I have had a plastic owl erected on my avery for a few years now, i took it down one day to get the hut painted, the stock birds were out in the avery at the time, i came in the house for a cup, heard all the fluttering from the stock doos, looked out the window and the hen was sitting on top of the avery looking in at the birds.
The owl was put back up, and touch wood i havent seen it since, i also have the radio playing when the birds are out, dont no if that makes any difference or not, but havent had any problems so far, so there is something working.