MY, HOW TIMES have changed. That’s what I keep thinking, looking around my own garden in recent years. I’ve been struck by the same thought over and over as I read “The Comfort of Crows: A Backyard Year,” the latest book by Margaret Renkl (illustrated with gorgeous collages by her brother, Billy Renkl, like the one above), which takes us through a year in her garden 1,000 miles to the south of mine in Nashville.