Aerial Survey Methods for Dall's Sheep

Most Dall’s sheep (Ovis dalli dalli) population monitoring efforts utilize intensive aerial surveys with no attempt to estimate variance or adjust for potential sightability bias.  We used radiocollared sheep to assess factors that could affect sightability of Dall’s sheep in standard fixed-wing and helicopter surveys and to evaluate feasibility of methods that might account for sightability bias.  Work was conducted in conjunction with annual aerial surveys of Dall’s sheep in the western Baird Mountains, Alaska, 2000 – 2003.  Overall sightability was relatively high compared to other aerial wildlife surveys, with 88% of the available marked sheep detected in our fixed-wing surveys.  Total counts from helicopter surveys were not consistently larger than counts from fixed-wing surveys of the same units and detection probabilities did not differ for the 2 aircraft types.  Our results suggest that total counts from helicopter surveys cannot be used to obtain reliable estimates of detection probabilities for fixed-wing surveys.  Groups containing radiocollared sheep often changed in size and composition before they could be observed by a second crew in units that were double surveyed.  Double-observer methods that require determination of which groups were detected by each observer will be infeasible unless survey procedures can be modified so that groups remain more stable between observations.  Mean group sizes increased during our study period and our logistic regression sightability model indicated that detection probabilities increased with group size.  Mark-resight estimates of annual population sizes were similar to sightability model estimates and confidence intervals broadly overlapped.  We recommend the sightability-model approach as the most effective and feasible of the alternatives we considered for monitoring Dall’s sheep populations.

Sightability Model Software

As a part of this project, we developed generalized software that uses aerial survey data to estimate parameters of a logistic regression sightability model and then uses that model with additional aerial survey data to estimate population size.  Sightability is estimated as a function of group size and confidence intervals are based on bootstrap methods as described by Udevitz et al. (2006).  The software consists of an R function, named SIGHTMOD, that provides a simple user interface for retrieving the data from Excel files and computing the estimates.  It does not assess suitability of the logistic regression sightability model for any particular application, but once it has been determined that this estimation approach is appropriate, SIGHTMOD is intended to provided a quick and convenient way to obtain routine estimates from these types of surveys.

 sightmod.R:  Text file containing the R function SIGHTMOD

 sightmod.pdf:  PDF file containing documentation and instructions for using SIGHTMOD

 example_marked_data.xls:  Excel file containing example data from a survey of marked animals for estimating parameters of a sightability model

 example_survey_data.xls:  Excel file containing example data from an aerial survey for estimating population size

 R Project for Statistical Computing:  Web site for downloading R software

References

Kleckner, C., M. Udevitz, L. Adams, and B. Shults. 2003. Demography of Dall's sheep in northwestern Alaska. Arctic Research 16:68-73.