Constraints on fruit-tracking by birds at the range edge

Aims: Biotic interactions may be disrupted at the range boundary if the involved species are rare in the most distant sectors. This study assesses fruit-tracking by seed-dispersing birds wintering on the border of the Palaearctic, a region where many species become scarcer before reaching the limits of their distribution in the Sahara. Since vagrancy improves fruit-tracking, this study tests if vagrant birds track fruit resources better than non-vagrant birds and if their tracking ability is reduced at the range edge. Location: The Iberian Peninsula and the Maghreb Methods: Seed-dispersing passerines (O. Passeriformes) were counted in 134 sampling sites along a 2000 km long latitudinal belt. Relationships between bird numbers and latitude, temperature, shrub cover and fruit abundance were explored by multivariate analyses. In addition, the habitat matching rule was used to assess the ability of birds to track fruit changes along the latitudinal belt. Results: Fruit abundance and temperature were the main drivers of the distribution of vagrant birds (all of them migratory species) while for non-vagrant birds (most of them sedentary species) distribution was weakly correlated to the study variables. Vagrant birds tracked the spatial-temporal variations in fruit abundance but showed a reduced ability to fit numbers to fruit abundance changes in southern sectors. Main conclusions: Vagrant, migratory birds are responsible for large-scale tracking of fruit outputs but face some constraints on fruit-tracking at the far edge of the Palaearctic. Since the movements of these birds southwards are declining due to climate warming effects, these results suggest a future weakening of their role as seed dispersers in the most peripheral areas of the Palaearctic.