Capsule Significant geographic variation in body size occurs throughout Eurasia and North Africa and this variation is best explained by Boyce's seasonality hypothesis rather than Bergmann's rule. Aims To investigate whether any of the four general ecogeographical hypotheses that have been used to explain geographic variation in the body size of birds (heat conservation, Bergmann's rule; heat dissipation, James' moisture-humidity modification of Bergmann's rule; Rosenzweig's productivity; Boyce's seasonality) apply to Chaffinches. Methods External measurements of museum specimens collected throughout the species' distribution range and specimens caught using mist nets in Turkey were used in principal component analysis to derive body size components. Geographic and climatic data were used to evaluate hypotheses which might explain geographic variation in body size, using an information-theoretic approach. Results Chaffinches from North Africa were larger than those from Eurasia, contrary to Bergmann's rule. This pattern might be explained by a continental effect, because North Africa differs from Eurasia in climatic and ecological features, leading to different selective pressures on these populations. Body size showed opposite relationships with geographical and climatic data in the two continents and this gave support to a continental effect on body size variation. Seasonality of primary productivity seemed to be the best explanation for body size variation in Chaffinches. Conclusion Of the four ecogeographical hypotheses the results were consistent only with Boyce's seasonality hypothesis, i.e. larger body size occurred in regions where food abundance exhibited more seasonal variation. This may be a buffer against periods of resource scarcity.
