On the importance ofg(0) for estimating bird population densities with standard distance‐sampling: implications from a telemetry study and a literature review

Abstract

Raw count data are often used to estimate bird population densities. However, such data do not consider detection probability. As an alternative, methods that model detection probability such as distance‐sampling have been proposed. However, standard distance‐sampling provides reliable estimates for absolute density only when the underlying assumptions are met. One of the most critical of these assumptions is that animals on a transect line or at an observation point have to be detected with certainty (the g(0) = 1 assumption). We radiotagged nine Orphean Warblers Sylvia hortensis and estimated their short‐distance detection probability. Birds were radio‐located in 264 cases in single bushes or trees. Their visual detection probability after a 5‐min search was only 0.58 (sd = ±0.14, range = 0.38–0.80), although the observer knew the bird's location. Furthermore, we carried out a literature review to assess how the g(0) = 1 assumption is handled in practice. None of the 28 standard distance‐sampling papers reviewed contained an estimation of g(0). In 57% of the papers, the g(0) = 1 assumption was not even mentioned. Nevertheless, none of the authors declared their estimates as being relative. Our empirical data show that the g(0) = 1 assumption would be severely violated for a foliage‐gleaning bird species at a desert stopover site outside the breeding season. The literature review revealed that the testing of the g(0) = 1 assumption is largely ignored in practice. We strongly suggest that more attention should be paid to the testing of this key assumption, because results may not be reliable when it is violated. If it is not possible to test the g(0) = 1 assumption or g(0) is less than 1, alternative methods should be used. Another possibility is to estimate detection probability by the means of radiotagged individuals.

Numbers, foraging and refuelling of passerine migrants at a stopover site in the western Sahara: diverse strategies to cross a desert

“…Less active birds were less often seen and caught. A telemetry study with radio-tagged Orphean Warblers in Ouadâne revealed that the visual detection probability was only 0.58 after a 5-min search and inactive birds were not detected in acacia trees even with a transmitter revealing their location (Bächler and Liechti 2007). Hence, we conclude that inactive untagged birds would often have remained undetected.…”

Response to human presence during nocturnal line transect surveys in fallow deer (Dama dama) and wild boar (Sus scrofa)

“…As noted by Bächler and Liechti (2007), "till now few attempts were made to validate distance sampling assumptions in birds studies" and this statement remains valid even when applied to terrestrial mammals. For instance, Ward et al (2004) showed that roe deer (Capreolus capreolus) were stationary on detection and did not change their behaviour during nocturnal observations.…”

Exploring the effects of truncated, pooled and sexed data in distance sampling estimation of breeding bird abundance

“…Buckland et al (2001) points out that the most critical assumption is that everything on the transect line is detected. Studies elsewhere shown that the g(0) = 1 assumption may be violated, even in very open habitats (Bächler & Liechti 2007). Density estimates can be corrected by using, for example, mark-recapture (Buckland et al 2004), radio-telemetry (Bächler & Liechti 2007), or double sampling (Bart & Earnst 2002), but this is often unrealistic for surveys with limited resources covering many species.…”

“…Although not tested formally, the SPTA survey was thought generally to fulfil these criteria. It is, however, worth pointing out that studies have shown the assumption g(0) = 1 to be untrue, even within open habitats (Bächler & Liechti 2007). At the beginning of the survey, field methods were standardized and fieldworkers were trained using range finders to estimate distances by eye.…”