New Approaches to the Analysis of Population Trends in Land Birds

Abstract

Although there is strong evidence that a few Neotropical migrant land birds have severely declining populations, we know of no evidence of general overall declines. One widely cited analysis of data from the Breeding Bird Survey (BBS), the only continental-scale monitoring program for land birds in the breeding season, indicates declining populations in eastern forests in the 1980's, but other analyses conclude that most species have had stable or increasing populations in the last 25 years.These conflicting results suggest that a new perspective is called for.Here, we propose two approaches to the analysis of BBS data that emphasize the detection of temporal and geographic variation in population trends.

The Importance of Analysis Method for Breeding Bird Survey Population Trend Estimates

“…Differences in the magnitude of route regression estimates were related to sample size (number of routes in the analysis); hence, we expect these differences to be smaller for estimates based on larger geographic areas or states/provinces with a greater density of routes (e.g., those in eastern North America). We do not know whether trend estimates from methods based on very different assumptions, such as nonlinear nonparametric route regression (James et al 1992) or Mountford's method (Mountford 1982), will show similar convergence with increasing sample size.…”

“…Population monitoring plays three important roles in the conservation of these species: 1) early identification of declining populations focuses research and management effort toward vulnerable species and habitats before they become critically endangered (Hagan 1992), 2) correlations with environmental factors allow the evaluation of preliminary hypotheses of causation (James and McCulloch in press), and 3) ongoing surveillance enables assessment of the effectiveness of management actions (Hellawell 1991). However, monitoring landbird populations is difficult, due to the extensive ranges of most species and the wide variation in population trends among different locations, habitats and time periods (James et al 1992;Sauer & Droege 1992;Peterjohn et al in press). The determination of large-scale changes in population size thus requires accurate trend information over a long time period and a large geographic area.…”

“…However, this method was shown to be unreliable (Geissler and Noon 1981), and more recent analyses have been based on the "route regression" approach developed by Giessler (1984). In addition, a number of independent researchers have suggested and implemented alternative approaches (Titus 1990;Moses and Rabinowitz 1990;James et al 1992;Böhning-Gaese et al 1993;James et al in press). These methods differ in the way they deal with the difficulties outlined above; however, despite some discussion in the literature (Geissler and Noon 1981;James et al 1990;Titus 1990;Peterjohn et al in press;van Strien et al in press), there is no consensus as to which is the most suitable (see Thomas in press).…”

Mark Plummer's Legacy: Leave No Orthodoxy Unquestioned

“…However, claims of an overall decline of neotropical migratory birds due to tropical deforestation were debunked by careful analyses (James, McCulloch, and Wiedenfeld 1996). In 2006, empty oceans were foretold due to over-fishing (Worm et al 2006), but a reanalysis of fisheries data later suggested the empty ocean forecast was grossly exaggerated (e.g., Branch 2008).…”

Understanding the Risk to Neotropical Migrant Bird Species of Multiple Human-Caused Stressors: Elucidating Processes Behind the Patterns.

“…The abundance of some bird species in otherwise suitable habitat has been shown to be negatively correlated with the atmospheric wet acidic deposition, such as acid rain , Dekhuijzen and Schuijl 1996, James et al 1996, Zang 1998, Hames et al 2002a). Acidic wet deposition may leach bases from well-drained soils such as calcium, which is needed by forest songbirds to produce a clutch of eggs when breeding (Blancher and McNicol 1991, Graveland 1996, Graveland and van der Wal 1996, Graveland 1998, Bures and Weidinger 2003.…”