BIRD POPULATIONS - Birdpop.org

DAVID J. ZIOLKOWSKI JR., KEITH L. PARDIECK AND JOHN R. SAUERfrom the Central region suggest that conditionsin that portion of the continent generally favoredstability more so than did conditions in the Westand East.Although the causes of such declines cannotbe divined from analysis of BBS data alone, it ispossible that extreme weather patterns during2007-2008 may have played a role. Mild El Niño– Southern Oscillation (ENSO) conditionsdeveloped in early Autumn 2006 (Arguez 2007),as indicated by a warming of the sea surface inthe eastern and central tropical Pacific Ocean(Larkin and Harrison 2002). These warmedwaters had cooled by early winter, changingwind patterns, and yielding relatively cool anddry winter conditions in the Western region, cooland wet conditions in the Central region, andwarm and dry conditions over much of theEastern region. The 2007 BBS field season metwith drier than average summer conditions inthe Western and Eastern BBS regions, including41% of the contiguous U.S. in moderate toextreme drought (Levinson and Lawrimore2008). Portions of the Central region, however,received record amounts of precipitation duringthis time. As the Pacific surface watersprogressed to a hypercooling phase in autumn2007 (Peterson and Baringer 2009), a strong LaNiña produced relatively cool winter conditionsin the Western and north-Central regions, warmand dry winter conditions in the southeast, andnotably warm and wet conditions in thenortheast. These conditions prevailed in the 2008BBS field season, with below normal summerprecipitation confined to the Western region andthe South-East.North American land bird population changeshave been correlated to earlier ENSO events(Morrison and Bolger 2002; Sillett et al. 2000;Nott et al. 2002) and previous BBS summaryreports have noted increases in Western birdpopulations and/or reductions in regionalmigrant groups in the subsequent 2-yr periods(Peterjohn et al. 1994, Pardieck and Sauer 2000,Pardieck and Sauer 2007). Comparison betweenEl Niño and La Niña is complicated given that‘typical’ weather patterns represent averageeffects, and actual effects can vary considerablydepending on the time of year of occurrence(Ropelewski and Halpert 1986). Also, othersubstantial atmospheric and oceanic phenomena,such as the Pacific/North American “teleconnection”pattern and the North AtlanticOscillation (Wallace and Gutzler 1981, Nott et al.2002), are likely influencing bird populations aswell. During 2007-2008, amongst the threeresident/migratory guilds, short-distancemigrants in the Western and Eastern regions andpermanent residents in the West experienced thegreatest proportion of declines. Neotropicalmigrants, on the other hand, were relativelystable across all regions, suggesting that winterweather patterns may have been an importantfactor contributing to declines in birds winteringin the United States. The relationship betweenland bird populations and large-scaleatmospheric and oceanic patterns deservesadditional study.The pattern of trends in the 2007-2008 periodwas similar to that seen in the 1966-2008 trends.Over the long-term, a species was 1.5 times morelikely to exhibit a significant decline than asignificant increase (Appendix 1). Life-historygroups during this time were also nearly threetimes more likely to contain a significantlygreater proportion of species with negativetrends (Figs. 1A-M). Only six of the 23 regionallife-history groups that deviated significantlyfrom 50% did so in the positive direction, and allsix occurred in the Central region.Overall, the tendency during the 2003-2008period was for all species to experiencesignificant range-wide population gains. All 13of the regional life-history groups with resultsthat deviated significantly from 50% during thisperiod were positively oriented. This is adeparture from the pattern seen in the previoustwo BBS summary reports, where mid-term andlong-term results demonstrated proportions ofincreasing species that were both similar to oneanother and lower than the 2-yr term results(Pardieck and Sauer 2000, Pardieck and Sauer2007). Whether or not the period of generaldecline during 2007-2008 reflects a meaningfulbiological correction following the 2003-2008growth spell is uncertain but worthy of furtherevaluation.Most grassland bird species have experiencedsteep long-term continent-wide declines andonly 5% of Eastern and 21% of Westernpopulations demonstrated positive growthduring 1966-2008 (Fig. 1B). The fact that the 6-yr[98]