Observer effects occur when estimating alert but not flight-initiation distances

Context. The estimation of alert (vigilance) and flight-initiation (escape) distances (AD and FID, respectively) has underpinned theoretical and applied studies of the escape behaviour and management of disturbance to wildlife. Many studies use multiple observers, and some conduct meta-analyses; these efforts assume no observer effects in the estimation of these distances. Aims and methods. We compared the estimates of FID and AD under ideal conditions (i.e. of black swans, Cygnus atratus, a large species with obvious behaviour, and at a location where swans allowed close approaches in open habitats), by one experienced and four inexperienced observers. Key results. FID did not differ among observers but AD differed between the experienced and all inexperienced observers, and among inexperienced observers. Thus, FID estimates appear more repeatable than those of AD. Experience apparently results in more conservative estimates of AD. Conclusions. FID represents a repeatable measure that is consistent across observers. This study supports its broad application in the study of wildlife escape behaviour. Implications. We recommend the use of FID rather than AD for comparative analyses that involve multiple observers, because FID is more reliably measured.