Very complicated question hammer33. In the Southeast, where weather is not a big player in fawn survival, Natural Selection is free to work on best estrus timing to produce the best fawn drop timing for the local conditions (more about that later). In this type of environment, herd dynamics can play a significant role in estrus timing. The presence or absence of older bucks in the population (buck age structure), and the local adult sex ratio, and even at times the doe age structure, can influence how long the rut lasts and when it occurs.
IF all herd dynamics are in order (close to natural buck and doe age structure and adult sex ratio), most - around 95% - of does will be bred in a 5-6 week period. Time and again when conducting fetal conception date data from herds with excellent herd dynamics we found this to be true. However, these conceptions are not evenly spread across the 5-6 week period. Conceptions start out slow the first week, explode in the 2nd into the 3rd week, and then trickle down to nothing by the end of the 5th or 6th week. In essence, conceptions are very "front loaded" in the process, and in most locations you can find a 10-day window during which 50% of conceptions occur. Below is a real-world data graph showing the percent of conceptions we found on a population of deer with exceptional herd dynamics. In the 2nd and 3rd week of breeding (14 days) 65% of conceptions occurred, even though the entire process took 6 weeks.
Of course, that's considering herds with great herd dynamics. But what if a local herd doesn't have great herd dynamics? In the Southeast, the rut can get really messy, and that's where you see "trickle ruts," where a little breeding is occurring over a long period of time. We have studied such herds, and I've seen conceptions dates spread across 120+ days (that's more than 3 months!). These herds had skewed sex ratios (often close to 3:1 pre-hunt), and buck age structures with few older bucks.
And here's another wrench in the machine - annual food sources. In areas where deer are heavily "acorn driven" (rely on acorn crops to load up on fat for the winter), an acorn failure year can so reduce the health of the local population that does enter estrus later than normal. My local deer population, that I have been tracking in great detail for many years, is very acorn driven. In any year with even a decent acorn crop, peak dates to see bucks chasing does are almost the exact same between years. However, in an acorn failure year, we will see peak chasing delayed about 7 to 10 days. This year was almost a total acorn failure year (we had a few in the early season but they were underdeveloped and wormy, and what few good ones existed were vacuumed up immediately). And this year our peak chasing was about 7 days late. In 2022 we had a total acorn failure, and peak chasing was 10 days late.
And remember when I said in a herd with good dynamics, 95% of does are bred within 5-6 weeks? Well what about the other 5%? They can and will be bred at some crazy times! I believe a few does get bred one estrus cycle (28-30 days) prior to the main estrus (peak of the rut). This is where the idea of a "false rut" in October comes from, I believe one or two does come into heat then and hunters see a flurry of rubbing, scraping and chasing at this time. In addition, some does breed at the normal time, but conception doesn't occur, so their bodies will go back into estrus 28-30 days after the normal peak. This is where the second rut comes in. And lastly, and something few hunters think about is that some does are incapable of conceiving for a variety of reasons, but their bodies will still keep going into estrus over and over (and they will be bred over and over) every 28-30 days until the breeding window closes. I've seen these type of does in estrus as late as early April. They are probably on their 5th or 6th estrus cycle that year, and bucks act accordingly!
