It seems to me that a stepper motor may not be the right motor for your application. You are not looking for precise positioning. Rather, it seems that you could get by with a reversible DC gearhead motor (driven by the arduino using an H-bridge circuit), a position encoder (to count revolutions of the motor shaft in either direction in order to verify proper operation), and perhaps some kind of limit switch (to detect when the scarecrow is in its home position, and maybe another one to detect when the motor is completely retracted - see discussion below).
The position encoder need not have great precision, either. Something like an optical interruptor that gives you one pulse per revolution might be sufficient.
I'm not sure that a wire or string-based tensioning design (if I understand your plan correctly) would be the best choice, though - have you considered using something like a leadscrew driving a nut (that pushes on the end of a lever that pivots about a fixed fulcrum) to move the scarecrow into position (where it would presumably be latched into place via a solenoid or a ratchet-like mechanism that will later be used to release the scarecrow to the force of gravity), and then withdraw the leadscrew nut after the latching is complete?