Depends where you are.
I have hunted muzzy the last 5 years. And we often hear a bull screaming in early am hours. They are still herded up and always have cows. Since we are hunting cows or spikes we dont call alot.
But I know of at least two spikes called in with cow calls.
The weather I think plays a big part. Last year was dry. No bulls screaming.
Looks like this could be another no fire hot hunt this year. Hunt the water holes.