Sexual selection can result in extreme development of multimodal mate-attracting traits, including complex constructions.  Male great bowerbirds build bowers for attracting females.  Bowers contain a thatched twig tunnel (avenue) opening onto two courts covered with decorations.  Males displaying on a court are seen by a female from within the avenue.  She sees and hears displays through the avenue entrance but can only see the male's head and objects in his bill as it passes repeatedly across the entrance.  Because the bower may affect the auditory as well as the visual parts of the multimodal male display we investigated bower acoustic properties by playing standard sounds from multiple court positions, recording the resulting sounds at the female's head position within the avenue.  Bower geometry results in a limited zone at the avenue entrance where his vocalisations can be heard with maximum intensity; this corresponds to his typical display position.  Experiments show that court decorations increase the intensity of some frequencies and reduce the intensity of others. Bower structure simultaneously affects both visual and auditory male display components and could be important in sexual selection.  It is important to consider more than one sensory mode, especially in the context of built signalling structures.

This is the data, corrected for position 1, used in the analysis. It also includes a sample R script to run an analysis to show how the GAM was done.  The original article has been accepted to Behavioural Ecology: Acoustic effects complement visual displays of Great Bowerbird bowers, by John A. Endler, Selina Meehan, Aida Rodrigues, and Vicki Hallett, Behavioral Ecology (in press 2024).

We played spectrally flat standard sounds from various positions on the bower court, where the male displays visually and auditorally, with a microphone at the female's head position within the bower avenue.  This yielded sound spectra and total intensity as a function of direction and distance from the bower avenue entrance as well as what was on the bower court, with locations with no bower as controls.

The standard sounds were either white noise or a spectral sweep, bandwidth 2 to 20 kHz, both generated in MATLAB.  The bandwidth includes all frequencies recorded in natural male soundss recorded by us and by others.  These sounds were played back from a Marantz PMD661 digital recorder.

For bowers the speaker (Second Marantz PMD661 experiments 1,3, and Bose Revolve II SoundLink experiments 2,4) was placed at the avenue entrance (position 1) and at two more distant positions on the court (25 and 50 cm from position 1) at 0 degrees (avenue axis), 45 degrees left and right and 90 degrees left and right.  Males call just in front of the avenue entrance, mostly at the ends of the walls so the speaker was also placed in those two positions.  The Marantz speaker shows less than 0.5db change when within ±20° of perpendicular to the microphone (Fig. S2), and our avenue entrance aiming error was less than 5°.  The Bose was omnidirectional within our measurement error.

Recordings were made inside the bower avenue at the female's head position and height.  In experiment 1 recordings of the played sounds were made with an AudioTechnica AT8035 directional microphone connected to a second Marantz PMD661 recorder set to record 16 bits/sample at 44.1 kHz.  The input gain was set to 4.5 for clipping at maximum -8dB.  In experiment 2,  We placed a matched pair of FEL Communications Clippy XLR EM272 Stereo Microphones back-to-back within a 5 cm diameter Styrofoam ball with their faces flush with the ball surface, with close to 360° solid angle reception.  This is larger than a bowerbird head but the microphone lengths set the minimum width and the ball diameter is smaller than the female head excursions whilst watching the male.  We oriented the ball at female head height with its left and right microphones facing the left and right avenue walls.  The microphone pair was connected to both channels of a Marantz PMD661 digital recorder with the same settings as in experiment 1. Both channels were converted from dB to power, added, and converted back to dB before further analysis. This configuration accounts for sound entering the avenue from the entrance, but also through the roof, through the avenue walls and the opposite entrance.  The Clippy pair sound spectrum was similar to but slightly flatter than the AudioTechnica microphone.  Experiment 1 looks at the effects of sound arriving at the female from the entrance next to the calling male and experiment 2 also includes sound coming around the back to the second avenue entrance and through the top of the avenue.

For all experiments we also recorded with the same geometry but a short distance away where there was no bower--this servs as a control for enviromental structure and also microphone receptive geometry. 

Recordings were converted from power to frequency spectra (dB vs sound frequency), using the MATLAB 2021b functions audioread, pspectrum, and db.  All recordings were converted to relative spectra by subtracting the spectrum from position 1, and this was done for all bowers and controls.  This eliminates variation and the need for absolute microphone calibation.  This is analagous to our use of a white standard in visual spectrum analysis.

For more details see methods section of paper Acoustic effects complement visual displays of Great Bowerbird bowers, by John A. Endler, Selina Meehan, Aida Rodrigues, and Vicki Hallett, DOI: 10.1093/beheco/arae070.