Alexandra H. answered 03/16/19
Compassionate Educator Specializing in K-6th, English, and Test Prep
"As I walked in using the cobbled path, a squadron of pigeons nestled upon the roof observed my entry, their heads crooked towards me like a sniper’s gun is crooked on an enemy fighter."
If you were to say "their heads were crooked towards me," then yes, you would have a complete thought, and would need a semi-colon after the word "entry." As it is, you're just fine - it is, as it stands, a dependent clause.
I would recommend removing "is" in the second clause.
Happy writing!