I didn’t realize this until quite recently, but the casques on the head of a hornbill are NOT hollow. I had just assumed they were because everything else about birds is surprisingly hollow and light. The big long toucan bills? Hollow. Those big strong eagle bones? Hollow. Those massive feathers on peacocks? Hollow.
Hornbills? The birds named after animals that have big hollow horns? Solid casques as dense as ivory.
Of course, while the rhinoceros hornbill is doing okay, some hornbills are being slaughtered for their “ivory” casques, which are carved and sold to rich collectors.