The lack of attention to avian osteology.
Went to a park, they had osteological/taxidermy specimen from three different fish, a bounty of mammals, about three reptiles, and only one bird skull, a turkey vulture, with an informational sheet saying they are now officially classified as storks.
Mammal skull IDing is often seen as standard skill for wildlife tracking and monitoring, and there are countless resources both in print and online to help naturalists ID mammal skulls and other skeletal elements. The only guide I have found for avian bone IDing that even begins to hold a candle to the wealth of information on mammalian osteology is directed at archaeologists, not naturalists or zoologists (Osteology for the Archaeologist, written by archaeologist Stanley J. Olsen. The guide is fantastic, but I find it interesting that there are no comprehensive sources for avian osteology directed towards those more interested in “the natural world”).
The inattention to bird bones is very apparent on iNat. Confusing Strigidae skulls for Accipitriformes is a common mistake even though they’re definitely distinguishable (owl’s skulls are highly adapted for a nocturnal lifestyle). It’s very common perfectly identifiable bird bones get down voted as “cannot be improved” when the ID definitely can be improved, sometimes down to species. Off the top of my head I can probably think of a dozen regular mammal bone IDers, while I can count all the regular avian bone IDers on a single hand.