I know this will create "controversy" but I don't care-I speak from experience regarding this area. Every 5 to ten years crows change their pattern. On the border I saw a gradual decline of crows until it's not worth going anymore. There were not less crows moving through but they changed their pattern despite wide food availability. While some still follow the old pattern it is insignificant compared to "Days gone by". THe trick is to find their old new pattern. I am not sure if it's thatmosphic pressures or earths' magnetic core or what? This is why there appeared to be no crows one year but the same amount turning up in their winter roosts! By my calculations I saw 5 really good years before it tapered off dramatically. I attributed it to an influx of other and all types of hunting. That did or had to play a major part in the dramatic shift in the big crow area for what that is worth given New Hampshire. So ultimately we saw less in the traditional areas because they used a different flyway. If you don't know where it is you undoubtedly will see less. One factor that figured in for me was traditional farms dried up. No food no crows. JUst prior to that too many newcomers. So there you have it. One very good reason is too many crow shooters in too small a state regarding the lack of accessessibility to hunting areas and the crows didn't go there anymore. Just need to figure out the new flyway and get under it somewhere. Tricky indeed! Good luck!
When the roost moves to a different location the flyways will usually change.
When the feed runs out in certain areas the birds patterns will change to meet the new conditions. It's a full time job staying a head of the curve if you want to get any shooting.
The more hunting pressure by other crow hunters makes it harder as the season wears on! This is why the last thing a crow hunter wants is another crow hunter in his area !