Fine‐tuned travel planning for hazardous journeys

Abstract

Few recent research papers radiate the feeling of largescale geographic and biological coherence and synthesis typical of Moreau’s (1972) seminal book on the migration of landbirds between Eurasia and Africa. A study in this direction, however, is that by Rubolini et al. (2002) on barn swallow Hirundo rustica fuelling patterns before southward migration published in this issue of the Journal of A ian Biology. Within the context of the Europe-wide ‘EURING Swallow Project’, Rubolini and co-workers describe the increases in body mass and fat score from late August to early October in barn swallows at 19 roost sites in Spain and Italy. In both first-year and older birds there were consistent differences in body mass and fat score between Spain and Italy, with the birds captured at Spanish roost sites showing smaller maximum fuel stores than the birds in Italy. Consistent with the assumption that barn swallows left the areas of the roost site for southbound long-distance flights across both the Mediterranean Sea and the Sahara, the authors found a positive correlation between the amount of fuel stored at each of the sites and the lengths of the ensuing flight across the two presumed ecological barriers. That the correlation between fuel stores and distance was weaker when calculated for a crossing of the Mediterranean only, was taken as evidence for the fact that after leaving the stopover areas in Spain and Italy, barn swallows do little feeding in North Africa before crossing the Sahara (Fig. 1). Given that swallows would in principle be able to feed on aerial arthropods while in migratory flight, many of us have assumed that aerial foragers do not face ecological barriers to the same extent as specialised landand waterbirds. As a consequence, aerial foragers would not need to build up fuel stores to the same extent, something that would come handy given all the ecological costs associated with such stores (Witter and Cuthill 1993). An earlier finding that barn swallows in north-western Europe show rather little fuel storage in late summer (Ormerod 1989) seemed to confirm this preconception. However, it has been found that barn swallows carry substantial fuel stores prior to a Sahara crossing, both in Algeria (Bairlein 1988) and in Italy (Pilastro and Magnani 1997). The new study of Rubolini and co-workers adds importantly to the story not only by confirming that fuelling takes place, but also by showing that the total fuel store deposited at any one site seems to be a function of the width of the barrier ahead. That the relatively hospitable North African rim can be considered part of an ecological barrier for barn swallows during southward migration, comes as a surprise. Another surprise is that barn swallows at the northernmost sites apparently initiated the big jump from farther away than seemingly necessary. To accomplish this they put on more fuel. This runs contrary to the finding by Pilastro and Spina (1997) in their exciting interspecific comparison of residual fuel stores of birds that have crossed the Sahara in spring. The fuel stores