How does an animal learn what a predator is?
To answer your question, you really have to specify what animal your talking about, some animals simply won't live long enough for learning to be an effective mechanism. 

If there are few predators, evolution will favour animals who are less vigilant about checking for potential predators generally, since this costs time (Google search vigilance behavioral ecology).

Do address JeromeJ theory in that comment, teaching is fairly rare in the animal kingdom, even amongst Apes. I don't know any examples of teaching about predator detection, although it is believed that Meerkats teach their young how to kill poisonous scorpions and ants teach each other the location of new sources of food (see http://psychology.wikia.com/wiki/Social_learning_in_animals) 

Rather than explicit teaching, it might be likely that birds socially learn infomation about predators simply from observation, although i'm not sure how often this occurs. A nice little example shows that wild crows learnt to avoid and "mob" specific humans using facial identification http://rspb.royalsocietypublishing.org/content/early/2011/06/20/rspb.2011.0957

I don't know enough to give a more detailed response though!