Landbird Counting Techniques: Current Practices and an Alternative

Abstract

Counting techniques are widely used to study and monitor terrestrial birds. To assess current applications of counting techniques, we reviewed landbird studies published 1989–1998 in nine major journals and one symposium. Commonly used techniques fell into two groups: procedures that used counts of bird detections as an index to abundance (index counts), and procedures that used empirical models of detectability to estimate density. Index counts rely upon assumptions concerning detectability that are difficult or impossible to meet in most field studies, but nonetheless remain the technique of choice among ornithologists; 95% of studies we reviewed relied upon point counts, strip transects, or other index procedures. Detectability-based density estimates were rarely used and deserve wider application in landbird studies. Distance sampling is a comprehensive extension of earlier detectability-based procedures (variable-width transects, variable circular plots) and a viable alternative to index counts. We provide a conceptual overview of distance sampling, specific recommendations for applying this technique to studies of landbirds, and an introduction to analysis of distance sampling data using the program DISTANCE.

Estimating the effects of detection heterogeneity and overdispersion on trends estimated from avian point counts

“…However, if the heaping effect occurred, it did not appear to have a great effect on inference about abundance, probably due to the relative length of our total time series compared to the three-year period during which heaping may have occurred. Nevertheless, our results support the recommendation of Rosenstock et al (2002) that when performing variable-distance circular point counts, an unlimited distance ring should be included even if the corresponding data are not used. Our results reinforce this recommendation regardless of whether distance sampling per se is the intended methodology.…”

“…If real, this heaping effect may be a good example of the potential for methodology to interact with observer psychology to result in misclassification of birds as being within the 100-m detection radius (Rosenstock et al 2002). However, if the heaping effect occurred, it did not appear to have a great effect on inference about abundance, probably due to the relative length of our total time series compared to the three-year period during which heaping may have occurred.…”

“…A sufficient condition underpinning the robust estimation of trends from uncorrected point counts is that the number of birds counted is strongly correlated with the actual number of birds present, e.g., E(C ) ¼ pN, where C is the number of birds counted, p is the detection probability, and N is true abundance (Johnson 2008). The validity of this assumption is often asserted to depend, in turn, on constant detection probability (Barker and Sauer 1995, Nichols et al 2000, Rosenstock et al 2002, Williams et al 2002. However, Johnson (2008) shows that two weaker conditions are sufficient for valid trend inference: (1) that p be independent of N, and (2) that the variance of p be ''small'' in relation to the variance of N (Johnson 2008).…”

“…Due in part to these difficulties, it is rarely possible to count all animals present in a survey area; typically, we do not know what proportion of the total number of individuals present are actually counted. Thus it is easy to question the utility of incomplete count data (Rosenstock et al 2002).…”

Multi‐trophic Invasion Resistance in Hawaii: Bioacoustics, Field Surveys, and Airborne Remote Sensing

“…Therefore, we established a simple analytical technique that extracts relative abundance of avian community size from bioacoustic recordings, and that is ready to be automated. Our analysis technique offers no quantifiable information on species composition and it does not account for differences in detection probabilities among species, habitats, or sampling periods (Rosenstock et al 2002). It may be suitable for detecting gross differences or changes in avian abundance when detection probabilities are constant.…”