Zebra finches produce intralaryngeal laminar flow whistles during panting

Birds and mammals converged upon the same physical mechanism of vocal fold vibration to produce their broad range of voiced sounds critical to communication1. The frequency range of vocal fold vibration is limited per species by biophysical constraints to 3-4 octaves2. However, recent work reported vocalizations in zebra finches with apparent fundamental frequencies of 7-11 kHz3 that far exceed the range of regular calls and song (0.5-1.5 kHz)4,5. These "heat" or "incubation" calls are suggested to have close-range communicative relevance in the global temperature rise context3,6, but their acoustics are poorly described and by what biophysical mechanism they are produced remains unknown. We recorded heat calls in adult zebra finches in vivo and show they are extremely soft, frequency-modulated calls with source levels of 13.9 {+/-} 3.3 dB SPL at one meter with dominant frequencies of 6.8 {+/-} 0.6 kHz. Through a series of in vitro experiments, we establish that these calls are aerodynamic whistles produced inside the avian larynx, not syrinx, during inspiration. Respiratory air flow during whistle production is an order of magnitude higher than song and consistent with thermal panting for evaporative cooling6,7. Laryngeal geometry and dimensional flow analysis suggest that these whistles are laminar flow whistles that occur when a flow boundary layer is in a transition phase from laminar to turbulent flows8,9. Birds, like some rodents10-12, are thus able to produce both voiced sounds and aerodynamical whistles in their vocal tract.