Clue: Area 51 fact: army uncloaked nine aliens, originally left of the birds in a region (9)
Answer: AVIFAUNAL
Explanation: Definition: of the birds in a region

**A** (area) + **V** (5) + **I** (1) + **F**(act) **A**(rmy) **U**(ncloaked) **N**(ine) **A**(liens) [originally] + **L** (left)