Extracting long‐term patterns of population changes from sporadic counts of migrant birds

Abstract

AbstractDeclines of many North American birds are of conservation concern. For almost 40 years, experienced birders have kept daily counts of migrant landbirds during visits to Seal and Brier Islands, both of which are off Nova Scotia's southern tip. Here we assess the utility of Generalized Additive Models (GAMs) to extract patterns of population change of a common migrant to Seal Island, the Ruby‐crowned Kinglet, while controlling for other influences including season, weather and effort. We also demonstrate, using counts of the Kinglet from Brier Island as well as counts of another common migrant, the Yellow‐rumped Warbler, how our GAM methods can combine data from different geographic areas or distinct species. Most existing analyses of similar long‐term data sets have used linear models to estimate trends. Our results and comparisons suggest that GAMs are a powerful way of extracting more information from such data. Copyright © 2009 John Wiley & Sons, Ltd.