Purely anecdotal, but for what it's worth:

I hunt deer and grouse in upstate Pennsylvania, primarily in the state forest lands where Potter, Lycoming and Clinton Counties meet. My brother and I have observed that the number of ravens we see while deer hunting has dropped off markedly. Where we used to see (or hear the wingbeats or calls of - the woods are stunningly quiet if the wind is still and the sound of a raven's wings carries a long way) a dozen or more a day, it's a good day if we see (or hear) two or three. The situation with grouse is similar. Hiking in the woods used to routinely flush grouse - not shootable flushes, mind you, but flushes nonetheless. Driving down the forest service roads resulted in regular sightings, usually of birds gathering gravel for their crops. It's a rare thing to see one today - rare enough that we've all but given up hunting them.

I have no idea what the cause of either bird's population drop might be, but it's clear that the drop has occurred. Nor, I suspect, does the PA Game Commission know.

As a side note, the hunter population during the traditional rifle deer season has also plummeted and aged as well. Seeing someone younger than in his 50s is quite unusual. Where there used to be 10-15 people seen walking through the woods in the average day, I now see none other than my brother, with whom I hunt. As a result, the deer move less. The Game Commission did institute more stringent rules about legal bucks (minimum of 3 points on one antler; 4 points in a couple of areas) so the trophies are more impressive. But the limitations have also discouraged the young and those with shorter attention spans from deer hunting. License sales have dropped precipitously. Gun sales seem to focus on semi-automatic rifles, which are not legal for deer hunting in PA.

Some combination of the protection of raptors, the decline of trapping, and the removal of nesting cover and shelter, particularly with farmers planting boundary line to boundary line, has resulted in the total elimination of the wild pheasant population. The only pheasant hunting remaining in the state is put and take.

All things considered, what was once the No. 1 or No. 2 state in number of hunters seems to be in an accelerating downhill trajectory.