Population cycles in the Varied Thrush (Ixoreus naevius) Jeffrey V. Wells, Kenneth V. Rosenberg, Diane L. Tessaglia, and Andre A. Dhondt 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 mori 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. RCsumC : Les populations de la Grive B collier (Ixoreus naevius), une espkce endCmique du nord-ouest de 1'AmCrique du Nord, subissent des variations annuelles spectaculaires de leur abondance au cours de l'hiver. Ces changements n'ont jamais CtC examinks systCmatiquement B partir de donnCes normalisCes. Les donnCes du projet FeederWatch (surveillance aux mangeoires) de 1988- 1989 et 1994- 1995 ont permis de constater que l'abondance des grives subit un changement cyclique bisannuel dans la plus grande partie du domaine rCgulier d'hiver. Ce pattern cyclique a Cgalement CtC mis en lumikre B l'analyse des donnCes des dknombrements de Noel et des inventaires des oiseaux reproducteurs au cours de la mCme pCriode. Ni l'Ctendue, ni la position du domaine d'hiver n'ont variC d'une annCe h l'autre, et la rkpartition des grives en fonction de l'altitude n'a pas variC entre les annCes de forte abondance et les annCes de faible abondance. Plus de grives isolCes ont visit6 les mangeoires au cours des annCes de forte abondance. Les donnCes B long terme (dknombrements de Noel et inventaires des oiseaux reproducteurs de 1969- 1970 B 1994- 1995) ont mis en lumikre une variation encore plus importante de l'abondance avec des sommets d'abondance tous les 2 ou 3 ans. Le nombre de grives errantes venant de l'est de 1'AmCrique du Nord, selon les relevCs saisonniers d'hiver publids dans Audubon Field Notes, n'Ctait pas en corrClation avec l'abondance dans les domaines rCguliers d'hiver ou les zones de reproduction, mais Ctait relic5 au nombre de grives recensCes au cours des ddnombrements de Noel dans le sud de la Californie. Ces rksultats indiquent que l'errance chez les Grives B collier est indkpendante des variations dCmographiques dans les aires rkgulikres d'hiver . [Traduit par la RCdaction] Introduction Varied Thrushes (Ixoreus naevius) undergo large fluctuations Many bird species show marked variation in abundance from year to year. The most striking examples are some northern or boreal species that exhibit cyclic changes in abundance associated with cycles in food availability (Lack 1954). These include finches (Bock and Lepthien 1976) and predatory birds (Lack 1954; Garrison 1993). Other species exhibit irregular irruptions and patterns of vagrancy that may be caused by food shortages or other factors (Lack 1954; Perrins 1979; Moss and Watson 1991 ; Dobson and Hudson 1995). In the Pacific coast region of western North America, in abundance within the traditional wintering area (Grinnell and Miller 1944; Bent 1949; Gabrielson and Lincoln 1959). Some populations are thought to undergo altitudinal migration, with birds moving to lower elevations by October (Bent 1949; Jewett et al. 1953). Also, large numbers of Varied Thrushes are often seen along roadways, beaches, and human habita- tions after unusually heavy snowfalls (Bent 1949; Ransom 1950; Martin 1970). Wintering birds are associated with dense forests of conifers or oaks, where their primary foods are acorns and various berries (Grinnell and Miller 1944; Bent 1949; Martin et al. 1951). Varied Thrushes also regularly visit backyard bird feeders within ,the winter range (Waldon Received January 18, 1996. Accepted June 7, 1996. 1992). Despite these anecdotal references to population fluc- J.V. Wells, K.V. Rosenberg, D.T. Tessaglia, and tuations, no regular pattern or cycle of abundance has been A.A. Dhondt. Bird Population Studies, Cornell Laboratory described. In addition to fluctuating winter abundance in ,the of Ornithology, 159 Sapsucker Woods Road, Ithaca, Pacific Northwest region, Varied Thrushes regularly stray NY 14850, U.S.A. across the northern U.S.A. and Canada to the Atlantic coast, Can. J. Zool. 74: 2062 -2069 (1996). Printed in Canada 1 ImprimC au Canada Can. J. Zool. Downloaded from www.nrcresearchpress.com by Depository Services Program on 03/20/14 For personal use only.