Population cycles in the Varied Thrush (Ixoreus naevius)

Abstract

The Varied Thrush (Ixoreus naevius), an endemic species of the Pacific Northwest of North America, shows dramatic year-to-year changes in wintering abundance. These changes have not, however, been systematically examined using standardized data. Using Project FeederWatch data from 1988–1989 to 1994–1995 we found that Varied Thrushes showed a biennial cyclic change in abundance within most of their normal wintering range. This cyclic pattern was also evident in Christmas Bird Count and Breeding Bird Survey data from the same time period. There was no difference in the extent or location of the winter range or in the distribution of elevations of Varied Thrush records between years of high and low abundance. More feeders had single Varied Thrushes in years of high abundance than in years of low abundance. Longer term (1969–1970 to 1994–1995) Christmas Bird Count and Breeding Bird Survey data showed a more variable pattern, with abundance peaks every 2–3 years. The number of vagrant Varied Thrushes from eastern North America reported in Audubon Field Notes winter season reports was not correlated with abundance in the normal wintering and breeding areas but was correlated with the number of Varied Thrushes tallied on southern California Christmas Bird Counts. These results suggest that patterns of vagrancy in Varied Thrushes are largely independent of population changes within the normal wintering area.