If you've spent any time around roosters, you know that their "morning" crowing can be loud. That distinctive cock-a-doodle-doo is piercing: if you happen to be standing near a rooster sounding off, you're hit with a sound wave that's about 100 decibels. That's unpleasantly loud, like the whir of a chainsaw. If one cock-a-doodled right in your ear, the sound is even louder over 140 decibels. Sounds that loud can cause damage in less than a second and are just shy of shattering your eardrum.
In fact, roosters are so loud that it's surprising they aren't deaf from their calls. So Belgian researchers looked into it and found that they have special ears that allow them to crow to their hearts' content without losing their hearing.
Loud sounds (above 120 decibels) can cause permanent hearing losses because the intense air pressure waves can damage and even kill the cells that translate sounds into neurological signals. At more than 130 decibels, it takes less than a second to inflict lasting damage. So you'd think that roosters crowing every day would slowly squawk themselves to total deafness. Since they don't, scientists figured they must have some way of protecting their ears when they crow.
When the researchers looked at their ears, they saw how the animals are able to be so loud without going deaf: their ears are blocked when they crow. When the animals opened their beaks fully, their external auditory canals completely closed off. So basically, roosters have built-in earplugs.