Cave crawling in zebra Finch skulls: Which anatomical structures constitute the functional intraaural Canal?

The middle ears of birds are acoustically coupled through an air-filled interaural canal, often illustrated and modelled as a simple tube. It allows sound to propagate through the skull from one ear to the other and considerably enhance the cues for directional hearing by interaction with the external sound field driving the eardrum vibrations. Theoretically, different combinations of frequency dependent gains and delays of sound in the interaural canal can produce very different directionalities of the ears but it still remains uncertain how interaural transmission gain and delay can be shaped by anatomical adaptations during evolution. A closer inspection of the zebra finch cranium using micro-CT scanning reveals that not only is IAC trabeculated and irregularly shaped but it also communicates with a set of highly complex, air-filled canals in the skull extending to the base of the beak. We tested the possible influence of these communicating cavities by measuring eardrum directionality and interaural transmission before and after filling the frontal cavities with dyed fat but found no dramatic effects. We will discuss what function the cavities serve and whether the ICA should be represented by a simple tube in future models