In my experience.......
and I have tried everything, from singing string to wires to "birds of prey" to whirling vanes to.....well just about anything you can think of.
Each seems to work for about a week, then the pesky critters get used to that method and you have to change to another.
Overall covers seem to work "better" but even then.....
I used to jet spray my boat every few days until I looked back at my boat after one wash, and saw a row of seagulls just sitting and waiting. It was almost as if they were saying to each other " that human has just washed our toilet clean again, now that it's nice and pristine we can go and use it again".
"Ribbing-the most expensive way of travelling third class"