The Battle Between Sound and Habitat: How Habitat Influences the Transmission of an Acoustic Signal?

Contributed by Alexis Billings

Successful communication is relatively simple: (1) a sender encodes and transmits information through a signal. (2) The signal is sent through an environmental space where is it corrupted and degraded. And (3) the signal is received and decoded by a receiver. Most of the research in animal communication has focused on part (1) and (2), the senders and the receivers. Therefore, there is a huge gap in our understanding of animal communication in regards to the signal itself. In acoustic signals this is incredibly important because sound waves are extremely susceptible to numerous degradations from habitat structures.

My research focuses on mobbing calls, which are acoustic signals given by numerous bird and mammal species in response to predators. I am interested in each component of communication, including how these signals actually transmit through the environmental space and reach the receiver intact. To get at this component I have combined wave transmission theory from physics with the ecology and biology surrounding these signals.

Using black-capped chickadee “chick-a-dee” mobbing calls, I played exemplars through three different habitats (open, semi-open, closed) and re-recorded the signal at 1, 10, 25, 50 and 100 meters. I calculated how much relative power (measured in decibels) each frequency bin (23 Hz per bin) actually lost between 1 meter and the other distances. I then calculated the predicted loss of relative power for each frequency bin using wave spreading loss and attenuation equations from wave transmission theory. From these two calculations I can compare the predicted loss of power per frequency bin and the actual loss of power per frequency bin.

Below is a filter function of the transmission of a part of a black-capped chickadee ‘dee’ note through a closed habitat at 100 meters. The x-axis is frequency or pitch (measured in Hertz) and the y-axis in the loss in power (measured in decibels). Above the graph is a

sample of a black-capped chickadee “dee” note flipped horizontally to show frequency bands. Finally, the graph contains the measured (green, irregular graph) and calculated (blue, parabolic graph) loss in power. From graphs like this, I found that the frequency bins between 4000 Hz and 12,000 Hz are the most robust in the chickadee call and are also the most strongly degraded over distance in the closed habitat (the natural habitat of the black-capped chickadee). So, it seems that black-capped chickadee mobbing calls are optimally structured to combat the degradations imposed by their natural habitat so the signal can successfully reach the receiver.