Most abundance estimation methods assume that all sampled individuals are identified correctly. In practice, this assumption may be difficult to meet and can bias abundance estimates, especially when morphologically similar species overlap in range. Over the past 2 decades, Kittlitz's Murrelet (Brachyramphus brevirostris) populations appear to have declined across parts of their Alaskan range, where they co-occur with the Marbled Murrelet (B. marmoratus). Recently, the reliability of Kittlitz's Murrelet declines has been questioned due to variability and uncertainty in species identification between the 2 species. We conducted a field experiment to quantify misidentification and partial identification (identification to genus [Brachyramphus] level only) of Kittlitz's and Marbled murrelets during abundance surveys, and to evaluate the relative impacts of environmental and observational factors on misidentification and partial identification. We applied these results to previously collected survey data to measure the potential bias of abundance estimates resulting from varying identification rates. Overall, the misidentification rate during our field experiment was 0.036 ± 0.004 (SE), with observer experience best explaining the variation. Abundance estimates adjusted for misidentification reflected little bias. The overall partial identification rate was much higher than the misidentification rate (0.211 ± 0.007 SE). Partial identification rates increased in choppy sea states, with greater observation distances, and when murrelets exhibited diving behavior; rates decreased with increased observer experience and when murrelets exhibited flushing behavior. Because observer experience was an important driver of both misidentification and partial identification, we stress the importance of conducting rigorous observer training before and during surveys to increase confidence in species identification and precision in abundance estimates. The methods developed in this study could be modified for any at-sea survey scenario to measure identification rates and the factors influencing these rates. Results may reveal important relationships for adjusting survey protocols to increase confidence in species identification and thereby to increase the precision of abundance estimates.
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