Why would the presenseof carrion birds suggest that a battle is amminent?

Carrion birds feed on dead flesh. At the scene of a battle, they are ubiquitous as they would descend on the battle field after the fighting had ceased to feast on those left behind.

It may have been a bit of poetic license to say that the birds gathered before the battle, however. It is logical to think that the noise of several thousand men, dwarves, elves, and whatnot would have been enough to frighten away most wildlife.