Irruptions of Pine Grosbeaks pulled by Rowanberry peaks in southern areas

Abstract Several species of owls and seed-eating birds breeding in boreal forests may occur irruptively areas south their regular wintering range. One common explanation is that they be forced to move if food supplies have crashed (push hypothesis). Little attention has been devoted the alternative idea are drawn towards abundant further (pull hypothesis), even could overwinter north. I evaluate these two hypotheses or pull) using data on winter irruptions Pine Grosbeak ( Pinicola enucleator ) one main type food, seeds berries Rowan Sorbus aucuparia ). numbers during southern Norway Sweden usually peaked at 2–3 year intervals 1959–2020, Rowanberries 1980–2020. peaks coincided with Rowanberry most years, annual number grosbeaks was positively correlated index. However, irruption years moved north already January–February, inconsistent push hypothesis. Furthermore, masting Finland had synchronous cycles, implying were triggered when Rowans large crops close likely origin irruptions, opposite what expected by Thus, available evidence consistent pull hypothesis: Grosbeaks winters high abundance exploit this resource until it gone mid-winter, then return northern areas.