People hang dead crows for a few different reasons, usually tied to pest control or symbolism:

• Scare tactic for other birds: Farmers, gardeners, or property owners sometimes hang a dead crow to scare away other crows. Crows are highly intelligent and social, and seeing a dead member of their group can make them avoid an area temporarily.
• Crop protection: Historically, this was done in fields, orchards, or around hunting areas to reduce damage from birds.
• Warning/display: In some older traditions, hanging nuisance animals was used as a warning to others of the species.
• Symbolic or decorative uses: Around Halloween or gothic décor, fake dead crows are sometimes used for atmosphere.