DISTRESS/EXPELLER CALL UNIT A bird expeller or distress unit emits specific distress calls of the pest bird to discourage them from wanting to be around the area. It is most effective if installed prior to nesting. Depending on the unit, the expeller/distress unit can come with one or up to 4 speakers to maximize coverage. The bird expeller/distress unit does make noise that may affect the customer or surrounding structures. There is a volume control; however, that may affect the overall outcome of effectiveness.