Distance sampling to estimate densities of four native forest bird species during multi-species surveys

The suitability of line-transect-based distance sampling to robustly estimate population densities of bellbird (Anthornis melanura), kererū (Hemiphaga novaeseelandiae), North island tomtit (Petroica macrocephala toitoi) and tūī (Prosthemadera novaeseelandiae) during concurrent multi-species surveys was investigated. Densities were estimated annually from 2006 to 2009 at three sites within the Coromandel Forest Park, New Zealand. The line-transect distance sampling method appeared to be suitable for estimating population densities of kererū, tomtit and tūī for little additional cost than a single-species survey. Potential violation of the three most important distance sampling assumptions was expected to have been minimised for these species; however, distance estimation errors were most likely to bias density estimates. In this study, the line-transect distance sampling method was not found to be suitable for estimating bellbird densities.