Auditory capabilities of birds in relation to the structural diversity of the basilar papilla.

The basilar papilla length increases systematically with body mass for 41 species from more than 10 avian orders and this relation does not differ between phylogenetic groups. Audiograms of 25 non-strigiform and 12 owl species, normalized relative to best frequency and best threshold, were used to compare audiogram shapes. The analysis revealed that the high frequency flank of the audiogram was remarkably similar across non-strigiform species. The high-frequency limit was on average 1.1 octaves above the best frequency, the low-frequency flank was less steep and showed much more species dependent variability. Audiogram shape in owls was much more variable. Morphological gradients along the basilar papilla revealed a small species dependent variability for the basal region of the basilar papilla and an increasing degree of variability towards the apex. In non-strigiform species, frequency selectivity for 2 and 4 kHz varied systematically with the space on the basilar papilla devoted to processing the corresponding frequency range. Space on the papilla did not vary systematically with frequency selectivity at 1 kHz. This difference between test frequencies might be related to the transition from electrical hair-cell tuning, that dominates below 1-2 kHz, to micromechanical tuning at higher frequencies.