Edward J. Walsh – Edward.Walsh@boystown.org
JoAnn McGee – JoAnn.McGee@boystown.org
Boys Town National Research Hospital
555 North 30th St.
Omaha, NE 68131

Cara E. Whalen – carawhalen@gmail.com
Larkin A. Powell – lpowell3@unl.edu
Mary Bomberger Brown – mbrown9@unl.edu
School of Natural Resources
University of Nebraska-Lincoln
Lincoln, NE 68583

Popular version of paper 1pABa2 Hearing sensitivity in the Greater Prairie Chicken
Presented Monday afternoon, May 18, 2015
169th ASA Meeting, Pittsburgh

The Sand Hills ecoregion of central Nebraska is distinguished by rolling grass-stabilized sand dunes that rise up gently from the Ogallala aquifer. The aquifer itself is the source of widely scattered shallow lakes and marshes, some permanent and others that come and go with the seasons.

However, the sheer magnificence of this prairie isn’t its only distinguishing feature. Early on frigid, wind-swept, late-winter mornings, a low pitched hum, interrupted by the occasional dawn song of a Western Meadowlark (Sturnella neglecta) and other songbirds inhabiting the region, is virtually impossible to ignore.

The hum is the chorus of the Greater Prairie Chicken (Tympanuchus cupido pinnatus), the communal expression of the courtship song of lekking male birds performing an elaborate testosterone-driven, foot-pounding ballet that will decide which males are selected to pass genes to the next generation; the word “lek” is the name of the so-called “booming” or courtship grounds where the birds perform their wooing displays.

While the birds cackle, whine, and whoop to defend territories and attract mates, it is the loud “booming” call, an integral component of the courtship display that attracts the interest of the bioacoustician – and the female prairie chicken.

The “boom” is an utterance that is carried long distances over the rolling grasslands and wetlands by a narrow band of frequencies ranging from roughly 270 to 325 cycles per second (Whalen et al., 2014). It lasts about 1.9 seconds and is repeated frequently throughout the morning courtship ritual.
Usually, the display begins with a brief but energetic bout of foot stamping or dancing, which is followed by an audible tail flap that gives way to the “boom” itself.

Watch the video clip below to observe the courtship display

For the more acoustically and technologically inclined, a graphic representation of the pressure wave of a “boom,” along with its spectrogram (a visual representation showing how the frequency content of the call changes during the course of the bout) and graphs depicting precisely where in the spectral domain the bulk of the acoustic power is carried is shown in Figure 1. The “boom” is clearly dominated by very low frequencies that are centered on approximately 300 Hz (cycles per second).

FIGURE 1 (file missing): Acoustics Characteristics of the “BOOM”

Vocalization is, of course, only one side of the communication equation. Knowing what these stunning birds can hear is on the other. We are interested in what Greater Prairie Chickens can hear because wind energy developments are encroaching onto their habitat, a condition that makes us question whether noise generated by wind turbines might have the capacity to mask vocal output and complicate communication between “booming” males and attending females.

Step number one in addressing this question is to determine what sounds the birds are capable of hearing – what their active auditory space looks like. The golden standard of hearing tests are behavioral in nature – you know, the ‘raise your hand or press this button if you can hear this sound’ kind of testing. However, this method isn’t very practical in a field setting; you can’t easily ask a Greater Prairie Chicken to raise its hand, or in this case its wing, when it hears the target sound.

To solve this problem, we turn to electrophysiology – to an evoked brain potential that is a measure of the electrical activity produced by the auditory parts of the inner ear and brain in response to sound. The specific test that we settled on is known as the ABR, the auditory brainstem response.

The ABR is a fairly remarkable response that captures much of the peripheral and central auditory pathway in action when short tone bursts are delivered to the animal. Within approximately 5 milliseconds following the presentation of a stimulus, the auditory periphery and brain produce a series of as many as five positive-going, highly reproducible electrical waves. These waves, or voltage peaks, more or less represent the sequential activation of primary auditory centers sweeping from the auditory nerve (the VIIIth cranial nerve), which transmits the responses of the sensory cells of the inner ear rostrally, through auditory brainstem centers toward the auditory cortex.

Greater Prairie Chickens included in this study were captured using nets that were placed on leks in the early morning hours. Captured birds were transported to a storage building that had been reconfigured into a remote auditory physiology lab where ABRs were recorded from birds positioned in a homemade, sound attenuating space – an acoustic wedge-lined wooden box.

FIGURE 2 (file missing): ABR Waveforms

The waveform of the Greater Prairie Chicken ABR closely resembles ABRs recorded from other birds – three prominent positive-going electrical peaks, and two smaller amplitude waves that follow, are easily identified, especially at higher levels of stimulation. In Figure 2, ABR waveforms recorded from an individual bird in response to 2.8 kHz tone pips are shown in the left panel and the group averages of all birds studied under the same stimulus conditions are shown in the right panel; the similarity of response waveforms from bird to bird, as indicated in the nearly imperceptible standard errors (shown in gray), testifies to the stability and utility of the tool. As stimulus level is lowered, ABR peaks decrease in amplitude and occur at later time points following stimulus onset.

Since our goal was to determine if Greater Prairie Chickens are sensitive to sounds produced by wind turbines, we generated an audiogram based on level-dependent changes in ABRs representing responses to tone pips spanning much of the bird’s audiometric range (Figure 3). An audiogram is a curve representing the relationship between response threshold (i.e., the lowest stimulus level producing a clear response) and stimulus frequency; in this case, thresholds were averaged across all animals included in the investigation.

FIGURE 3 (file missing): Audiogram and wind turbine noise

As shown in Figure 3, the region of greatest hearing sensitivity is in the 1 to 4 kHz range and thresholds increase (sensitivity is lost) rapidly at higher stimulus frequencies and more gradually at lower frequencies. Others have shown that ABR threshold values are approximately 30 dB higher than thresholds determined behaviorally in the budgerigar (Melopsittacus undulates) (Brittan-Powell et al., 2002). So, to answer the question posed in this investigation, ABR threshold values were adjusted to estimate behavioral thresholds, and the resulting sensitivity curve was compared with the acoustic output of a wind turbine farm studied by van den Berg in 2006. The finding is clear; wind turbine noise falls well within the audible space of Greater Prairie Chickens occupying booming grounds in the acoustic footprint of active wind turbines.

While findings reported here indicate that Greater Prairie Chickens are sensitive to at least a portion of wind turbine acoustic output, the next question that we plan to address will be more difficult to answer: Does noise propagated from wind turbines interfere with vocal communication among Greater Prairie Chickens courting one another in the Nebraska Sand Hills? Efforts to answer that question are in the works.

tags: chickens, mating, courtship, hearing, Nebraska, wind turbines

References
Brittan-Powell, E.F., Dooling, R.J. and Gleich, O. (2002). Auditory brainstem responses in adult budgerigars (Melopsittacus undulates). J. Acoust. Soc. Am. 112:999-1008.
van den Berg, G.P. (2006). The sound of high winds. The effect of atmospheric stability on wind turbine sound and microphone noise. Dissertation, Groningen University, Groningen, The Netherlands.
Whalen, C., Brown, M.B., McGee, J., Powell, L.A., Smith, J.A. and Walsh, E.J. (2014). The acoustic characteristics of greater prairie-chicken vocalizations. J. Acoust. Soc. Am. 136:2073.