Resonance dynamics of pilot whale clicks recorded by dorsally-attached tags. (a) The slow decay of on-animal recorded clicks is due to discrete resonances evident in the spectrogram as narrow horizontal bars following the broadband click (2048 pt FFT, Hamming window, 50% overlap, 192 kHz sampling rate, 500 m depth). Colour in all spectra plots shows the signal-to-noise ratio in dB. (b) Stack plot of SNR spectra (47 Hz resolution) for successive clicks during a deep dive descent showing consistent dynamics in resonance frequency: the lowest resonance frequency reduces gradually during each click sequence but makes a step increase after each of the 3 pauses shown. (c) A low frequency noise transient is often audible during pauses in clicking (here at 1.5 s), presumably due to air movement (2048 pt FFT, 192 kHz sampling rate, 550 m depth). (d) Predicted air space volume for regular click sequences at 240 m (blue regression line, R2 = 0.97) and 590 m depth (red regression line, R2 = 0.99), and for buzz clicks at 514 m depth (green regression line, R2 = 0.97). Air volume per click (the slope of the regression lines) increases with depth and reduces sharply in the lower amplitude buzz clicks.