Numerical response of Bullfinches Pyrrhula yrrhula to winter seed abundance

The main aim of our study was to investigate the coupling between the winter abundance of a seed-eater, the Bullfinch Pyrrhula pyrrhula, and the abundance of rowanberry Sorbus aucuparia seeds. We detected a numerical response of Bullfinches to temporal and spatial variation in the abundance of rowanberry seeds throughout the Cantabrian Range in northern Iberia. Data on the abundance of the Chaffinch Fringilla coelebs, a closely-related species were used to account for the effect of factors other than rowanberry production on Bullfinch abundance. Chaffinch did not show such an associa-tion. The changes observed in Bullfinch numbers were consistent with variation in rowanberry abundance throughout and across several spatial and temporal scales: local-ity, fruit-rich and fruitless sites within a given locality, and year and month. Our results suggest that rowanberry abundance may influence the timing and extent of the altitudinal movements undertaken into lowland areas by the Bullfinch populations that breed in the montane areas of Iberia.