Original sentence: Weaverbirds are famous for their grass nests that hang on a branch.
Simple correction: Weaverbirds build grass nests that hang from branches.
Why this is better (easy explanation):
- Your sentence is almost correct. The small change is the preposition: we usually say something hangs from a branch, not hangs on a branch.
- Using build makes the sentence clearer for young readers: it tells who makes the nests.
- Using branches (plural) sounds natural because weaverbirds make many nests on many branches.
Other good options:
- Weaverbirds are famous for their grass nests that hang from branches.
- Weaverbirds build hanging grass nests on tree branches.
All of these are correct. The version above is the easiest to understand: Weaverbirds build grass nests that hang from branches.