Capturing Sound from the Loudest Birds on Earth

Larson Davis Sound Level Meters are ideal for a wide range of applications. Read about Dr. Jeffery Podos' use of the SoundAdvisor™ Sound Level Meter Model 831C in his research on the sound amplitude of the mating song of the white bellbird in his own words.

My collaborator and I used a Larson Davis SoundAdvisor Sound Level Meter (SLM) to measure the amplitude of mating songs of two exceptionally loud bird species found in the Amazon rainforest - the screaming piha and the white bellbird. Prior research had documented the sound amplitudes of screaming pihas. Bellbirds have long been thought to be even louder, perhaps even the loudest birds in the world. Yet no good calibrated measures of their songs had yet been reported.

We conducted our research on a remote mountain towards the northern edge of the Brazilian Amazon, not far from Brazil's border with Venezuela and Guyana. Our sampling sites ranged from the base of the mountain to over 1 km in altitude. Our hired team spent a week with machetes opening trails up the mountain, and preparing sites for camping. The rainforest at this site is pristine, supporting an amazing diversity of plants and animals, including many monkeys, snakes, large mammals, and loud birds. It was an incredible privilege to spend time on this mountain. We were able to record the amplitudes of the songs of 3 pihas and 8 bellbirds, and also to make many observations of the mating behavior of bellbirds.

Our ability to document the song amplitudes of these very loud birds was greatly facilitated by the Larson Davis SLM, in at least four ways. First, this SLM offers very high temporal precision in capturing and recording amplitude values. We set the SLM to record amplitude at 20 ms intervals, which means 50 recordings per second. As a result we were able to generate and publish precise waveforms that capture rapid amplitude changes. Prior generation SLMs were not sufficiently responsive, even on "fast" settings, to capture rapidly changing amplitude variations, as is characteristic for many animal sounds.

Second, we took advantage of the fact that the SLM generates simultaneous recordings of both average and peak amplitude values (Leq and Lpeak), and also across multiple weightings (A, C, and Z). It is really nice to have access to all of these values for later analysis.

Third, this SLM allows for the simultaneous, synchronized recording of amplitude profiles and sound as .wav files. In essence, one is able to sample sounds simultaneously at 50 Hz (amplitude calibrated) and 48 kHz .wav format. Having access to the audio file provides a helpful reference for working with amplitude value spreadsheets, and also for saving and recovering voice annotations that accompany recording.

Fourth, the Larson Davis SLM has a very impressive dynamic range, meaning that we are able to record both very soft and very loud sounds without loss of accuracy and with no need to adjust the meter's window of amplitude sensitivity. This is a really nice feature when one is recording a very loud animal.

Once we had our data in hand, in order to generate validated measures of amplitude, we followed two additional steps. First we subtracted out baseline levels of background noise, as averaged for 0.5 s before each song. This allowed us to account for the fact that background noise can artificially inflate inferred measures from signals. Second, we accounted for the distances of our recordings, between us and the singing birds. We took distance measures using laser range finders, of the type favored by golfers. Measuring distances allowed us to account for the fact that sound gets softer with distance (dropping by 6 dB with every doubling of distance). Following the lead of other biologists, we standardized our amplitude values to what they would have been 1 m from the singing birds.

Our analyses confirmed the song amplitudes of screaming pihas, as had been documented in that prior study. We also found that the white bellbird, with its louder of two song types, exceeds the pihas in song amplitude by about 9 dB. This makes these two birds the loudest bird yet documented on earth. There are almost certainly one or two other species louder than the white bellbird; but for now these are the two loudest species yet registered worldwide, as far as we know.

Photo Credit: Anselmo d'Affonseca

Reference:
Podos, J. & Cohn-Haft, M. (2019). Extremely loud mating songs at close range in white bellbirds. Current Biology 29: R1055-R1069