The sound of a rooster crowing can reach 142 decibels (when measured at the rooster’s head). To put that in context, a chainsaw produces about 120 db and a jet taking off 25 metres away is about 150 db and can lead to eardrum rupture. Scientists in Belgium questioned how could a rooster crow so loudly and not damage its own hearing.
They discovered that when a rooster opens his mouth fully to crow, the auditory canal (the pathway from the outer to inner ear) is shut off. This means that while a rooster is crowing, he can’t hear anything. In contrast when a hen opens her beak completely, there is only a slight closure of her auditory canal. What scientists haven’t explained is how nearby hens can tolerate the loud noise.
Source: Raf Claes, Pieter Muyshondt, Joris Dirckx and Peter Aerts. Do high sound pressure levels of crowing in roosters necessitate passive mechanisms for protection against self-vocalization? Zoology. February 2018. Volume 126. Pages 65-70.