Agonistic behaviour of Palaearctic passerine migrants at a stopover site suggests interference competition

Records of interspecific agonistic behaviour of Palaearctic passerine migrants from their Afrotropical wintering grounds are rare. There are, however, no detailed observations from stopover sites where individuals might concentrate and depress resources that are critical for fat-depleted birds in times of high energy demand. We recorded intraspecific and interspecific interactions of Palaearctic migrant passerine birds at Ouadâne, Mauritania, a stopover site in the Sahara desert. In spring 2003 we made casual records of all aggressive behaviour we observed, whereas in spring 2004 we used focal sampling of foraging birds to record the frequency of aggression. We found that interspecific interactions occurred more often than intraspecific interactions but their relative frequency differed between species. There were also mass-dependent species hierarchies. A comparison with other studies showed that in Willow Warblers the frequency of interspecific aggression was significantly higher at the stopover site than in the wintering grounds farther south. We cannot assess the adaptive advantage of agonistic behaviour, as our approach does not consider the allocation of aggressive interactions in relation to other activities and its influence on the birds' energy balance. Nevertheless, our observations suggest interspecific interference competition because inferior individuals are temporarily deprived of food, which may influence the fitness of migrants.