Need to look at my gameplay again, but might have seen a bird, but large wildlife would stay away from Warzone areas. Continuous fighting between the Fallen, Hive, and us Guardians make large amounts of noise.
Like our cities, wildlife stays away for the most part, but hearing gunshots, explosions, and alien creatures making large amounts of noise will halt any animal from visiting. Including Nova bombs thown, Fists of Havoc shaking the ground, and Guns of pure light exterminating anything it hits will strike more fear to any animal increasing our dominance in the food chain.
You think any deer will dare to stroll into Old Russia at war. The deer is either on crack, or tied on the front of a Redneck Guardian Speeder.