From Kate Sutherland (Aug 05):

In browsing through your website, your question about squirrels caught my
eye. I'm no squirrel expert, but my office window looks out on a big tree
and one of my chief modes of procrastination is watching squirrels chase one
another up and down the tree, then onto and over my roof. When they're
chasing one another in frenzied fashion, it looks like reflex and
improvisation. But they definitely have regular routes when they go about
their day-to-day business. They managed to completely peel away the
insulation round one part of the drainage pipe from our air conditioner,
because they grabbed on to the same precise spot again and again when
jumping from the tree to the house. At least, that's what the pest control
guy told us when he came round to seal up a hole by which they were getting
into our attic. (The squirrels are very entertaining through the window, but
not so much inside the house.)

From Stan D (Aug 05):

No two trees are the same like no two stairways are the same.  Reflect on what's happening when you walk briskly up or down a staircase you've never
walked before. I think the squirrel is doing the same thing.
 
"This staircase is a variation of all the staircases I've encountered
before. While it may be very different, it still follows some basic
principles and patterns." So it is with trees.