We are talking about chicks here. Rats are a real danger for chicks. I’ve experienced the killing abilities of a rat a few years ago. Two chicks of 8 got killed until I repaired the opening the rat came through.
I do hope @Lins_BE takes more precautions than relying on the ultrasound.
I remember now that my mother in law had bought a ultrasound against mice. It seemed to work a bit for a while, but it didn't work properly.
And I’ve searched here as wel. All say ultrasound devices (for different animals) work only temporarily.
I didn’t read the whole thread. But since there was no reaction on the ultrasonic as far as I’ve seen, I feel the urge to warn you.
The hearing of chickens is better and different as human hearing. Possibly you're chickens go mad. ?
I would prefer to make the coop safe with hardware cloth to...