There are several possibilities for acacia to shed hair:
1 normal molting alopecia.
If birds live in a bad environment, don't clean their droppings often, don't disinfect their cages in time, or eat too salty, and have skin diseases and endocrine disorders, they will lose their hair and may not be able to grow feathers again.
Mosquito bites can also be infected with viruses, but they should grow feathers in winter.
Birds often bump into cages and their heads get hairy.