In-air and underwater auditory thresholds were measured in diving bird species, using behavioral and electrophysiological techniques. In the first set of experiments, the auditory brainstem response (ABR) was used to compare in-air auditory sensitivity across ten species of diving birds. The average audiogram obtained for each species followed the U-shape typical of birds and many other animals. All species tested shared a common region of greatest sensitivity, from 1000 to 3000 Hz. The audiograms differed significantly across species. Thresholds of all duck species tested were more similar to each other than to the two non-duck species tested. The red-throated loon (Gavia stellata) and northern gannet (Morus bassanus) exhibited the highest thresholds while the lowest thresholds were observed in the duck species, specifically the lesser scaup (Aythya affinis) and ruddy duck (Oxyura jamaicensis). In the second set of experiments, both the ABR and psychoacoustics were used to measure in-air auditory sensitivity in one species of diving duck, the lesser scaup. Both approaches yielded audiograms with similar U-shapes and regions of greatest sensitivity (2000-3000 Hz). However, ABR thresholds were higher than psychoacoustic thresholds at all frequencies. This difference was smallest at the highest frequency tested using both methods (5,700 Hz) and greatest at 1,000 Hz, where the ABR threshold was 26.8 dB higher than the behavioral measure of threshold. In the third set of experiments, psychoacoustic techniques were used to measure in-air and underwater sensitivity in one species of sea duck, the long-tailed duck (Clangula hyemalis). Underwater auditory thresholds were measured for the first time in any diving bird species. Long-tailed duck in-air sensitivity was greatest at 2000 Hz. The ducks responded reliably to sound stimuli underwater, and correctly responded to high intensity stimuli (greater than 117 dB re 1 μPa) with over 80% accuracy at frequencies between 0.5 kHz and 2.86 kHz. The large differences in diving behavior and physiology across bird orders suggest these studies should be extended to other diving bird species in order to understand how well birds hear underwater. These first measurements highlight the need for further investigation into underwater hearing in diving birds, in order to understand underwater hearing mechanisms and begin to develop hypotheses as to how the introduction of man-made noise sources into the aquatic environment may impact these species.