Clear frosty mornings typically have the birds more vocal than any other time in Northern and Central Il, may differ the further south you go, I have killed many birds on cold mornings, they love to get on eastern facing slopes in steep terrain or a sunny field edge to warm up!
Killed a bird on the opener a couple years back, it was 17 degrees and had snowed over night, the birds were hammering, had 6 toms on one ridge, finally got into an argument with an old hen and she brought em all to my set up!