optimum cochlear outer hair cell design

The present work examines the hypothesis that cochlear outer hair cell (OHC) properties vary in precise proportions along the tonotopic map to optimize electromechanical power conversion. We tested this hypothesis using a very simple model of a single isolated OHC driving a mechanical load. Results identify three non-dimensional ratios that are predicted to optimize power conversion: the ratio of the resistive-capacitive (RC) corner to the characteristic frequency (CF), the ratio of nonlinear to linear capacitance and the ratio of OHC stiffness to cochlear load stiffness. Optimum efficiency requires all three ratios to be universal constants, independent of CF and species. The same ratios are cardinal control parameters that maximize power output by positioning the OHC operating point on the edge of a dynamic instability. Results support the hypothesis that OHC properties evolved to optimize electro-mechanical power conversion. Identification of the RC corner frequency as a control parameter reveals a powerful mechanism used by medial olivocochlear efferent system to control OHC power output. Results indicate the upper-frequency limit of OHC power output is not constrained by the speed of the motor itself but instead is probably limited by the size of the nucleus and membrane surface area available for ion-channel expression.

Auditory sensitivity and vocal acoustics in five species of estrildid songbirds

Comparative studies of acoustic communication in clades with diverse signal features provide a powerful framework for testing relationships between perception and behaviour. We measured auditory sensitivity in five species of estrildid songbirds with acoustically distinct songs and tested whether differences aligned with species differences in song frequency content. Species were chosen based on phylogeny and differences in song acoustics. Behavioural audiograms were obtained using operant training and testing. Adult audiograms were compared across species and between sexes within a species. Juvenile and adult audiograms were compared in one species. The audiograms of adults reared by their own species and those reared and tutored by another species were compared in one species. Results showed that audiograms were similar across species and similar to previous reports of songbird auditory sensitivity. Species differed in the highest frequency detected and the frequency of peak sensitivity. While hearing frequency range was not correlated with song frequency bandwidth, the frequency of peak sensitivity was highly corelated with the frequency of peak energy in song. Sensitivity did not differ based on sex, age or tutoring experience. Our findings suggest that adaptations in songbird auditory sensitivity are largely constrained by shared peripheral and central encoding mechanisms, with species-specific perception appearing only at peak sensitivity.

Tonality over a broad frequency range is linked to vocal learning in birds

Many birds emit tonal song syllables even though the sound sources generate sound with rich upper harmonic energy content. This tonality is thought to arise in part from dynamically adjusted filtering of harmonic content. Here, we compare tonality of song syllables between vocal learners and non-learners to assess whether this characteristic is linked to the increased neural substrate that evolved with vocal learning. We hypothesize that vocal learning ability is correlated with enhanced ability for generating tonal sounds, because vocal production learners might also have an enhanced ability to articulate their vocal tracts and sound source for producing tonality. To test this hypothesis, we compared vocal learners and non-learners from two groups (186 passerines and 42 hummingbirds) by assessing tonality of song syllables. The data suggest that vocal learners in both clades have evolved to sing songs with higher tonality than the related, non-vocal learning clades, which is consistent with stronger roles for broadband dynamic filtering and adjustments to the sound source. In addition, oscine songs display higher tonality than those of hummingbirds. A complex interplay of vocal tract biomechanics, anatomical differences of the sound source as well as increased motor control through vocal learning facilitates generation of broad tonality.

Transmission of high-frequency vocalizations from hummingbirds living in diverse habitats

Some species of Andean hummingbirds produce high-frequency vocalizations which exceed the vocal range of most birds. They also challenge our understanding of the role of habitat structure in the evolution of vocal signals because these hummingbirds live in strikingly different habitats, ranging from cloud forest to high-altitude grasslands. Although these vocalizations are produced at high frequencies, they exhibit considerable variation in frequency content and temporal structure. The calls of the hummingbirds from the cloud forest are simpler and have a narrow frequency range compared to the complex song of the grasslands hummingbird. We hypothesized that each of the three high-frequency vocalizations is adapted for transmission in their habitat. We characterized the transmission of high-frequency vocal signals in the cloud forest and in the grasslands. All vocalizations attenuated and degraded substantially at short distances, suggesting that they are adapted for short-range communication. The simple vocalizations of the cloud-forest species transmitted better in both environments compared to the complex song of the grasslands hummingbird, probably due to relaxed constraints for high-frequency sounds in open habitats.