Data from: 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.