Ibis (2007), 149, 693– 700 © 2007 The Authors Journal compilation © 2007 British Ornithologists’ Union Blackwell Publishing Ltd On the importance of g(0) for estimating bird population densities with standard distance-sampling: implications from a telemetry study and a literature review ERICH BÄCHLER* & FELIX LIECHTI Swiss Ornithological Institute, CH-6204 Sempach, Switzerland 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 prob- ability 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. Bird population trends are good indicators of biodiversity and environment (Gregory et al. 2005). Within this context, numerous monitoring pro- grammes for birds have been initiated in recent years. But despite all efforts that have been made to improve monitoring techniques, obtaining reliable estimates of bird abundance still remains a difficult challenge. As a first step in the estimation procedure, birds have to be counted. The most commonly used counting techniques are territory mapping, line transects, point counts and mist-netting. Count data are often used directly as an index of population density. Such index values do not consider detection probability of the birds and therefore may not represent absolute population densities (Emlen 1977, Burnham 1981), yet they are often used as relative values to make comparisons between datasets (e.g. Hutto et al. 1986). For such comparisons to be valid, mean detection probabilities of the studied animals have to be constant over the dimensions to be compared (e.g. Wilson & Bart 1985, Farnsworth et al. 2002, Norvell et al. 2003). However, detection probability of birds is affected by numerous factors (Farnsworth et al. 2002, Rosenstock et al. 2002) and therefore the assumption of stationary detectability is rarely met (Norvell et al. 2003, Kéry et al. 2005). To convert count data into estimates of absolute density, numerous methods that model detection probability have been developed. The most important *Corresponding author. Email: erich.baechler@vogelwarte.ch