Some of the boats on the swinging mooring where I am have strung together old CD's a bit like bunting, work well. When you pass the boat you can ask for a request.
Presumably birds which live in the area long-term, will always grow accustomed to anything which doesn't actually threaten their well-being from moment to moment.
I've seen those hawk-shaped kites at work lately, and everyone agreed how lifelike and natural they seemed to be, in form and in movement on the wind...but their appearance seems only to affect the uninitiated...the hardcore local winged crew still evacuate greenish muck from themselves, all over everything the kites were intended to protect.
I wonder if a flashing laser-pointer, connected to a heat-seeking guidance system, could pester sitting birds so unrelentingly as to make them move on?