The answer to the first part of the question can be found in the first few paragraphs of chapter 13. Brian calls the birds "foolbirds."

He was looking for one of the foolish birds—he called them foolbirds—and there was a flock that lived near the end of the long part of the lake.

Much of the rest of this chapter doesn't discuss much more about these birds. Chapter 13 and 14 are heavy with Brian's internal thoughts about mistakes he had made and the dangers of making more mistakes. There is also a lengthy section describing how focused Brian needed to become about food. We are told that food was "everything." In chapter 15, Brian's thoughts return to getting food. Specifically, he tells readers about the foolbirds.

He calls them foolbirds for two reasons. They are exceptionally well camouflaged, so they fool his vision.

He just couldn't see them, couldn't figure out how to locate them before they flew, because they stood so perfectly still and blended in so perfectly well.

The second reason he calls them foolbirds is because Brian assumes they are stupid.

And what made it worse was that they were so dumb, or seemed to be so dumb, that it was almost insulting the way they kept hidden from him.

As for what you might call them, that is up to you. Personally, since the birds are so difficult to see and move so quickly they are tough to target, I might call them "Ninjabirds."