And strangely enough, there weren't threads about this matter. Or i wasn't able to find them (at least, searching for scarecrows gave me almost nothing).
So what i propose would be a scarecrow, made with some hay, cotton cloth (or a piece of clothing, maybe?) and a full-grown pumpkin. They would work in a radius (aka, a square) and they could require some item to continue working properly (something like a couple autumn grass per day, for example). Obviously, their effects wouldn't be stackable.
Their functionality could depend on whether crows or similar birds are implemented or not.
If they aren't implemented, they could simply work as a small plenty buff per day for the crops covered.
Assuming that birds or such are implemented, though, they could have an alternative use. Let's assume that birds will peck seeds and plants, hereby reducing plenty. Scarecrows could work as a primary target or simply make so that birds stay away.
Another method to scare birds would be, well, attacking them. They could either flee or attack back, though.