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