Bulls bugle more when cows come into estrus, usually in September, and early October. Sometimes a cow comes in the second or 3rd cycle in November/December and bulls bugle. I have heard bulls bugle in July playing around, not rutting. Also heard them in December. In Utah the peak of the rut is around September 20th most years. Herd dynamics, temperature and weather does have a role. Rutnbuck is correct, shortness of days stimulate hormonal changes, that brings on the rut.