Population Management:
Population Inventory
Records are important in wildlife management just as they are in other forms of management. Accurate records are necessary to detect changes, measure progress, evaluate management success, and predict future needs. Records and survey data provide a factual basis for management decisions. Without them, deer management is based on guesswork.

Harvest records, spotlight surveys, and cruise surveys are the most practical means to inventory or monitor most Cross Timbers deer populations. Many other techniques have been tried in the Cross Timbers, but generally have been less efficient for estimating deer population parameters.

Harvest Records
Deer harvest records should include sex, age, weight (consistently either dressed and/or live weight), and buck antler measurements. This is the minimum information that should be recorded for every deer harvested. It is important that all harvest information be measured accurately because otherwise the information is worthless. Antler measurements (Figure 5) should include number of points at least 1 inch long, basal beam circumference of one or both main beams, outside length of one or both main beams from the burr to the tip, and maximum inside spread between the main beams.

Age is the most difficult of this harvest information to accurately measure. White-tailed deer ages are normally estimated according to the replacement and wear of teeth in the lower jaw. A manager should be aware that a good deer biologist consistently assigns deer to age-groups according to specified patterns of tooth wear and replacement, but the actual ages may be somewhat different. Some recent studies at the NFWU and other places examining wild deer of known ages indicate considerable variation between actual ages and estimated ages of deer over 2 years old using this technique. Nevertheless until something better comes along, the "age" information from the traditional tooth wear and replacement technique is still very useful for classifying similar groups of deer for comparison purposes, especially concerning the young and very old age-classes.

Almost anyone can learn to classify fawns, yearlings, and older deer (Figure 4 - see PDF) with less than an hour of study and practice. However, consistently aging deer over 2 years old requires much more study and practice. Most managers find it simplest to collect a mandible (lower jawbone) from each deer harvested and ask a deer biologist to age it.

Harvest records alone do not provide the complete picture of a deer population, but they provide enough useful information to definitely justify the time and effort necessary to collect them. Analysis of harvest records, such as weights and antler dimensions per age-class, provides insight about the relationship of a deer population to carrying capacity. Harvest records can also provide clues to population parameters such as buck age structure. Harvest records combined with spotlight surveys and/or cruise surveys provide a relatively well rounded view of a deer herd.

Deer Surveys
Currently, there are no true deer census techniques because it is not possible to obtain an exact count of a wild deer population. Instead, land managers and biologists must use population estimation procedures to estimate population density, sex ratio, and fawn crop from samples of a population. Several techniques are used to estimate population density in various regions of the country such as the spotlight survey, Hahn deer cruise survey, deer drive count, sex-age-kill method, pellet survey, trail counts, total area survey with a helicopter, and strip survey with a helicopter or airplane. Aerial surveys are usually more efficient than ground surveys in open or low brush situations. However, the types of timber predominant in the Cross Timbers afford poor visibility of deer from the air where wooded areas make up a significant portion of the habitat. The density estimation technique probably best suited for the needs of most Cross Timbers land managers is the spotlight survey.

Spotlight surveys are usually conducted during September, but can be used during the period from mid August to early October. The local game warden and sheriff should be informed about the location, date, and time of each spotlight survey so they will not confuse the activity with illegal night hunting. A spotlight survey is usually initiated about 1 hour after sunset but can begin later as long as the same general start time is used for all the surveys.

It is best if all-weather roads are used for the route to minimize the interruption of spotlight surveys during wet periods. The spotlight route should transect the various plant communities occurring on a tract of land in proportion to their relative abundance as best as possible. Surveying areas that do not appropriately represent the overall management area will misrepresent the deer population. For example, if a spotlight survey includes the best deer habitat or includes fields with crops that attract deer in greater proportion than they occur on the overall management area, the survey will likely overestimate deer density.

A spotlight crew consists of one or two observers and a driver. Observers should stand in back of a pickup, jeep, or similar open vehicle, or sit on an elevated platform to have better visibility over herbaceous vegetation and uneven terrain. The observers search the countryside with spotlights from each side of a vehicle driven along the predetermined route at approximately 5 to 8 miles per hour.

When two observers are used, each observer should concentrate on a separate side of the route. When one observer is used, only one side of the route should be surveyed. While looking for deer during the survey, a spotlight should be continuously and smoothly moved from the center of the route (0o) to about 140o on each side of the vehicle. It is acceptible to overlap the front center up to 5° and occasionally look behind the vehicle. When deer are sighted, the vehicle and spotlights stop just long enough to identify all deer in a group or until observers decide all the deer cannot be identified from a vantage along the route. While the survey is underway, the vehicle should not leave the route to locate or identify deer.

Every deer visible within 200 yards (some people use 250 yards) of the vehicle should be recorded as buck (male over 1 year old), doe (female over 1 year old), fawn, or unidentified deer. Deer seen outside this range are not included in the density estimate. Most managers also record whether bucks are spikes, physically mature bucks, or young bucks with multi-pointed antlers. Care should be taken to avoid misidentification of poorly viewed deer. Small antlers on bucks are difficult to see and fawns without nearby adults are difficult to identify, so an observer must be careful to accurately classify identifiable deer and report the rest as unidentified deer.

To estimate the acreage surveyed, the perpendicular distance that a deer could be sighted (primarily determined by cover and topography) should be measured or estimated at 0.1 mile intervals along the spotlight route. When two observers and spotlights are used, distances from both sides of the vehicle should be recorded at each interval. When one observer and spotlight are used, only the distance from the side searched with a spotlight should be recorded. Distance should not be measured or estimated during an actual spotlight survey because the process will alter the survey time and probably scare some deer away before they can be recorded. It should be done after a spotlight survey is completed or on a different day (night) during the same period as the spotlight surveys. The distances are averaged to calculate average visibility for the route.

The average visibility multiplied by the length of the spotlight route gives the sample area. The sample area divided by the average number of deer seen during a season’s spotlight surveys produces a deer density estimate. The ratio of does to bucks is a sex ratio estimate. The ratio of fawns to does is a fawn crop estimate.

Population parameter estimates from a single spotlight survey are usually not very meaningful because too much variability exists. But population parameters derived from several spotlight surveys during one season and trends established over several years are generally meaningful. At least three (five is a much preferred minimum) spotlight surveys should be conducted during a single season to generate information for management decisions.

Consistency is essential to obtain relatively precise information from spotlight surveys or any other population estimation procedure. Each time a spotlight survey is conducted, it should be performed in the same manner, along the same route, at the same time of year and night, with the same number of observers, with similar elevation of observers and spotlights, at the same speed, and under similar conditions. It is important to carefully plan a spotlight route initially to avoid changes.

Many deer seen during spotlight surveys are recorded as unidentified deer because positive identification as buck, doe, or fawn is often not possible in limited visibility with a spotlight at night. Ideally, a manager should strive to identify a sample of at least 100 deer (not necessarily 100 different individuals) each survey season from the management area for calculating sex ratio and fawn crop. Sometimes five or more spotlight surveys do not produce 100 deer identified as buck, doe, or fawn. Under these circumstances, cruise surveys (also called mobile line technique) can be used to gather additional information for calculating sex ratio and fawn crop. Cruise surveys may be the only type of survey needed in some situations. Deer density information is not essential for some management programs so carefully replicated spotlight surveys may not be necessary.

During cruise surveys, a person simply rides around the management area several times looking for deer. All deer sighted are examined using binoculars, identified if possible, and recorded. Cruise surveys should be conducted at the same time of year as spotlight surveys. They can be performed day or night. The first 2 hours of daylight, the last hour of daylight, and nighttime with a spotlight are most productive. Most deer managers utilizing cruise surveys ask their employees to carefully identify and record all deer seen in September during the course of their normal travel on the management area. This minimizes the amount of time devoted specifically to cruise surveys. Care should be taken to adequately sample the overall management area. If too many recorded deer sightings come from a relatively small portion of deer habitat on the overall management area, the data may misrepresent the population.

Real life survey results are not perfect. A person has to temper interpretation of survey results with other knowledge about a deer population and environmental factors. Table 5 presents a real set of data from spotlight and cruise surveys at the NFWU. This data set should help emphasize the point that such surveys do not exactly measure population parameters; they only approximate them.

According to survey information in Table 5, deer numbers dramatically increased from 1982 to 1983 and dramatically decreased from 1983 to 1984, indicating large fluctuations in population levels during this period. While deer numbers probably increased and then declined, the fluctuations were probably not as great as indicated by the survey data. Variability in the spotlight surveys may have caused the 1982 estimate to underestimate the actual population level and the 1983 estimate to overestimate the actual level. More replications of the spotlight surveys probably would have improved the estimates.

The probable increase in deer numbers from 1982 to 1983 was most likely due to better habitat quality on the NFWU than surrounding lands. NFWU habitat quality dramatically improved from 1982 to 1983 due to reduced cattle competition and the implementation of a prescribed burning program. Surrounding habitat quality deteriorated due to heavy cattle grazing during this same period. These factors encouraged more deer to spend more time on the NFWU.

The apparent population decline from 1983 to 1984 may be related to the effect of the 1983-84 severe winter on a relatively dense deer population. The 1983 population level appeared near or slightly above carrying capacity. The lowered carrying capacity during the severe 1983-84 winter probably resulted in increased natural deer mortality. When deer exceed carrying capacity, they are more susceptible to weather extremes.

The 1989 surveys provided insufficient data. The NFWU staff is not comfortable with the population parameters estimated with the 1989 surveys for the reasons given in the table’s footnotes. More replications of the spotlight surveys probably would have improved these estimates.

In many situations, it is not always possible to obtain a sample size of 100 identified deer every year. Due to other work priorities, rainy weather, or robust herbaceous vegetation (concealing deer), the NFWU staff achieved the desired goal of 100 identified deer during September in only 5 out of 9 years. A manager has to work with what is available realizing its limitations.

Though limitations exist, the surveys provide much useful information. From incidental deer observations, hunter success, tracks, and other indications, the NFWU staff believes deer were most abundant in 1983 and least abundant in 1986 and 1987. The spotlight surveys support this.

A relatively large number of does were removed from the NFWU for the food habitat and physiology studies during August 1985 through May 1987. The survey data demonstrate the depressing effect of the heavy doe harvest on the population. They also indicate a quick recovery afterwards that is typical of most white-tailed deer populations. Doe-only harvest was practiced at the NFWU during 1982-1987. The survey data indicate a dramatic shift in sex ratios as a result.

Through management, the NFWU deer carrying capacity improved from 1982 through 1990. Increased vegetative production is confirmed by the trend of decreasing visible area along the spotlight route. Fawn crops were much better in 1988 and 1990 relative to the fawn crops of 1982, 1984, and 1985 when densities were somewhat similar, indicating improved habitat conditions.