Steve,
I think you will find more variation in success between days within a month than you will see between given months within a year.
If the coyotes are there, if they are responsive, and if you give them an offer they can't refuse, they will usually respond.
I call and kill coyotes in every month on the calendar about equally. The key is locating them first. What the summer months give up in responsiveness during hot weather, they gain in territorial aspects of coyote behavior during those same months.
Like others have stated, you will find coyotes to be generally more concentrated in Sept. and Oct. but they will respond year round.
I think it would be easier to point out the times when calling can be more difficult.
This would be during hunting seasons when they are getting run all over the place or in cold weather when they may be camped on a carcass.
The honest answer is "it depends on many variables".
Sorry I couldn't be more specific.
Wiley E
