I tackled all the engineering printing and electronics as that's what I'm best at,

 and my partner did all of the 3D sculpting painting and sewing.

I found this gif of Reciprocating motion on instructables,

and it was exactly what I wanted to make the Scarecrow move his arms with one motor.

After a few iterations and refinements the mechanism was reliable and ready for dressing

The wiring is very simple I used a USB-C battery management system, 

I upcycled the battery from a vape pen that has been thrown out onto the street.

 I pick up discarded vape pens wherever I can, the batteries in them are very useful.

from the battery management system we go through the switch straight to the motor as easy as electronics come.

and that's the story of the Pigeon scarer.