Variability in grassland bird counts related to observer differences and species detection rates

Duane R Diefenbach, Daniel W. Brauning, Jennifer A. Mattice

Research output: Contribution to journalArticle

145 Citations (Scopus)

Abstract

Differences among observers in ability to detect and identify birds has been long recognized as a potential source of error when surveying terrestrial birds. However, few published studies address that issue in their methods or study design. We used distance sampling with line transects to investigate differences in detection probabilities among observers and among three species of grassland songbirds: Henslow's Sparrow (Ammodramus henslowii), Grasshopper Sparrow (A. savannarum), and Savannah Sparrow (Passerculus sandwichensis). Our review of 75 papers published in 1985-2001 found that the most commonly used methods were fixed-width transects (31%, 23 papers) and fixed-radius point counts (20%, 15 papers). The median half-width of fixed-width strip transects used by researchers was 50 m, but our results indicated detection probabilities were <1.0 at distances >25 m for most observers and species. Beyond 50 m from the transect line, we found that as many as 60% of birds were missed by observers and that the proportion missed differed among observers and species. Detection probabilities among observers ranged from 0.43 to 1.00 for Henslow's Sparrow, from 0.44 to 0.66 for Grasshopper Sparrow, and from 0.60 to 0.72 for Grasshopper Sparrow for birds detected within 58-100 m of the transect line. Using our estimates of detection probabilities for Henslow's Sparrows among six observers in a computer simulation of a monitoring program, we found that bird counts from fixed-width transects required an additional 2-3 years of monitoring to detect a given decline in abundance compared to density estimates that used a method to correct for missed birds. We recommend that researchers employ survey methods that correct for detection probabilities <1.0.

Original languageEnglish (US)
Pages (from-to)1168-1179
Number of pages12
JournalAuk
Volume120
Issue number4
DOIs
StatePublished - Jan 1 2003

Fingerprint

grasslands
grassland
bird
Passeriformes
line transect
grasshopper
birds
transect
researchers
songbird
survey method
monitoring
songbirds
methodology
computer simulation
surveying
rate
detection
experimental design
Ammodramus savannarum

All Science Journal Classification (ASJC) codes

  • Ecology, Evolution, Behavior and Systematics
  • Animal Science and Zoology

Cite this

Diefenbach, Duane R ; Brauning, Daniel W. ; Mattice, Jennifer A. / Variability in grassland bird counts related to observer differences and species detection rates. In: Auk. 2003 ; Vol. 120, No. 4. pp. 1168-1179.
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Diefenbach, DR, Brauning, DW & Mattice, JA 2003, 'Variability in grassland bird counts related to observer differences and species detection rates', Auk, vol. 120, no. 4, pp. 1168-1179. https://doi.org/10.2307/4090287

Variability in grassland bird counts related to observer differences and species detection rates. / Diefenbach, Duane R; Brauning, Daniel W.; Mattice, Jennifer A.

In: Auk, Vol. 120, No. 4, 01.01.2003, p. 1168-1179.

Research output: Contribution to journalArticle

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