POPULATION-CYCLES IN THE VARIED THRUSH (IXOREUS-NAEVIUS)

The Varied Thrush (Ixoreus naevius), an endemic species of the Pacific Northwest of North America, shows dramatic year-to-year changes in wi ntering abundance. These changes have not, however, been systematicall y examined using standardized data. Using Project Feeder Watch data fr om 1988-1989 to 1994-1995 we found that Varied Thrushes showed a bienn ial cyclic change in abundance within most of their normal wintering r ange. This cyclic pattern was also evident in Christmas Bird Count and Breeding Bird Survey data from the same time period. There was no dif ference in the extent or location of the winter range or in the distri bution of elevations of Varied Thrush records between years of high an d low abundance. More feeders had single Varied Thrushes in years of h igh abundance than in years of low abundance. Longer term (1969-1970 t o 1994-1995) Christmas Bird Count and Breeding Bird Survey data showed a more variable pattern, with abundance peaks every 2-3 years. The nu mber of vagrant Varied Thrushes from eastern North America reported in Audubon Field Notes winter season reports was not correlated with abu ndance in the normal wintering and breeding areas but was correlated w ith the number of Varied Thrushes tallied on southern California Chris tmas Bird Counts. These results suggest that patterns of vagrancy in V aried Thrushes are largely independent of population changes within th e normal wintering area.