Moon Watch

Moon Watch

C. J. Winand

Wildlife biologist C.J. Winand regularly pens this magazine’s “Huntin’ Whitetails” column.

Can watching the moon help you see more deer? One researcher says yes.

NOWADAYS IT SEEMS we have moon charts for everything. These charts tell us everything from the best time to have children to the best place to set up for deer. If you ask fishermen about the effects of the moon on fishing, you’ll likely get as many negative as positive responses. But, ask a police officer, bartender, or emergency room worker, and you’ll probably hear that the moon definitely influences human behavior.

Astronomers will tell you the moon’s gravitational pull is stronger on some days than others, and some people believe this gravitational pull affects animal movements. During the past 50 years, many wildlife biologists have wondered if the moon plays a role in deer movements. Some very prominent outdoor writers have said, “Ignore moon phases.” Others argue, “Those who ignore moon phases should spend more time in the woods and less time in the library.” And still others say, “Moon phases might have some effect on deer activity, but there are too many other variables, such as weather and hunting pressure, to sort it all out.”

So, what are hunters to believe? Can wildlife biologists and hunters make any sense of the moon phenomena?

The answer is yes, and the tool that many hunters and biologists now use is called a Deer Activity Index (DAI). The DAI was developed by Dr. Grant Woods as he was completing his Ph.D. in Forest Resources at Clemson University. To date, it is the only scientifically and statistically reliable index we have for predicting daytime deer activity.


Now, before you get all excited that the DAI is the panacea for all your deer hunting problems, please remember that the DAI is only an index, which shows trends, not absolutes. Dr. Woods will be the first to tell you that the DAI is only a guide or tool for predicting movements of deer to increase your odds. It carries no guarantees, and, as always, nothing beats putting time in at a well-placed treestand.

Obviously, if we could hunt every day, we would not need this tool, because we eventually would be Out there on the best days. But many of us can get out only once a week, or every other weekend, and the DAI could possibly be of value, because it rates every day during the deer season on a scale of 4 to 10. Days rated a 4 are predicted to have the lowest daylight deer activity, while days rated 10 are predicted to have the highest. If you have only limited time to spend afield, you can use the DAI to pick the potentially best days.

Does this mean a 10 guarantees you’ll see deer? No. Does it mean you should not hunt on days rated a 4? Again, no. The index simply tells you which days give you the highest, and lowest, odds. Although, your best chances to see deer are on days rated 8, 9, or 10, obviously any day, 4 through 10, could produce for you.


With any system like this, the obvious question is: How did the DAI come about? Back in 1991, Woods and his fellow biologists started to log data on every buck, doe, fawn, and unidentified deer they saw after every morning and afternoon hunt they conducted on free-ranging deer at various locales around the country. Since 1991, Woods and his research associates have collected data from more than 10,000 deer observations. In addition, they have data on more than 900 deer harvested for research purposes. From a statistical perspective, this sample size is very impressive.

Now, before looking further into the data, we must understand some basic moon characteristics. Woods worked with astronomers at various observatories around the country. He found that the moon’s orbit is different every day, based on three parameters. First, the distance from the Earth to the moon varies on a daily basis. Second, for North American hunters the declination (the number of degrees the moon’s orbit is to the equator) is always between 28 degrees north or south of the equator. Third, and most obviously, is that the moon’s illumination changes from full to waning to waxing to new (dark) once a month.

What this all means is that each day the moon is in an entirely different position. For example, if we assume that an October full moon is 12 degrees north of the equator and 241,899 miles from your area, next month’s full moon could be completely different. In fact, a November full moon could be 27 degrees north of the equator and only 219,786 miles from your prized honey-hole. The point here is that the moon’s position in relation to Earth is always changing, even from one full moon to the next. This applies to all other moon phases as well. The limitless variables probably explain why hunters have so many opinions concerning moon phase and the best times to hunt!

I vividly recall my Pap explaining to us kids that the full moon period is the best time to hunt. Although it may not be significant, Woods also found this to be true. But Woods’ data show a big overlap between deer sightings and the different full moons, plus other moon phases. Remember, not all full moons are created equal. This could explain why some hunters see the most deer during a full moon, while others, as Woods says, “would be better off writing a thesis on the breeding behavior of squirrels.”

With all the fluctuating data, Woods spent many a late night computing numerous combinations of the moon’s orbit, distance, illumination, and other parameters, and he finally came up with a set of seven factors that started to make sense. (In case you’re wondering, yes, I did ask him exactly what these parameters were. Woods explained that he might share his raw data some day, but for now, he’s keeping that to himself.) The end result for hunters is a continuously updated DAI, which Woods has incorporated into a handy calendar to help hunters maximize their days afield. Overall, Woods says, the DAI has been 71 percent accurate since its conception.


Like many people, I was very skeptical about the value of the DAI calendar, so my associates and I tried applying it to one of our managed properties in Maryland, a 100-acre farm just outside Baltimore, Maryland. Hunting that property, six of us hunters recorded all of our deer sightings and correlated our data to that of the DAI. Out of 103 hunts, we saw 474 deer and harvested 25, and we observed a buck-to-doe ratio oft buck to 7.6 does (fawns were included in the doe count).

Then we split all the deer sightings into three groups relative to DAI rankings: 4,5, 6 days; 7 days; and 8,9, 10 days. Comparing our sightings with these groupings, we saw 4.66, 4.50, and 4.57 deer per hunt, respectively. Thus, it would appear that we had an approximately equal chance of seeing deer on any DAI-value day.

But, when we grouped the data into the number of buck sightings per hunt (we assumed hunters would see more deer, especially bucks, on the higher value DAI days) the DAI proved itself with 1.86, 3.07, and 2.59 sightings, respectively. These observations make sense given the high number of fawns observed. Research with self-triggering cameras has shown that fawns are more active than adult deer throughout daylight hours, regardless of other factors.

Bill Fehon (of the United Bowhunters of Pennsylvania) and I work with a group of bowhunters called the Bryn Athyn Marksmen’s Association (BAMA), located in suburban Philadelphia. When we compared deer harvests to the specific DAI value, we found a strong correlation. From October 19, 1998 to January 9, 1999, the BAMA group killed 74 deer. Again, as in Maryland, we compared hunter success data to the DAI predictions, and we found that the DAI not only predicted deer movements correctly, but paralleled the harvest data.


Although this is impressive, surely some variables offset the DAI? For example, you might expect food availability to influence DAI predictions. Given a bumper crop of acorns, you’d assume that deer would remain relatively stationary, because they can find food just about anywhere they are. Theoretically this makes sense. But Woods still found that during a bumper acorn crop, “Researchers observed an average of 5.44 deer per hunt on the 8,9, or 10 DAI days, compared to 2.38 on the 4, 5, or 6 DAI days.” Evidently, then, food won’t totally override the DAI. Woods says, “When a bumper acorn crop is present, deer seem to move more before dark during the high DAI versus low DAI days. On low acorn years, deer observation averages may be higher since deer must search longer for food. The bottom line is that food changes the degree of precision of the DAI, but not the accuracy of the DAI.”

How about the affects of weather on the DAI? Temperature, wind direction, wind chill, humidity, dew point, and cloud cover all must have some impact on deer movements, right? Woods states, “Abnormal climatic factors, such as a rapidly falling barometer associated with storms, can have a greater influence on daytime deer activity than the DAI. But during ‘normal’ weather conditions, the DAI is a better predictor of daytime deer movements than is the weather. In addition, the weather is difficult to predict and cannot be used to plan vacation days, but the DAI can.”

We all know that hunting pressure in the form of hunter numbers, hunter activity, and habitat type can influence deer movements. Woods found that, excluding opening days, the DAI also appears to outweigh hunting pressure as a predictor of daytime deer activity. Peaks in hunting pressure seem to affect deer activity more than constant hunting pressure. It is important to remember that Woods’ data were actually collected during hunting seasons from various locales across the United States.


Does the DAI give us an indication as to the rut timing? Biologists know that rut timing varies from one herd to the next. Within deer herds with proper age structures and balanced sex ratios, the rut will be short and intense. Given this scenario, Woods believes the rut will probably outweigh the DAI. This is not to say, however, that the DAI is without value. Woods has found that the DAI indicates whether the majority of rutting activity will take place during daylight or nighttime hours.

In many parts of the country deer herds have lopsided age structures and unbalanced sex ratios. In these herds the rut normally lingers over a longer period of time and lacks the intensity observed in well-balanced herds. Biologists call this prolonged breeding season a “trickle rut.” In the trickle rut scenario, Woods found the DAI to be valuable, because the rut does not override the index. Sure, some rutting activity takes place during low DAI-value periods, but you’re less likely to observe it because it could be taking place at night. Remember, the DAI only predicts daytime deer activity, not rutting or nighttime movements. It is also important to remember that the DAI does not tell you where to hunt, but rather when you can expect to see maximum daytime activity.

Grant Woods will be the first to admit that no one has a magic potion for predicting deer movements. However, he has shown that the DAI reliably indicates trends. Of course, if you can hunt every day, you don’t need this help. But if you have only limited time, the DAI can help maximize your time afield. And that’s a good start toward successful deer hunting.

COPYRIGHT 1999 PRIMEDIA Special Interest Publications

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