The Air Force says there is an optimum height, which I don't know exactly, but is around 4-7 inches. Too short, and they find it easy for landing and eating bugs. Too tall, they can roost in matted-down areas.

The middle optmum height is too tall to easily land on, not tall enough to hide in, and difficult to forage for bugs (plants make it difficult to scratch for bugs and worms).

Then, they tend to avoid it.

About two years ago, we had a seasonal swarm of crane-flies, the things that look like giant mosquitos. Swallows were everywhere. I was in the RSU, and we called out the USDA bird-depredation guy.

They fired the bird cannons; no effect. Next, he blasted two of them with a shotgun.

Most of them left immediately.

Then, he did something fascinating. He zip-tied the two dead ones, and hung them on the wind-sensor tower...a jury-rigged scarecrow.

The birds were gone for a week. Word apparently spread fast among them.

Maybe they use twitter......