Not Everyone's A Fan
It's hard to believe that anyone could not like my cats, but the squirrels that traverse our backyard are not big fans. During the warmer months, when I get home from work, I take the cats out into our fenced backyard and the squirrels will sometimes give us an earful. They can easily circle the edges of the yard by sticking to the top of the fence (as this one is doing), or by showing off their acrobatic prowess and jumping through the trees.
Like other urban areas in Oregon, the tree squirrels you'll find in Portland's neighborhoods and parks are likely to be introduced species like eastern grays and eastern foxes.