I was looking at the Wiki for crows and realized that scarecrows aren't worth it at a certain point. According to the Wiki "For every 16 crops (rounded down), a crow may try to eat a crop. This is maxed at 4 crows. Each crow has a 30% chance of attempting to eat." So that means that if you have 64 or more crops then you should lose, on average 1.2 crops a night. This number does not change though if you have 64 or 3000+ crops. Its kind of an edge case, but if your the kind of player that fills their entire field with crops then I think its actually better to not use scarecrows.