Bullfinches in northern Iberia are affected by the amount of rowanberry seeds available in winter. The researchers found that Bullfinch numbers change based on the abundance of rowanberry seeds, while a similar bird species, the Chaffinch, did not show the same pattern. This suggests that rowanberry availability influences Bullfinch movements to lower areas during the winter months. The study looked at different locations and times to see how Bullfinch populations responded to changes in rowanberry seed availability.