Big Buck · Deer · 1 comment ·Jan 23, 2012

Solving the Buck Movement Mystery - Part I

Hunters have likely debated the factors influencing buck movement patterns ever since Native Americans first began chasing white-tailed deer more than 10,000 years ago. In the late 1950’s our knowledge regarding whitetail movements took a giant leap forward when the radio-transmitting collar was invented. Dr. Larry Marchinton was likely the first person to put one of these new radio collars on deer in the early 1960’s in Florida.

During the decades that followed, literally tens of thousands of whitetails have been captured and fitted with radio collars. Researchers and DNR staff from nearly every state inhabited by whitetails have tracked the movements of radio-collared deer at one time or another.

Unfortunately, the vast majority of these deer were either females or young bucks less than three years old. This is because throughout most of the whitetail’s range (1) adult sex ratios are unnaturally skewed toward females due to an overharvest of the buck segment of the herd; (2) buck age structure is unnaturally young, resulting in very few middle-aged (3-4 years old) and mature (5-6 years old) bucks (also due to an overharvest of bucks); or (3) middle-aged and mature bucks are present, but too difficult to capture because tree height precludes the use of the helicopter and net gun. As a result, the average home range and core area size for middle-aged and older bucks was unknown until recently!

A Big Buck Movement Study Is Born
It was not until the early 1990’s that a study was specifically designed to address the movements of middle-aged and older bucks. I was the lucky Ph.D. student assigned to this project under the guidance of Drs. Marchinton, Karl Miller, and Charlie DeYoung.
The study took place on the 44,000-acre Faith Ranch in Dimmit and Webb counties. Our research goals were to determine the seasonal and annual movement patterns of different age classes of bucks and to document how movement patterns changed in individual bucks as they matured. During 1992-95, we captured 125 bucks using a helicopter and drive net or net gun. All bucks were captured at random to provide a representative cross section of the buck segment of the deer herd. All bucks were aged and placed into one of eight age categories based on tooth wear. All bucks were uniquely ear tagged, tattooed, measured to determine gross Boone and Crockett Club (BCC) score and body size, photographed, and most importantly, fitted with radio-transmitting collars.

The radio collars that were placed on the 125 bucks allowed us to use telemetry to determine the locations of each buck on a daily basis. With the help of 11 different interns, we radio-tracked the bucks during one of three eight-hour shifts each day for the duration of the three-year study. By the end of the study, we had amassed nearly 20,000 individual locations for the 125 bucks!

The Results
Unfortunately, several bucks died or their radio collars stopped working before we had enough locations to determine a seasonal home range or core area, so these bucks were excluded from the analysis. As a result, our sample size was reduced from 125 bucks to 96 bucks. However, our study included more middle-aged/older bucks than any other telemetry study on whitetails.

Before proceeding with the results, we need to get a couple of definitions out of the way. Home range is defined as “the area traversed by the buck during normal activities of food gathering and mating” over a given time frame. Most home ranges are defined by the season or calendar year. Core area is defined as “that portion of the home range where locations are most concentrated.” Nowadays, computer programs are used to calculate both the home range and core area. In this case, the home range includes 95 percent of a buck’s locations, while the core area usually includes the 50 percent of a buck’s locations that result in the smallest area.

A Surprise!
Prior to our study, most researchers and hunters assumed that older bucks had the largest home ranges. The theory was that older bucks were the most dominant and therefore the most likely to range over the largest area. If this were true then home range size should increase as an individual buck increases in age.

Surprisingly, we found the opposite to be true. Young bucks (1-2 years old) had the largest annual home range, while middle-aged and mature bucks had smaller but similar-sized home ranges, and old bucks (7+ years old) had the smallest home ranges (see Table 1). This same pattern held true for the average size of a buck’s core area as well.

Average annual home range size for young bucks was 2,278 acres. Middle-aged bucks had a home range that averaged 1,233 acres. Mature buck home range size averaged 1,366 acres, while old bucks had a home range that averaged only 1,055 acres - less than half that of young bucks. Average core area was 356 acres for young bucks, 180 acres for middle-aged bucks, 210 acres for mature bucks, and only 151 acres for old bucks.

In my opinion, young bucks have the largest home ranges because they are the least dominate antlered buck in the population. These young bucks begin life within their mother’s home range. Then, usually around 18 to 22 months of age, they abandon their mother’s home range or are forced out. They then go in search of a home range of their own. Because nearly all-suitable habitat in south Texas already has deer present, these young bucks no doubt encounter resident deer everywhere they go. As a result, I would assume that it is fairly difficult for a young buck to establish a home range and “fit in.” In fact, this process often takes several months. When a young buck finally does find a place to fit in, he must learn where all of the best food and water sources are located, all of which results in longer distance movements and larger home ranges than occur for older bucks.

The situation is the opposite for old bucks. They have lived in their home range for at least five years already, so they know every nook and cranny. They have likely reached dominant status by this point, so no other deer are forcing them to move. And, they are old! We all know that older-aged animals, just like humans, move slower and less often. All of this results in old bucks having the smallest home ranges.

Join us next week as we investigate more the buck movement mystery in part two of this four-part series.