Monitoring Trends in Deer Abundance

On many occasions people have inquired about the number of deer on Cary Institute grounds. It is difficult to quantify the number of deer present on an unfenced 2,000-acre property (809 hectares). Even if it were possible to account for all the deer on the property at a given point in time, unless all ensuing births and deaths were accounted for the number would be of little value.

Trends within the deer population are more telling than simple head counts. Managers are interested in knowing if the population is increasing, decreasing or stable. By looking at trends management actions can be assessed. In the absence of this knowledge, managers would have no way to gauge a program's effectiveness and make adjustments if necessary.

There are a number of methods to gauge deer population trends; techniques and costs can vary widely. Most managers pick an index technique that will quantify deer abundance in a cost-effective way that is well suited to their site. When dealing with large areas (townships or counties), the buck kill/mile2 in the legal harvest is a useful index. Where appropriate, deer/vehicle encounters or complaints from agriculturalists can reveal population trends. On smaller discrete parcels these indices are less useful and techniques like spotlight counts are more applicable.

At the Cary Institute, deer abundance is managed through annual controlled hunts, with the goal of stabilizing the resident population. For over 25 years I have been monitoring trends in deer abundance to assess if the controlled hunts are meeting their goal. Spotlighting surveys (1981-1999) have been conducted for most of my tenure at Cary Institute. Since 1987, bow hunters have recorded observations of all deer seen during their hunts. Both techniques have identified similar trends in deer abundance. Currently, Winchcombe used bow hunter observations to track the trend in deer abundance at Cary Institute.

Spotlight Counts

Spotlighting counts at Cary were conducted in late September through mid November (1981-- to 1998--). Using a 4-wheel drive pickup with two, 400,000-candle power spotlights, a specific route was driven several times each year beginning one hour after sunset. The same open fields and roadsides were carefully examined on each trip. Members of the spotlighting crew were volunteers associated with our deer management program, making the effort affordable. A minimum crew of one driver and two spotlight operators with binoculars is required, however we always had extra hands for recording data and locating and identifying deer.

A standardized protocol was followed, allowing data within and between years to be compared. Data for all nights were averaged and used to examine trends. When looking across all 18 years of the data set, the regression analysis indicates the deer population was growing at about a rate of 2% a year between 1981-- - 1998--. In the absence of a hunting program, the growth rate could have easily been 30-40%. Data for more recent years, between 1987-1998, show no discernable trend either up or down.

Whether the objective of management actions are to increase, decrease or stabilize a deer herd, effective assessment must include a reliable index of deer abundance. Spotlight counts, when properly applied, can serve as such an index. The data also revealed that spotlighting for as few as 2 or 4 nights can provide reliable information for managers. The reduced effort can save agencies and private landowners both time and money when using spotlighting for indexing deer abundance.

Hunter Observation Technique

Spotlight counts are a reliable and affordable way to index deer abundance trends. However, the special permits and scheduling to bringing people on site to collect data in the evening is time consuming. In 1987, in an effort to establish a new index, Winchcombe decided to have bow hunters collect data on the number of deer they observed while hunting. Hunters kept track of the days and hours hunted and the number of deer seen on data sheets. Each hunter monitored an exclusive piece of property, with the exception of a few cases where hunters shared areas.

Bow hunter observations for a given year were pooled and an observation rate was calculated (the number of deer seen/hour of hunting for each year). When these data were examined for correlations with spotlight counts for respective years, there was a statistically significant correlation between the methods. Bow hunter observations were adopted as a surrogate for spotlight counts because these data tell the same story as spotlighting but, are essentially cost free to collect. Bow hunter observation rates were regressed against year and like the spotlight data for the same period, there was no discernable trend. A third independent index, the buck kill/mile2 for the local township showed the same regression result, no trend.

Reliable population indices are essential for assessing management actions. Winchcombe believes observations by bow hunters are a reliable index. Hunter observation data is easy and inexpensive to obtain, areas with poorly developed roads precluding spotlighting can be surveyed and this activity allows hunters to participate in scientific as well as management efforts.

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