Winter survey data reveal rangewide decline in Evening Grosbeak populations

Once one of the most common species seen at bird feeding stations across much of North America in winter, the Evening Grosbeak (Coccothraustes vespertinus), appears to be experiencing a population decline. Like other finches associated with the boreal forest or high elevations, Evening Grosbeaks form flocks in winter, often irrupting out of their breeding range and frequenting supplemental feeding stations where populations may be monitored. We sought to quantify the extent of changes in the abundance and distribution of Evening Grosbeak populations using data gathered in winter by Project FeederWatch, a continent-wide monitoring program that began in 1987. FeederWatch data gathered between 1988 and 2006 indicated a significant surveywide decline in the mean flock size recorded over time. The proportion of sites reporting Evening Grosbeaks plummeted by 50% in 18 years. At locations where the species continued to be seen, mean flock size declined by 27%. Similarly, analysis of data from 391 sites reporting data for at least 10 consecutive years indicated significant declines in Evening Grosbeak abundance at 76 sites and increases at no sites. Although the Evening Grosbeak has experienced dramatic rangewide population declines in recent years, the mechanisms contributing to these changes remain unclear. Given the geographic extent and rapid rate of observed population changes, urgent investigation of the mechanisms driving these declines is warranted.