Squirrels can jump up pretty high on a tree trunk, it would have to be some tall flashing.
Discussion
This is true unless you layer it in a way they can't climb. They can also climb up brick so I'm rethinking this. Their gestation period is only about 46 days and they mate all year round. They can often become intrusive and best in attics, chewing wires and ducts. The get in through the facia and soffit, typically out of view from ground level. Homeowners insurance excludes rodent damage. They're a real pain and very smart.
Def can cause lots of damage.
and it only takes 1 exposed trunk for them to have access to the whole network