Reducing Biases Affecting At-sea Surveys of Seabirds: Use of Multiple Observer Teams

Abstract

However, a fundamental problem has been the lack of method standardization (see Woehler & Van Franeker 1995) combined with a lack of method validation, which together could ultimately allow results to be grouped across studies to attain a common goal. Lack of standardization reflects the varying approaches among researchers to reduce factors that bias survey counts—primarily the effect of directional bird movement that is random relative to the movement of the survey vessel (reviewed in Tasker et al. 1984, van Der Meer & Camphuysen 1996, Clarke et al. 2003), although responsive bird movement toward or away from the ship is also a major problem. The most serious problem, random directional movement (as opposed to nonrandom directional movement— e.g. birds that are attracted to the survey vessel; see “Discussion”) usually results in density overestimation because most species fly faster than survey vessels move. Densities of birds that fly slower than a survey vessel (e.g. storm-petrels) are often underestimated (Spear et al. 1992, Spear & Ainley 1997a; see “Discussion” for an explanation).