It appears to be that they are genetically programmed to do it when they do it, at the same time each season. Has NOTHING to do with a moon phase or weather parameters. Photoperiod appears to contribute to the timing, but there are other environmental factors that contribute to the predetermined time does come into estrous. Over most of their range from Canada to TX most breeding takes place in November. Then you get to the southern states. You can throw the photoperiod theory out the window altogether. In the lower coastal plain of South Carolina deer breed in September/October. In the lower coastal plain of AL and MS they breed in late January and February. The state of FL has a range of peak dates that covers nearly every month of the year. Deer breed when they are genetically programmed to do so.