Do scarecrows really keep away crows?

Not really. When crows find a field with a scarecrow, they might be scared off the first few times they see it, but they are smart enough to assess the situation from a distance until they realize that whatever the thing is in the field, it isn't moving and it isn't going too. Then, they let each other know through their caws that there is no danger in the field and then they all come to eat, totally ignoring the scarecrow. This is why when you see scarecrows around Halloween in illustration or decoration, they are often seen with a crow sitting defiantly on their shoulder.