What is the significance of Lyman's comparison of himself to "the contour bird" in Chapter 1?Since there is no such bird - and Stegner knew that -...

What a baffling question!  I can think of a couple of possibilities.  You are referring to the following sentence:

Increasingly, after my amputation and during the long time when I lay around feeling sorry for myself, I came to feel like the contour bird.

He goes on to say that he wanted to fly around the Sierra foothills backward--another impossibility for all birds, except the hummingbird.   Perhaps Stegnor is playing on the "tour" part of the word.  The narrator--not interested in his own life anymore--would like to tour about in someone else's life, hence the interest in his grandmother's letters.  "Contour" also works in this sense.  He would like fly about and fit the contours of the lives of his grandparents:

I'd like to live in their clothes a while, if only so I don't have to live in my own.

So we have several fantasies in this paragraph:  the contour bird, which does not exist; birds that fly backwards; living the lives of those dead and gone; and the last and most important, the narrator's wish to touch ground again--impossible because he is a double amputee.