Date of this Version
Johnson, D.H., J.W. Solberg, and C.L. Amundson. Countability of sandhill cranes in aerial surveys. In: Hartup, Barry K., ed., Proceedings of the Eleventh North American Crane Workshop, Sep 23-27, 2008, Wisconsin Dells, Wisconsin (Baraboo, WI: North American Crane Working Group, 2010), pp. 89-97.
Aerial surveys are used to monitor populations of many wildlife species, including sandhill cranes (Grus canadensis). In addition to the usual problems of detectability (involving both availability and perceptibility), aerial surveys of concentrated animals are subject to countability issues; from a rapidly moving aircraft, observers cannot count or accurately estimate the number of animals in a large group. Calibration is sometimes performed in an effort to adjust aerial counts for incomplete detectability and countability by calculating the ratio of animals actually in a group to the number in the group estimated from the aircraft. Here we explore alternative, model-based approaches to the analysis of those adjustment ratios using aerial survey data of sandhill crane concentrations during 1978-2007 in the Platte River Valley of Nebraska. Ratios varied by year and by observer. In addition, the ratio varied with the actual size of the concentration. Modeling can be used to develop improved estimates of the ratio.