What question did this study set out to answer?

The study aims to enhance the accuracy of population size estimates for Williamson’s and Red-naped Sapsuckers using the Partners in Flight methodology.

April 11, 2026Avian Conservation and Ecology0 citationsOpen Access

Estimating Williamson’s Sapsucker and Red-naped Sapsucker population sizes using the Partners in Flight method with data-derived adjustments

LGLes W. GyugLife Services (United States)

Key Points

The study aims to enhance the accuracy of population size estimates for Williamson’s and Red-naped Sapsuckers using the Partners in Flight methodology.
Applied the Partners-in-Flight (PIF) method to convert relative density data to absolute density.
Incorporated data-derived adjustments for pairs and detection distance based on playback survey results.
Introduced seasonal adjustments to account for lower territorial advertising during BBS count periods.
Conducted distance analysis to estimate Effective Detection Radius (EDR) for both species.
Revised PIF estimates increased from 300,000 to 870,000 for Williamson’s Sapsucker globally.
Revised PIF estimates increased from 180,000 to 490,000 for Red-naped Sapsucker in British Columbia.
Estimated Effective Detection Radii were significantly higher than PIF estimates, at 249m for Williamson’s and 212m for Red-naped Sapsuckers.
Confidence intervals were -28% to +34% for Williamson’s and -34% to +44% for Red-naped Sapsuckers.

Abstract

The Partners-in-Flight (PIF) method converts the relative density of North American Breeding Bird Survey (BBS) data into absolute density to estimate landbird population sizes. Here, I attempted to improve the accuracy and reliability of the PIF estimate for Williamson’s Sapsuckers (Sphyrapicus thyroideus) over their global breeding range, and for Red-naped Sapsuckers (S. nuchalis) in Bird Conservation Region (BCR) 9 in British Columbia. I substituted data-derived adjustments for the expert opinion adjustments for pairs and for detection distance. Sapsucker territorial advertising (drumming and calling) is lower during the BBS count period (late May to early July in B.C.) than earlier in the breeding season (late March through early May) so I added a seasonal adjustment to the PIF method based on the regression of playback survey results on BBS routes against long-term BBS route means. The pair adjustment for Williamson’s Sapsucker was 1.867 (SD 0.09) based on four separate sources, up from the PIF-estimated category of 1.5 (SD 0.13). Detection distance estimated as Effective Detection Radius (EDR) using distance analysis methods was much higher for Williamson’s Sapsucker (249 m) and for Red-naped Sapsucker (212 m) than the PIF-estimate of 121.5 m. The seasonal adjustment for Williamson’s Sapsucker was 11.78 (SD 0.09) and for Red-naped Sapsucker was 8.77 (SD 0.51), i.e., BBS long-term route means detected, on average, only 8.5% and 11.4%, respectively, of the birds known to be present based on playbacks. When these adjustments were applied, the rounded PIF estimates were revised upwards from 300,000 to 870,000 for the global range of Williamson’s Sapsucker, and from 180,000 to 490,000 for Red-naped Sapsucker in BCR 9 in B.C. Ninety-five percent Confidence Intervals relative to the estimates were -28% to +34% for Williamson’s Sapsucker and -34% to +44% for Red-naped Sapsucker. The largest contributor to the fairly wide confidence intervals of the adult population estimates was the between-route variance of the BBS data itself. Only modest improvements to the wide confidence intervals were possible even if seasonal, time-of-day, and pair adjustments could be estimated precisely with no associated error.

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