BIRD POPULATIONS - Birdpop.org

DAVID J. ZIOLKOWSKI JR., KEITH L. PARDIECK AND JOHN R. SAUERfrom the two approaches.Analyses of BBS data produce a singlecomposite estimate of population change, ortrend (an interval-specific geometric mean ofproportional changes in population size),presented as mean percentage change per year(Link and Sauer 1998). In the hierarchical model,composite trends are functions of the model’sparameters and hyperparameters, combinedusing abundance weights reflecting the typicalmagnitude of counts on routes in the surveyarea. We estimated trends at the scale of theentire survey area and separately for Eastern,Central, and Western BBS regions (Bystrak 1981),for the interval 1966–2008, noting that the surveydid not begin until 1968 in some parts of thesurvey area. Alaska, northern Canada (territoriesand northern portions of most provinces),Newfoundland, and northern Mexico wereexcluded from the analyses because ofinsufficient data to estimate long-term trends.As the analysis was conducted using Bayesianmethods, estimates of parameters and theircredible (Bayesian confidence) intervals are takenfrom the Posterior Distribution of theparameters; for mean change, we used the 2.5and the 97.5 percentiles of the distribution as ourestimate of the composite change and its 95%credible (Bayesian confidence) intervals. We usedwidth of the interval to measure precision of theresults and considered an estimate to beimprecise if the half-width of the credibleinterval was larger than 3%/yr change for thespecies (Sauer et al. 2003). Although themagnitude of a trend estimate may be quitelarge, if the credible interval includes zero thetrend is not considered significantly differentfrom zero. See the BBS Analysis and SummaryWebsite (http://www. mbr-pwrc.usgs.gov/bbs.html) for additional discussion about trendquality and precision. As in earlier BBSsummaries (Pardieck and Sauer 2007), we haveincorporated an overall Trend Quality (TQ) scoreto identify trends that contain certaindeficiencies. TQ is a ranked score ranging from1-3; we categorized each species’ results intoTQ1: acceptable, TQ2: use with caution, or TQ3:possibly questionable based on (1) precision [CInot small enough to exclude 0 for a 3%/yrchange = TQ2, or not small enough to exclude 0for a 5 %/yr change= TQ3]; (2) sample size [Nroutes < 15 = TQ2, or N routes < 5 = TQ3]; and(3) Relative Abundance [annual index in finalyear