Fluctuations in population size and migration of two species of crossbills in relation to seed crop size of spruce and pine: asymmetric importance of alternative food resources

Bird species depending on seeds from masting trees face large temporal and spatial variation in food availability and perform nomadic or irruptive movements to locate food. In northern Europe, the Common Crossbill (Loxia curvirostra) is specialized on seeds from Norway Spruce (Picea abies) which usually has peak years followed by crop failure. Common Crossbills breed in mast years and emigrate when spruce seeds are no longer available. On the other hand, the Parrot Crossbill (L. pytyopsittacus) is specialized on seeds from Scots Pine (Pinus sylvestris) in which seed production is more stable from year to year. The Parrot Crossbill is therefore thought to be more sedentary. However, both species may switch food if there is low abundance of their 'own' seed type, but little is known about the relative importance of crop size of the two tree species on breeding population size and migration of the two crossbill species. Here, we analyse time series (up to 36 years) of crossbills and crop size of spruce and pine in Norway and Sweden to investigate this. We found that breeding population sizes of both species were positively related to spruce crop size. Surprisingly, pine crop size had no positive influence, not even on the Parrot Crossbill. Emigration of both crossbill species (based on autumn migration counts in southern Sweden) increased when there were small seed crops of spruce, but pine crop size had no influence, not even on the Parrot Crossbill. There was no influence of population size on emigration. These results suggest that alternative food resources had an asymmetric effect, only influencing the Parrot Crossbill. This asymmetry explains why the two crossbill species often have synchronous eruptions despite being specialized on different tree species with different seed crop size dynamics.