Isn't there a book called something like: When the snales awake? - found it:
When the Snakes Awake: Animals and Earthquake Prediction by Helmut Tributsch

I haven't read it myself - I think the animals sense something coming - ants, crickets, birds but I don't have any scientific evidence to back that up. Maybe what they sense is the foreshocks...we don't feel the really little ones but maybe they do... or like EQF mentioned, maybe its electromagnetic.

Canie