Various scavangers?

Birds are (usually) small, and would be removed as a food source by many animals if the bird were stationary and on the ground- as dead ones often are.

How often do you see dead mice, voles, or other small animals?

It takes a while to finish eating a deer- hence the number seen dead along roads in this area.

I once watched ( near work) a deer carcass over a week being reduced to just a skull.