Cause and effect in population declines of migratory birds

Abstract

-Population declines have been reported for nearly one-third of all Nearctic migrant birds that winter in the Neotropics. Speculative explanations for these reports have been presented emphasizing the importance of events occurring during one portion of the annual cycle versus another. These explanations are difficult to test directly. However, certain predictions can be made regarding the characteristics of populations controlled by breedingground factors as opposed to those associated with migration or wintering-ground phenomena. These characteristics can be measured for and tested experimentally on a species-byspecies basis. We present 14 such predictions, and review data currently available to assess their relevance to observed declines. Based on these data, populations of many species of Nearctic migrants appear to be controlled by wintering-ground events. Received 30 September 1993, accepted 5 January 1994. DECLINES IN AT LEAST 109 species of Nearctic migrants have been reported by one or more survey methods (Droege and Sauer 1989, Morton and Greenberg 1989, Terborgh 1989, Robbins et al. 1989a, Askins et al. 1990, DeGraaf and Rappole in press). These figures are cause for concern despite conflicting regional population trends (Sauer and Droege 1992), and despite arguments that at least some purported evidence of decline may be the result of procedural errors (e.g. survey design and analysis flaws; Hutto 1988, James et al. 1990, 1992, Rappole et al. 1993a). If the declines are real, what are the