Knowing when the peak of the whitetail rut will occur can assist hunters plan their fall deer-chasing excursions. But, if you want to stir up hunters as they sit around talking about those hunting plans, bring up the timing of the rut. You’ll get various comments like the following. “It was late last year.” “It was cold early, so the rut was early.” “The full moon really messed up the rut.” I smile when I hear these types of comments because the simple, known-forever, fact is that the peak of the rut never changes. For most of the whitetails’ range, excluding areas of the South and Southwest, the peak time for doe conception is November 15, plus or minus a week.

That’s the peak. The reason the peak never changes is because it is determined by photoperiod (day length) and that never changes. Mother Nature has had this figured out for thousands of years. She knows that from the time of conception in November to the date of birth is between 193-205 days. Do the math, you’ll find that most fawns are born around June 1. Mother Nature knows the timing that provides fawns the best chance for survival. Since survival is key for deer, they cannot be breeding for months in the fall and in turn, birthing fawns throughout the spring. It just doesn’t make sense from a fawn mortality standpoint. That’s why photoperiod is the trigger for breeding and it never changes.

For regular readers of this column, you know I’m a data guy. In fact, there are tons of data to show that from year-to-year the peak of the rut doesn’t change. Just because peak breeding is the same every year doesn’t mean the amount of rutting activity isn’t affected by the moon, weather, habitat and other factors. These factors are what hunters are talking about when they comment about the timing of the rut. Though the peak is the same, several factors can cause bucks to scrape, rub and chase does at other times before or after the peak.

For example, weather might cause more movement at the beginning of the rut. Yes, you may get bucks chasing does in October, say while a cold front is moving through and some does will get bred at that time. Alternatively, it might be unseasonably hot into November, with little rut activity observed until the last two weeks of November. Regardless, the peak for does being bred is still the same as always. 

Rut Research 

Dr. Duane Diefenbach has been looking at buck and doe movements in the fall for many years. His results are posted on the Penn State University Deer Blog. I’ve noted this in previous columns; you can get The Deer Blog via email by signing up at their website. It’s worth your time. A significant finding is that the median conception date for does in Pennsylvania doesn’t vary much.

 

              YEAR /DATE CONCEIVED /FULL MOON

               1999     November 11        November 23 

               2000     November 15        November 11

               2001     November 10        November 01

               2002     November 16        November 20

               2003     November 14        November 09

               2004     November 12        November 26

               2005     November 09        November 16

               2006     November 14        November 05

 

His data showed that on average for the years 1999 thru 2006, only 20 percent of does were bred by the last week of October. In contrast, by the end of November 85 percent of does had been bred.

What weeks saw the most conceptions? The two weeks from November 5 (21 percent) thru November 19 (22 percent) saw the highest percentage of does bred. The next highest was the week of November 20 (15 percent of all conceptions) and the week of October 29 (12 percent of all conceptions). Thus, the peak centered on November 13 and 14. Although, there was a fair amount of breeding two weeks prior and two weeks after. Dr. Diefenbach found only four fawns conceived in September and one the first week of March. When looking at late breeders, he found does being bred every week from December to March. Not a lot, but some.

If a doe isn’t bred in her first cycle, then she may be 21-30 days later. And, if she isn’t bred during the second cycle, then may be 21 to 30 days after that. That’s one reason we get late drop fawns. The second reason is because doe fawns can breed, and they do so late. Doe fawns breeding is common in the Midwest, but relatively unusual in Pennsylvania.

One September, I was bowhunting in Wyoming, sitting at a pinch point coming out of a huge alfalfa field. Some mornings I saw at least 20 does with fawns cross within 30 yards of my blind. There was an amazing number of twin fawns; things were looking good for the future of that area. However, one morning a doe and an exceptionally small spotted fawn came by. The doe was smaller than most of the does I observed, and her fawn was unbelievable small compared to other fawns observed. That doe was probably inseminated as a fawn the previous fall.

Yes, as mentioned, doe fawns can breed. Actually, doe fawn breeding is a good sign because body weight is the key. If a doe fawn is eating well and gets to be around 80 pounds by December or January, then they may be bred during that second or third breeding cycle. If you have poor habitat, almost no doe fawns will be bred. In good habitat, a surprising number will be bred. And they’ll drop their fawns late, because they were bred late. To illustrate that nutrition is the key, one Iowa study showed that 80 percent of doe fawns were bred their first year. Even more impressive is the fact that a good number of those fawns delivered twins. Amazing!

You might ask, what evolutionary advantage would there be to having doe fawns breed, especially since their later-born fawns are probably at a disadvantage going into winter? I’m just guessing here, but one thought is that if some catastrophe hit the June crop of fawns, these late fawns could be important for the population. 

Other Rut Factors 

What about moon phase and the rut? There are many theories on the impact of moon phase on the timing of the rut. I’m not here to argue for or against any of them. Again, let me summarize data collected by Dr. Diefenbach and his graduate students on the full moon. It shows that the date for a November full moon varies considerably, but the median conception dates do not vary (see table).

To find out if the full moon decreased daytime activity, one of Dr. Diefenbachs graduate students took locations of does fitted with GPS satellite collars in the month of October in 2015 and 2016.

Movement data compared full moon (greater than 67 percent illuminated), partial moon (between 33-67 percent illuminated) and new moon (less than 33 percent illuminated) conditions. The data showed that moon phase had an almost no effect on doe movement. None of the moon phases caused deer activity patterns to increase or decrease.

Of course, deer moved more at dawn and dusk, but in this study, does moved seven yards more per hour when there was a new moon verses a full moon. For a partial moon, deer only moved four yards more per hour versus on a full moon.

According to the Penn State study, deer don’t move more at night when there is a full moon. Maybe it’s not moon phase that affects deer movement. Perhaps, it’s moon position. In recent years, this theory has gained support based on the idea that the moon’s gravitational pull is stronger when the moon is directly overhead. We know that this causes ocean tides, so does it also affect deer behavior? Some hunters say yes, while others are less sure.

Personally, I tend to bowhunt during a rising moon. Are bucks more active at that time? Who knows? As you plan for the exact week in November that you should focus on for your hunt, be flexible. You now know that the full moon doesn’t affect the peak of the rut, but weather may, and that’s something you cannot predict until a week or so before it occurs. The bottom line for me is come late October or early November, I want to be deer hunting with my bow, no matter what.