>>3324It's the insects. Insects were locked in a struggle with witches for centuries, being able to choke with sheer numbers any spellcasting hag that dared to lower her altitude too much. Not to mention what terrible fate befell a witch flying in full speed that acciddentaly ate a bee or hornet.
In order to combat insects, witches created a new form of shapeshifting, focused on airborne agility and insect-eating. This form came to be known as bats. They are nocturnal, because witches could only do their magic at night, as the common knowledge teaches us. Some witches were turning themselves into bats, others found some poor guys that were way too attracted to pointy hats for their own good.
At first the changes were more or less controlled (if not always voluntary) and people were human for most of the time and bat for the rest. As those changes were based on black magic, they required blood in order to function. This gave the basis for vampire myths, by the way. But after some time not all transformations were successful. If someone tried to turn into bat and they stayed human, they could try again later, but if they couldn't revert from bat to human... Well, their fate was to become a new animal species. Artificial and based from human genome, which explains how bats can fly and still be mammals, by the way.
Surviving witches, when they noticed that their colleagues turned into much simpler and nonmagical animals, went into panic mode. Most of them accepted supremacy of insects and flew only on maximal altitude, far away from the menace, but this proved fatal due to low temperatures (freezing to death mid-flight) and low oxygen (losing consciousness and falling to death). Over few generations, the art of flying became completely lost. Yearning for speed and wind in their hair once again, remaining witches were responsible for development of bicycles and motorbikes. The modern bikers and cyclists are all part of secret, occult cabal, still targeted by flying insects from time to time.
Anyway, this is all tangential, because birds do exist and they are cute. I wish that my children would come out of eggs, really. Now excuse me, I need to do my daily ride over the local leylines.