Ecological barriers shaping fuel stores in barn swallows Hirundo rustica following the central and western Mediterranean flyways

The crossing of ecological barriers is among the most energy-demanding and risky phases of migration. Scanty field data exist on the relationships between ecological barriers and pre-migratory fuel storing in songbirds. The aim of this study was to analyse whether the distance to be covered across ecological barriers can be considered as a factor affecting pre-migratory fuel stores in barn swallows Hirundo rustica following the two main western European flyways, funnelling through Iberia and Italy. Data refer to 13029 swallows and were collected during July-October in 1997-1998 at 19 roost sites, scattered over Spain and Italy (south of 43degreesN). During the post-breeding phase (PB, July-August) energy stores did not differ significantly between the two geographical areas, whereas during the pre-migratory phase (PM, September-October) swallows in Italy carried larger energy stores than those in Spain. Assuming that swallows leave for migration from the fuelling site, we found a significant positive correlation between the width of ecological barriers and an average index of energy stores for each roost site during the PM phase. The width of ecological barriers (along a N-S migration route) was expressed as (1) the distance between the roost site and the coast of North Africa (representing the Mediterranean Sea), and (2) the distance between the roost site and the southern margin of the Sahara desert (the total width of ecological barriers). The weaker correlations obtained when considering only the Mediterranean as a barrier suggest that swallows may cross the desert without substantial refuelling in North Africa. Hence, we showed that fuel stores have a degree of population-specific variability among Italian and Iberian barn swallows and that the extension of ecological barriers may play a role in determining the amount of stores needed for the migratory flight.