Analysis of North American Breeding Bird Survey (BBS) data requires controls for factors that influence detectability of birds along survey routes. Identifying factors that influence the counting process and incorporating them into analyses is a primary means of limiting bias in estimates of population change. Twedt (2015) implemented an alternative counting protocol on operational and nonrandom BBS survey routes in the southeastern United States. Observers on selected routes employed a time–distance protocol in which they recorded birds in 1–min intervals and in 2 distance categories. We hypothesized that processing and recording observations using this time–distance protocol could cause observers to count fewer birds relative to observers using the standard protocol. We used a hierarchical log–linear model with a categorical covariate associated with protocol (standard vs. time–distance) to assess whether use of the time–distance protocol had a measurable effect on counting birds along BBS routes. We applied this model to BBS data from portions of 8 states in which the time–distance protocol was implemented and estimated a protocol effect for 167 bird species. We documented a significant overall effect of the time–distance protocol on observers'counts of birds. On average, the effect of the time–distance protocol was a 10% decline in counted birds; 80% of species had lower counts when the time–distance protocol was used on a survey route. However, because the time–distance protocol was only used on a small portion of the operational BBS routes and for a limited time, including the covariate for the time–distance protocol data had insignificant effects on analysis of population change. Although the covariate controlled for the effects of the time–distance protocol in BBS data, the results emphasize the importance of standardization as well as a need to track and, if necessary, control in analyses for changes in counting procedures along BBS routes.
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Vol. 121 • No. 2