9633522 Rodenhouse This research is designed to test and evaluate the mechanisms (i.e., processes) determining population size in North American migratory songbirds. The study will help in understanding why some species are currently showing population declines. The work focuses on breeding populations of Black-throated Blue Warblers (Dendroica caerulescens) in the Hubbard Brook Experimental Forest in north central New Hampshire. Through monitoring and experimental studies, we will quantify the occurrence and strength of two types of population regulation, namely density-dependence and site-dependence. Each type is capable of generating negative feedbacks on population size. One test will involve evaluating the effects of experimental reductions of local population density, while monitoring will examine how habitat quality influences bird survival and reproduction. Habitat quality will be defined in terms of food abundance, weather, predator abundance, and vegetation composition and structure. The ability of these measures, singly and collectively, to predict territory-specific reproduction, survival, and dispersal of these species over broad spatial and altitudinal scales will be examined. Data from these studies will be employed in computer simulation models for comparisons of the regulatory strength of the two potentially important regulatory processes. Model projections and analyses will help to identify, assess, and integrate the factors and processes determining population changes and abundances of this migratory bird species. Results of this research will contribute not only to basic ecological theory, but also will have direct management applications for migratory songbirds, some of which are declining in abundance and are of conservation concern.
