Key points are not available for this paper at this time.
Jon E. Swenson, Stephen J. Knapp, Peter R. Martin, Thomas C. Hinz, Reliability of Aerial Cache Surveys to Monitor Beaver Population Trends on Prairie Rivers in Montana, The Journal of Wildlife Management, Vol. 47, No. 3 (Jul., 1983), pp. 697-703
Swenson et al. (Fri,) studied this question.