In April 2000, I wrote a News and Views article about using infrared-triggered cameras to monitor and estimate deer populations on the Noble Research Institute Wildlife Unit (NFWU). With another year of data collection under my belt, I thought that I would show some of that data to you and compare it with spotlight survey data collected on the NFWU. Although it is too soon to draw any conclusions, the developing trends are interesting.
The table outlines the population-characteristic estimates derived by using the two survey techniques.
Spotlight surveys consistently yielded higher population density estimates than did the infrared-triggered fixed-camera (ITFC) surveys, yet most research indicates that spotlight surveys tend to underestimate population densities. If this trend were true for the NFWU, one would conclude that the ITFC survey grossly underestimated the population density, which I don't believe is the case. We do know that the ITFC method does not photograph all of the deer, so the population density estimate derived from the ITFC survey is to some degree an underestimate. I believe, however, that the ITFC method detects a high percentage of the deer on the property and that the estimate derived by using this method more closely approximates the true situation on the NFWU than does the estimate derived by using the spotlight survey. This statement should not be generalized as being true for all locations, however.
Buck:doe ratio estimates are similar for both survey techniques. Each yielded estimates between 1 buck : 1 doe and 1 buck : 2 does. Neither technique consistently yielded higher or lower estimates.
Net fawn-crop estimates derived by using the ITFC survey were consistently higher than those derived by using the spotlight survey. In 1998 and 1999, differences were fairly large. Cover is excellent on the NFWU and some fawns don't follow their mothers closely, either of which could have made fawns difficult to observe during spotlight surveys, thereby causing the large differences between the survey types. Inability to observe and identify fawns would result in lower net fawn-crop estimates.
We cannot definitively say which survey technique is more accurate. However, the ITFC survey has tremendous potential. We are excited about using this tool and plan to continue identifying and defining its merits and limitations.