BIRD POPULATIONS - Birdpop.org

NORTH AMERICAN BREEDING BIRD SURVEY 2003-2008differ significantly from 50% in any region (Fig.1J). Results from the 6-yr period were similarwith exception of the Eastern region, where asignificant number of species (68%) showedpositive population trends. Noteworthy is that,although 1966-2008 and 2003-2008 results fromthe East and West are not statistically significant,the credible intervals are relatively short andsuggest that a nearly even proportion of speciesexperienced gains and losses.Significantly fewer than 50% of neotropicalmigrant species exhibited increasing populationtrends in the Western (40%) and Eastern (34%)regions over the long-term (Fig. 1K). In contrast,neotropical migrants were significantly morelikely to experience population growth in theCentral region during that period (55% speciesincreasing). All regions surpassed thatproportion during 2003-2008 with a significantpercentage of species in both the Western andCentral regions (66%) and in the Eastern region(59%) experiencing positive trends, yielding anoverall increase of 64% positive trends surveywide.No such pattern could be discerned duringthe 2-yr period because regional results were notstatistically distinguishable from 50%.The ground nesting bird guild was amongstthe groups most likely to experience declines(Fig. 1L). Many of the long-term declines weresevere, as evidenced in the significantly lowproportion of positive trends from Western (31%)and Eastern regions (27%) and the low 29% ofincreases demonstrated survey-wide. The 2003-2008 interval portrays a more positive picturewith 46%, 60%, and 51% of ground nestersincreasing, respectively, in the Western, Central,and Eastern regions. However, no significantdeviations from 50% were detected for anyregion during this period. Declines were againapparent in the more recent 2-yr interval butonly the results from the Western region werestatistically significant, where only 20% ofspecies experienced increasing populationtrends.During 1966-2008, mid-story and canopynestingbirds fared relatively well in the CentralBBS region (63% increasing trends). Populations,however, experienced significantly fewerincreases in the Western (38%) and Eastern (42%)regions (Fig. 1M). These opposing proportionslikely account for the survey-wide result beingindistinguishable from 50%, despite having anarrow credible interval. Western and Easternregional results were equivocal and notsignificant during 2003-2008, but populations inthe Central region demonstrated a significanttendency for increase during this period (60%increasing trends). More mid-story and canopynestingbirds experienced population declinesthan increases in all BBS regions during the 2-yrinterval but these results were not significant.DISCUSSIONThe 2-yr and 6-yr trends provide generalindication of how populations fluctuated duringthese intervals. These trends show temporal andspatial heterogeneity and thus should be viewedwith caution and only interpreted within thecontext of the long-term trend results. Althoughthe majority of significant long-term trendestimates had high trend quality (Table 1), thequality of the shorter interval trend estimatesvaries considerably among regions and species.Any analysis of BBS data should carefullyconsider the possible limitations of the data andpotential trend quality issues. Additionally, theBBS is not designed to determine causal factorsof population changes. Therefore, we wereunable to identify specific factors responsible forthe various temporal and regional patternsevident in this analysis. Nevertheless, it can beinstructive to examine the overall patterns withinthe context of previous analyses to identifygeneral tendencies that could provide insight forfuture research and conservation efforts.The 2007-2008 period was relatively poor formost species across the survey, producing one ofthe lowest proportions of positive 2-yr trends(39%) ever reported in the BBS. Trends are rarelyexactly zero, so when considering combinedtrends for life-history guilds, if a group wereessentially stable, one would expect 50% ofconstituent species to have increasing trends and50% to have decreasing trends simply by chance.In 2007-2008, all groups with results deviatingsignificantly from 50% had 2/3 or more of theirconstituent species in population decline. Threeguilds (open-cup nesters, short-distancemigrants, and ground or low nesting birds)experienced greater declines in the Westernregion and 4 (successional or scrub, urban, opencupnesters, and short-distance migrants)experienced greater decline in the East. Results[97]