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<p>The observer effect can influence the quantification of univariate metrics at both species and community levels. We examined which metrics significantly differ when calculated from the field work of two observers with comparable skills, recording breeding birds in a central Italian forest under similar environmental conditions and with standardized timing and methods. As expected, the observers differed in the number of individuals (abundance) and species (richness) they recorded. However, no significant differences emerged in mean abundance, richness, Shannon–Wiener diversity, Whittaker plots, or Simpson dominance. The only metrics showing substantial discrepancies were richness estimators (Chao 1 and Chao 2), driven by differences in the detection of occasional species—those recorded at only one (singletons) or two (doubletons) sampling points. Differences in observers’ ability to detect rare species appear to be the main source of the marked discrepancies. This diagnostic skill is likely more important than simply locating individuals of common species. When sampling to estimate community richness with multiple observers, it is essential to consider that observer bias primarily affects richness estimates and can distort rarefaction curves. Increasing the number of sampling points can reduce the influence of chance and improve data reliability. These findings also have implications for teaching field-based monitoring techniques in academic courses in community ecology.</p>

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Observer bias in bird census: in forest assemblages, richness estimates appear the most sensitive metrics – a cautionary note with educational implications

  • 期刊论文

  • 发表日期：

    2026年2月2日

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  • Corrado Battisti,
  • Francesco Bonucci,
  • Fabrizio Branca,
  • Antonio Gerardo,
  • Pietro Giovacchini,
  • Mirena Haxhiu,
  • Chiara Marescalchi,
  • Letizia Marsili

摘要

The observer effect can influence the quantification of univariate metrics at both species and community levels. We examined which metrics significantly differ when calculated from the field work of two observers with comparable skills, recording breeding birds in a central Italian forest under similar environmental conditions and with standardized timing and methods. As expected, the observers differed in the number of individuals (abundance) and species (richness) they recorded. However, no significant differences emerged in mean abundance, richness, Shannon–Wiener diversity, Whittaker plots, or Simpson dominance. The only metrics showing substantial discrepancies were richness estimators (Chao 1 and Chao 2), driven by differences in the detection of occasional species—those recorded at only one (singletons) or two (doubletons) sampling points. Differences in observers’ ability to detect rare species appear to be the main source of the marked discrepancies. This diagnostic skill is likely more important than simply locating individuals of common species. When sampling to estimate community richness with multiple observers, it is essential to consider that observer bias primarily affects richness estimates and can distort rarefaction curves. Increasing the number of sampling points can reduce the influence of chance and improve data reliability. These findings also have implications for teaching field-based monitoring techniques in academic courses in community ecology.