Species abundance and migratory status affects large-scale fruit tracking in thrushes (Turdus spp.)

Abstract

The mutualistic interactions between fruit-producing plants and seed-dispersing birds may be disrupted by the difficulties involved in tracking fruit resources, which are often patchily distributed in space and time. As a consequence, the strength of this interaction will rely on the ability of birds to distribute numbers according to the spatiotemporal patterning of the resource. This paper tests if wintering thrushes (Turdus) track inter-winter and inter-site changes of cone abundance in an area that encompasses the main range of the Spanish Juniper Juniperus thurifera in the Iberian highlands. We test whether the regional abundance and migratory status of birds affect this fruit-tracking process. Analyses were approached by using the habitat matching rule, a null model applied to explore the effect of spatiotemporal change of resources on bird distribution. All species followed inter-winter food variation over the study area but under-matched the changes. The strength of this bird–plant interaction increased with the regional abundance of individual species. When thrushes were analyzed according to their migratory status, migratory species (Turdus pilaris, T. iliacus, and T. philomelos) tracked the resource better than sedentary ones (T. viscivorus and T. merula). This suggests that in a time of rapid environmental change any reduction of vagrant and abundant migratory thrushes could weaken the strength of this bird–plant interaction.