Originally posted by Grimtempest:

I'll buy that i suppose since i've never thoroughly tested it and just had incidents of them spooking upwind for unknown reasons, i've often wondered why deer were the only mob that seemed to spoke based on upwind or not. Maybe other random environmentals caused it.

The rest i'm 100% sure of since i've spent alot more time fighting mobs then trying to sneak up on deer.

Deer will spook if you aren't behind them. They have a cone of sight ahead of them.