Your sentence is mostly good but can be slightly improved for clarity and grammar. Here's a corrected version:
"Illustrate autumn in a mixed forest, showing deciduous trees with vibrant hues of yellow and red. The leaves twirl as they fall down. The trees can feature occasional old nests and some tree hollows."
Explanation of changes:
- Added a comma after 'forest' for better flow.
- Changed 'show' to 'showing' to keep consistent verb forms.
- Added 'with' before 'vibrant hues' for clarity.
- Reworded 'leaves twirl falling down' to 'leaves twirl as they fall down' to improve sentence structure.
- 'an occasional old nest' changed to 'occasional old nests' to agree with 'trees' being plural.