Winter population trends of selected songbirds

Abstract

Many studies have found significant changes, primarily declines, in populations of breeding birds throughout the United States. Most studies have focused on birds that migrate to the Neotropics for winter. Speculations about causes of observed declines have primarily implicated habitat fragmentation and loss (e.g. deforestation) in Central and South America. The National Audubon Society's Christmas Bird Counts (CBC), begun in the winter of 1900-01, provide the data needed to discern consistent population trends in birds wintering throughout the United States. For this study we used the CBC data to examine population trends of songbirds with ranges that apparently are limited by lower temperatures in the North. We chose these species to track populations of birds that could be in peril in the future. These birds potentially will be more quickly affected by changing climate than other birds, and we need baseline information on them to document possible consequences of global climatic change. The species that are indeed declining need to be monitored because the possible synergistic effects of declining populations and changing climate could result in local and even regional extinctions.