Hi riejkecares,
For me, I use mostly weasel hair brush to do the beak and eye because this type of hair is stiff and can easily create the texture feeling of the beak and eyes.
As for the size of the brush, I sometimes use the Inkston gongbi brush which is made with weasel hair. Sometimes, I just use the standard weasel brush which is used also for small regular script calligraphy.
Inkston 0903Ws 花枝俏 “Flower Brush” Extra Fine Detail LangHao Wolf brush
Or bigger sizes,
Inkston 经典狼毫 Classic Lang Hao “Wolf” Brush Set