Dear Ecolog-L, Wilson et al. (2000; see citation below) compared two methods (using a paired design), namely line transects and point counts, for collecting data on migratory landbirds in forested wetlands. In their publication they stated the following:
"... Our goal was to compare species composition and abundance within each forest stand using these two survey methods. However, we did not overlay point counts on transects because of the potential to bias detections when observers had prior knowledge of species identification and location. Thus, we alternated survey methods between the parallel survey lines on subsequent visits. We varied the order in which we conducted survey methods to ensure that each method was applied to each survey line and that each method preceded the other on approximately half the visits. ..." I hope that is enough of a quote for my purposes. What I am most interested in is their decision to not overlay the point counts on the line transects because this would bias detections due to prior knowledge of individuals locations. Obviously this increased their sampling effort and (for now) let me say I was wondering if there is a way to control for this bias so the point counts and line transects could be overlain. It has been suggested to me that this bias can be controlled for statistically by taking the difference between each line transect and the overlain pooled point counts. I was told this eliminates the requirement of "independence". After further consideration of this guidance I remain unconvinced, because I believe a biased sample remains a biased sample regardless of the statistical procedure performed upon it. The point count almost certainly would improve detectability of individuals on the first and last 150m of the line transect, and I believe any "difference" computed between them would be flawed by that bias. Thank you for your thoughts, David Wilson, Twedt, and Elliott (2000). COMPARISON OF LINE TRANSECTS AND POINT COUNTS FOR MONITORING SPRING MIGRATION IN FORESTED WETLANDS. J. Field Ornithol., 71(2):345355
