Next time you have a gigantic black bird hovering over your head, make a note to watch your step. Just before the end of the novel, when Danny's depression is really getting bad, Pablo sees an omen that tells him the end is near:
"There he stood," Pablo always said. "I could just see him, leaning on the rail. I looked at him, and then I saw something else. At first it looked like a black cloud in the air over Danny's head. And then I saw it was a big black bird, as big as a man. It hung in the air like a hawk over a rabbit hole. I crossed myself and said two Hail Marys. The bird was gone when we came to Danny." (16.48)
The message here is pretty clear: the black cloud or giant black bird means that death is close to Danny. Gulp. The fact that it disappears after Pablo prays means that it's probably an evil spirit, since God has power over it. Whatever it is, it's plaguing Danny: it's almost a symbol of Danny's crazy inner state.
Steinbeck never takes a side in Tortilla Flat: we never know what he really thinks about supernatural things like Saint Francis or Hail Marys or the big black bird. These things aren't necessarily meant to be taken literally: they might just be there to make us think about the possibility that there are things or forces out there we don't completely understand.
They could even just symbolize aspects of our emotions we don't fully understand. Do you think Danny knows what is actually wrong with him? It's easy to label it depression, or something like that, but does Danny know what that actually means? Do we? Why do you think Danny gets into such a funk? What's actually wrong? Is it him? Is it his friends? Is it his society?
We don't know for sure, but the black bird stands in to remind us that it's an important question.