Why does the nanny bird have no tail?

There's not enough time. It hasn't grown yet.

Nanny bird, also known as white-waisted wenbird, fortune-telling bird and cereal bird, belongs to the golden-bellied finch family and is a domestic species of wild white-waisted wenbird. Most cultivated varieties are imported. It is said that the Ten Sisters are a new breed that was crossed by a white-waisted golden-bellied bird and another closely related bird centuries ago. Because it is easy to raise, it gradually breeds into various forms and then makes various improvements to produce species.