NEW APPROACHES TO THE ANALYSIS OF POPULATION TRENDS IN LAND BIRDS

Although there is strong evidence that a few Neotropical migrant land birds have severely declining populations, we know of no evidence of g eneral overall declines. One widely cited analysis of data from the Br eeding Bird Survey (BBS), the only continental-scale monitoring progra m for land birds in the breeding season, indicates declining populatio ns in eastern forests in the 1980s, but other analyses conclude that m ost species have had stable or increasing populations in the last 25 y e These conflicting results call for a new perspective. Here, we propo se two approaches to the analysis of BBS data that emphasize the detec tion of temporal and geographic variation in population trends. The fi rst approach uses nonlinear regression to detect temporal changes in p opulation trends, and permits the construction of graphs that compare nonlinear trends among regions. The second approach provides statistic al tests of multispecies patterns of geographic variation in populatio n trends. We report analyses of BBS data for 1966 to 1992 for the 26 s pecies of wood warblers (Parulinae) for which the most data are availa ble and the chance of obtaining reliable estimates is best. Using pool ed data for all 26 species for eastern and central North America, nonl inear regression indicated declining populations in the Appalachian Mo untains and the Eastern Foothills in the 1970s, and stability elsewher e. As examples of the use of this first approach in single-species ana lyses, we give maps, graphs, and statistical tests to illustrate varia tion in population trends by physiographic strata in the Blue-winged W arbler (Vermivora pinus) and the Ovenbird (Seiurus aurocapillus). As a n example of the second approach, we applied a probit-normal model to a matrix of increases and decreases in the 26 species in 37 geographic strata. The analysis identified specific highland areas as having unu sually high proportions of declining populations. These results sugges t that correlates of elevation should be considered as factors possibl y regulating populations of warblers and other land birds. Our perspec tive is that focus on studies of temporal and geographic variation in population trends of all birds, including Neotropical migrants, could help conservation biologists identify where species and groups of spec ies are in the most trouble.