What's wrong with the sparrow tail hair?

The sparrow tail hair fell off, indicating that it was changing the hair, and soon it came out.This bird will change twice a year, once in April in spring, and the other in September in autumn.When changing hair, do a good job of hygiene and keep the cage clean and tidy, so that birds are not easy to get sick.When it grows new hair, it must also supplement nutrition to promote the growth of new feathers.