Drivers of winter population cycles in the Varied Thrush Ixoreus naevius

The drivers of year-to-year difference in winter abundance patterns, particularly dramatic in the “eruptions” of many boreal seed-eating birds, are poorly understood. Varied Thrush (Ixoreus naevius (Gmelin, 1789)), endemic to the Pacific Northwest of North America, is a boreal species that exhibits pronounced, often biennially cyclic, changes in winter abundance within most of its normal wintering range. Although the drivers of this variability have not previously been explored, it has been suggested that differences in acorn abundance, a key winter food resource, might be important. Here, we examine three hypotheses for the drivers of this pattern: the acorn crop within the bird’s normal winter range, weather within the bird’s winter range, and weather during the previous breeding season within the bird’s breeding range. Analyses supported the importance of breeding season conditions, particularly breeding season rainfall, with more birds wintering following wetter years. No support was found for the hypotheses that winter conditions, neither the acorn crop nor winter weather, correlate with winter abundance patterns. For this forest species, year-to-year differences in winter abundance patterns are apparently not driven by the “pull” of winter food supply or winter conditions, but by environmental factors during the prior breeding season that presumably affect reproductive success and subsequent population size.