Here are two correct ways to say your sentence, depending on whether you mean one chick or more than one:
- Singular: When a rock partridge chick stays still, it looks like a stripey stone.
- Plural: When rock partridge chicks stay still, they look like stripey stones.
Why:
- Use stays with one chick and stay with more than one chick (subject-verb agreement).
- Use it for one chick and they for many chicks (pronoun agreement).
- "Stripy" is fine; you can also say "striped" if you prefer.
Good job noticing the idea — just match singular/plural words so the sentence flows correctly!