ECOGRAPHY 25: 428–438, 2002 Seasonal changes in the numerical responses of predators to cyclic vole populations Kai Norrdahl and Erkki Korpima ¨ki Norrdahl, K. and Korpima ¨ki, E. 2002. Seasonal changes in the numerical responses of predators to cyclic vole populations. – Ecography 25: 428 – 438. Theoretical models predict that a delayed density-dependent mortality factor with a time lag of ca 9 months is able to drive 3–5-yr population cycles of northern voles. We studied numerical responses of predators in western Finland during 1986 – 92, in an area with 3-yr population cycles of voles. Abundances of small mammals were monitored in several farmland areas (each 3 km 2 ) by snap-trapping in April, June, August, and October (only in 1986 – 90), and the abundances of avian, mammalian, and reptilian predators by visual censuses during trapping occasions. The 3-yr cycle studied was a cycle of Microtus voles (field vole M. agrestis and sibling vole M. rossiaemeridionalis ) and their small-sized predators (small mustelids and vole-eating birds of prey). The numerical responses of both migratory avian predators and small mustelids to changes in vole densities were more alike than different. In late summer (August), the time lag in the numerical response of all main predators was short (0–4 months), whereas longer time lags prevailed from spring to early summer. The length of the time lag in spring appeared to be related to the length of the winter, which indicates that strong seasonality may create longer time lags to the numerical response of predators at northern latitudes than at more southern latitudes. Our results suggest that, from spring to early summer, predation by migratory avian predators may act in concordance with mustelid predation to produce the long time lag necessary to drive the 3-yr cycle of voles, whereas almost direct density-dependent predation by all major predators in late summer may dampen spatial variation in prey densities. K. Norrdahl (kai.norrdahl@utu.fi) and E. Korpima ¨ki, Sect. of Ecology, Dept of Biology, Uni. of Turku, FIN-20014 Turku, Finland. Periodic multiannual population oscillations of small mammals at northern latitudes are characterized by a fairly regular period (the interval between successive density peaks) despite large variation in the amplitude (the ratio of maximum over minimum population density) of fluctuations (e.g. Akcakaya 1992, Hanski et al. 1993, Ho ¨ rnfeldt 1994). In arctic tundra and in northern Fennoscandia, as well as in parts of Siberia, where voles, lemmings and their predators form the bulk of vertebrates that undergo a cyclically fluctuat- ing population size, short-term cycles with a period from 3 to 5 yr predominate (Elton 1942, Lack 1954, Hansson and Henttonen 1985, 1988). In Fennoscan- dia, there is a latitudinal gradient both in the period and the amplitude of cycles: pronounced cycles with a period of 4–5 yr are typical at northern latitudes whereas seasonal fluctuations characterize vole popu- lation dynamics in the southern parts of Fennoscan- dia (Hansson and Henttonen 1985, 1988, Hanski et al. 1991). Between these two zones, cycles with a pe- riod of 3 yr prevail. Tens of hypotheses and hundreds of papers have been published on the causes of these cycles (for re- views, see e.g. Akcakaya 1992, Batzli 1992, Krebs et al. 1992, Stenseth and Ims 1993, Norrdahl 1995, Krebs 1996, Korpima ¨ki and Krebs 1996, Stenseth Accepted 2 November 2001 Copyright © ECOGRAPHY 2002 ISSN 0906-7590 ECOGRAPHY 25:4 (2002) 428