Do diurnal migrants follow the Guadiana River when crossing dry sectors of SW Iberia?

Abstract

Little is known on the importance of water courses as migratory corridors. Here, we present the results of an investigation on the movements of diurnal migrants associated with the Guadiana River when crossing dry and inhospitable regions of southern Iberia. Direct observations were carried out along the river and at control sites away from it, in late August-November 2006. The number of birds seen was extremely small (<< 1 individual per hour for most species) and most records probably involved individuals in local movements, as indicated from similar rates of birds moving up and downriver. The only exceptions were hirundines which, within our study area, were strongly associated with the Guadiana both when foraging and when engaged in medium to long-distance daily movements. House Martins Delichon urbicum were particularly numerous, and 98.7% of the birds seen in directed movement (n = 4122) were flying above the river. However, passing hirundines (mostly Barn Swallow Hirundo rustica and Sand Martins Riparia riparia) were scarce and did not show the same degree of association with the Guadiana. Our results suggest that there is no significant migratory corridor along the lower Guadiana during summer-autumn, but hirundines strongly associate with this river when foraging and when commuting between foraging grounds and roosts.