Accounting for detectability when estimating avian abundance in an urban area
- Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand
Urban areas can support significant bird populations, including species of conservation concern, but urban ecologists have been slow to apply detectability-based counting techniques. We compared abundances and relative abundances of eight urban birds, derived using two commonly applied techniques (fixed-radius point and strip sampling) and distance sampling. We evaluated the influence of habitat and two covariates (observer and whether birds were seen or heard) on detectability. Due to built-up structures in urban areas, point counts are appropriate. Unavoidable and sometimes complex but necessary interactions with multiple property owners may compromise the number of points able to be counted and therefore the precision of estimates. Abundances from strip and fixed-radius point counts were on average only one-third (strip) and less than one-half (fixed-radius point) those obtained using distance sampling, with interspecific variation in the degree to which densities were underestimated. Rankings of relative abundances were mostly similar, although distance sampling ranked silvereye (Zosterops lateralis) and grey warbler (Gerygone igata) relatively higher in residential habitat. Habitat did not appear to influence detectability for most species, but the two covariates (observer and seen/heard) improved model fit for a number of species, indicating it is useful to record this information. Well-standardised non-detectability-based counts could provide useful information on community structure and relative abundances in urban areas, but distance sampling is necessary to track the population status of species, although it cannot usefully be applied to rare species.