| | Latitude and the Rut
The change of the angle at which sunlight enters the deer's eyes is what triggers antler developement and the rut. It has to do with stimulating hormone release. At the higher latitudes (up north) the angle gets lower earlier in the year, so everything up that way happens sooner. Occasionally, you'll hear guys say that the first snow triggers the rut, but if that were the case, I guess there wouldn't be any deer in Alabama. A good scientific explanation can be found in chapter 11 of Richard Goss's book, Deer Antlers, Regeneration, Function and Evolution.