I've done research on this and still I cannot find why it is that scarecrows are symbols associated with Halloween. I do, however, have some conclusions of my own.
First, let us note that scarecrows are not very good at scaring crows. Clearly they are some atavistic throwback to a time when an agricultural deity or sacrifice was employed to make the land fruitful.
Second, let us note that we see artistic depictions of scarecrows very often associated with harvest festivals. Why should this be? If the purpose is to make the land fruitful and/or scare away pests, then once the harvest is in they are of no use.
1. Scarecrows are not easily seen when the corn is full and tall, but once harvested there he stands, alone in the field, looking creepy by himself with nothing to do.
2. He gets to come in and join the party. Sort of a 'well done, old boy, thanks for the help' at harvest time.
3. Scarecrows are bloody creepy.
That third one is the most important to me. I give credence to the other two of my hypotheses (I did make them up, after all) because the human mind associates things easily. We see one thing and associate it to another and presto, we have a tradition. Wherefore would we ever put a carved pumpkin on the top of our scarecrow as a head? Yet we have seen this as a part of Halloween seasonal art for a century. Let's face it: scarecrows are creepy.
Guardian of the fields or demonic entity waiting to stalk out of the back 40 and into your home in search of human flesh when the corn has been reaped? You decide.
This is Chauncey....
He is the Messianic guardian of my front lawn...
Or is he?
I swear, his hat is moving. He's been on his post for a mere 3 days now and that hat is moving. Probably at night. When he stalks the yard...