I was looking around the web today, trying to find out which plants/trees are okay for squirrels to chew on. I looked up cottonwood and crape myrtle since I have that growing in my yard.
I came across this list of toxic and non-toxic plants for animals, so I thought I'd share it if it will help others as well.
http://www.aspca.org/pet-care/poison-control/plants/