Building A Scarecrow

Scarecrow at a festival in New Jersey

A scarecrow is a decoy or mannequin usually in the shape of a human. It is dressed in old clothes and placed in gardens or open fields to discourage birds such as crows or sparrows from feeding on recently sown seed and growing crops.

As its name suggests, it’s supposed to scare away crows from the field or garden.
Mostly they're for decoration, I think.

The first scarecrows in recorded history were made by the Egyptians along the Nile River to protect wheat fields from flocks of quail.  
Egyptian farmers put wooden frames in their fields and covered them with nets.  The farmers hid in the fields and scared the quail into the nets.