Irruptions of Bohemian Waxwings in relation to population density and food availability

Abstract

Bird irruptions are thought to be triggered by a combination of lack of food in the regular distribution range and high population density (push factors). Recent studies have also suggested that irruptions may be drawn towards areas with high food availability (pull factor). The Bohemian Waxwing (Bombycilla garrulus) is an irruptive species with large annual variation in the numbers reaching southern areas during winter. I used long-term (up to 41 years) data on annual number of waxwings observed during breeding in Finland and Sweden, during winter in Norway, Sweden, Denmark, UK and Poland, and data on their main winter food, Rowanberries (Sorbus aucuparia; masting occurs at 2–3 year intervals) from Norway (southern areas) and Finland (overlapping with northern breeding areas) to test the push and pull hypotheses. Winter numbers fluctuated in synchrony over large parts of northern Europe. In years with low food availability in Finland waxwings arrived to southern areas early, and early arrival to southern areas was linked to larger winter numbers in some countries. However, controlling for arrival time to southern areas, numbers were larger when there were more rowanberries in Finland. Thus, irruptions generally occurred with high rowanberry abundance, but sometimes occurred with the combination of low rowanberry abundance and early arrival. There was mixed evidence that irruptions were linked to high breeding population density. There was no support that irruptions were drawn towards high rowanberry abundance in southern areas. In conclusion, waxwing irruptions had complex relationships with food availability, but most irruptions occurred in years with high rowanberry abundance in autumn in northern areas. This is opposite to expectations from the current view that irruptions are triggered by lack of food, but irruptions departing late may still have been triggered by abundant food finally becoming depleted late in winter.