Solving the Buck Movement Mystery - Part II
Seasonal Home Range
The bucks in our study had the smallest home range during spring, which we defined as April 1st through May 31st. At this time, average home range size was only 694 acres, an amount half as big as the other seasons. The next smallest seasonal home range was during summer (June 1st through September 30th) at 1,154 acres. Surprisingly, home ranges were largest during the prerut period (October 1st through November 31st) at 1,292 acres. Average home range size during the rut (December 1st through January 10th) was 1,236 acres. During post-rut (January 11th through March 31st), average home range size was 1,273 acres.
I would guess that spring home ranges were smallest because this is typically when forage conditions are best, thanks to the flush of new plant growth that occurs each spring. Even without rain, brush plants will leaf out, flower and often put on new growth during spring. Spring showers, when they do occur, also result in forbs (broad-leafed weeds) carpeting the landscape. Forbs are, by far, the most preferred plant type for deer. Obviously, with improved forage conditions, bucks do not have to move as much to fill their stomachs, their main concern during spring.
Summer home ranges were relatively small, likely because of the high temperatures south Texas experiences during this time of year. Bucks no doubt reduce movements during the hottest portion of the day, resulting in smaller home ranges.
Unexpectedly, home range size was largest during pre-rut. During pre-rut, bucks break apart from their summer bachelor groups and travel alone until the post-rut when they begin to reform bachelor groups in late January. During pre-rut, a sudden increase in the hormone testosterone causes the antlers to harden and a buck’s focus to shift from forage to females. Now, instead of being most interested in filling their stomachs, their interest shifts to scent marking trees, making scrapes, and sparring with other bucks to establish loose breeding ranges.
Mature bucks especially, increased their movements and resultant home range sizes during the pre-rut. Average home range size for a mature buck during summer was 1,218 acres. This jumped to over 1,800 acres during pre-rut, an increase of 50 percent. Then, during rut, home range size decreased to only 1,216 acres, an amount nearly identical to their summer home range size. During post-rut, home range size increased again to 1,396 acres.
The pre-rut home range size for mature bucks was 113 percent larger than the home range size for old bucks, 68 percent larger than the home range size for middle-aged bucks, and 28 percent larger than the home range size for young bucks. The only plausible theory that I can think of for this is that mature bucks, more than any other age class, were in their prime. These bucks were at their peak in body and antler size and therefore the most likely to be dominant. As a result, they encountered the least resistance when traveling the perimeters of their home range to scent mark new areas.
We expected home ranges to be largest during the rut when bucks were most actively chasing females. However, in our study, home range size during the rut was smaller than for either the pre-rut or the post-rut. My only guess for an explanation is that the buck segment of the deer population on this particular ranch was very well balanced. In fact, 40 percent or more of the bucks observed during the annual helicopter survey were estimated to be mature! The high density of older-aged bucks may have resulted in a more “structured” rut than typically occurs because a high percentage of bucks had prior rut experience. These bucks may have had a more defined dominance hierarchy as well that resulted in less movement.
Smaller than expected home ranges during the rut may also have been due to our sampling technique and may not have been a true indication of what was happening during the rut. Standard telemetry is very time consuming and as hard as we tried, we were only able to get two to four locations per buck per week. The newer GPS collars on the other hand, work off satellites and allow for the gathering of locations as frequently as every 15 minutes. This translates to almost 700 locations per week versus only two to four!
The infrequent sampling schedule for the telemetry method that we used may have resulted in missing long distance buck movements, especially if those movements only lasted a day or two. In fact, preliminary results of an ongoing buck movement study involving GPS collars, indicates a high percentage of bucks embark on “excursions” during the rut. These excursions can involve traveling straight-line distances of 15 miles or more and typically only last a day or two.
Buck home ranges were surprisingly large during the post-rut. We expected smaller home ranges, thinking that most bucks should have been worn out from the rigors of the pre-rut and rut and less likely to move. However, in hindsight, bucks may have moved more at this time because they were worn out. On average bucks lose around 30 percent of their body weight during the rut. During post-rut, testosterone levels drop causing the antlers to cast and the focus to shift from females back to forage. It is likely that the “need to feed” was highest during post-rut, causing the increase in buck movements during this time of year.
Site fidelity is a fancy term used to describe the percent overlap in an individual buck’s home range from one year to the next. We tracked the movements of several bucks over multiple years, which allowed us to measure site fidelity. Surprisingly, bucks did not shift their home ranges much from one year to the next. In fact, home ranges for old bucks overlapped 73 percent from year to year. Mature bucks overlapped 66 percent and middle-aged bucks overlapped 61 percent.
The above pattern of increased site fidelity as a buck gets older makes sense because the older the buck, the more familiar he is with his home range and areas nearby his home range. This familiarity should allow an old buck to move less to meet his day-to-day needs. Older bucks should also be more established in the local buck dominance hierarchy, also allowing for reduced movements. Finally, much like humans, older bucks would naturally be expected to move less simply because they are older. The opposite would be true for young bucks and to a lesser degree for middle-aged bucks.
Seasonal site fidelity was highest during the summer, which means that bucks were more likely to stay in the same summer location from one year to the next than any other season. Lowest seasonal site fidelity occurred during the rut. This means that individual bucks altered their year-to-year home range more often during the rut than any other season. Apparently, bucks explored new areas most often during the rut, which was what we had predicted. This change in home range during the rut was no doubt influenced by the need to search for females, what I call the “need to breed.”
Individual Bucks Are Highly Variable
What the above averages do not show is how variable home range and core area size is among individual bucks. If you control for the influence age has on buck movements by only considering bucks within the same age class, there is still a lot of variability from one buck to the next. In fact, our more recent GPS data show that annual home range size can vary for two bucks of the same age from a low of only 300 acres to well over 10,000 acres! Therefore, it is important to keep in mind that although the averages indicate general trends, an individual buck may move a lot more or a lot less than the average.
From a hunting standpoint, the most difficult buck to harvest is the buck with the smallest home range because this buck moves the least, reducing the chance of an encounter while hunting. Obviously, bucks that have the largest home ranges and that move the most, are the bucks most susceptible to harvest. As a result, our data support what every hunter already knows… young bucks are easier to kill than middle-aged bucks, which are easier to kill than mature bucks. Old bucks that are seven years old or older are the hardest to kill, not because they are necessarily any smarter than a younger buck, but because on average, they move the least and have the smallest home range.
Tune in next week for the third part of this four-part series on solving the buck movement mystery.
Posted by Dr. Mickey W. Hellickson
Photo by Hardy Jackson