What do birds symbolize in "All the Pretty Horses"?

Birds were mentioned quite a few times in all the pretty horses and I was wondering what they symbolized.

A few examples...

"small desert birds flew chittering among the dry bracken"

"a few last chitterings of birds"

"they passed a stand of roadside cholla against which small birds had been driven by the storm and there impaled. Gray nameless birds espaliered in attitudes of stillborn flight or hanging loosely in their feathers."

"A flock of small birds rose up and passed back over him with thin calls."

"desert sky where there was no cloud, no bird."

These are just a few of the many times they were mentioned...

What do they symbolize?