A Scarecrow is a craftable item that prevents crows from eating crops. Scarecrows can be crafted at Farming level 1. Scarecrows are not needed to protect crops grown from Wild Seeds, since crows do not attack them.

So after having some jerkass crows steal some crops for the first time I immediately put up a Scarecrow, since it'll keep the crows away. However, it has a limited range of 20 tiles, as seen in the

Displays the range of sprinklers, scarecrows, bee houses, and junimo huts so you can ensure your crops are watered, won't be eaten by crows, and your bee houses will produce special honey!