Rainfall at African wintering grounds predicts age-specific probability of haemosporidian infection in a migratory passerine bird

In migratory species breeding in temperate zones and wintering in tropical areas, the prevalence of blood parasites may be affected by migratory strategies and winter habitat choice. We explored whether African winter habitat was linked to the probability of haemosporidian infection in the House Martin Delichon urbicum breeding in Spain, and tested for potential differences between age-classes. As a proxy for winter habitat features, we analysed stable isotope (delta H-2, delta C-13 and delta N-15) values of winter-grown feathers moulted in tropical Africa. Rainfall at the African winter grounds was related to the probability of being infected with haemosporidians and this effect differed among age-classes. We found that haemosporidian prevalence was similar for young and experienced birds wintering in habitats of higher rainfall (H-2-depleted), whereas there were great differences in winter habitats of lower rainfall (H-2-enriched), with young having a much higher prevalence compared with experienced birds. Likewise, experienced birds wintering in habitats of higher rainfall had a higher probability of haemosporidian infection compared with experienced birds wintering in habitats of lower rainfall. By contrast, young birds wintering in habitats of lower rainfall had a higher probability of haemosporidian infection compared with young birds wintering in habitats of higher rainfall. These outcomes highlight the interaction of age with haemosporidian infection in the migratory ecology of the House Martin, which may drive carry-over effects in this long-distance aerial insectivore.