During spring and autumn migration 2008, 6538 birds were examined for tick infestation at Lista Bird Observatory in Southern Norway, and 822 immature I. ricinus were collected from 215 birds. Large variations were found in the contributions of the different bird species to the number of ticks collected. Ticks were found on 34 of the 85 bird species examined. The bird species most commonly infested by ticks were Turdus spp., Anthus trivialis, Fringilla coelebs, Sylvia spp., Prunella modularis and Erithacus rubecula, consistent with previous Norwegian studies [7, 8, 15, 16]. These avian species are ground-feeding, which puts them at risk of tick infestation.
The prevalence of ticks on the birds was higher during spring migration compared to autumn migration, with 6.2% and 2.7% of the birds infested, respectively. This may be explained by different tick activity during spring and autumn migration and/or by different tick population densities along the birds' migration routes in to Norway (spring) compared to in their Norwegian breeding grounds (autumn). However, it is possible that the observed difference is due to the much higher number of birds caught during autumn migration compared to spring migration (5503 and 1135 birds, respectively), which left less time for the bird observatory staff to examine each bird for ticks. However, the mean intensity of tick infestation was higher during autumn migration compared to spring migration, with 4.1 and 3.2 ticks per infested bird, respectively. The reason for this is unknown.
Few ticks were found in the immediate distance (<0.5 km) from the bird observatory, however, high I. ricinus densities were found in other sites in the region, and local tick recruitment cannot be excluded. Future studies should investigate this, for example by studying genetic variation in I. ricinus ticks along migratory routes as previously described .
The prevalence of B. burgdorferi in ticks collected from birds was 4.4% (4.0% and 4.5% in ticks collected during spring and autumn migration, respectively). In ticks collected during spring migration, B. burgdorferi s.l. were detected in 5.4% of nymphal ticks, whereas in ticks collected during autumn migration, the spirochetes were detected in 3.4% of larvae and 7.5% of nymphs, however, these differences were not statistically significant. The most prevalent genospecies were B. garinii (77.8%), followed by B. valaisiana (11.1%), B. afzelii (8.3%) and B. burgdorferi s.s. (2.8%). Other Nordic studies have reported findings of similar infection rate and genospecies composition in ticks collected from migrating birds [3, 4]. B. burgdorferi genospecies composition in host-seeking ticks in Southern Norway was described in a previous study . Although local variations were observed, the overall prevalence of B. burgdorferi s.l. in I. ricinus was 22.3%, and the general pattern was a dominance of B. afzelii, followed by B. garinii, B. burgdorferi s.s, and B. valaisiana. The low prevalence of B. burgdorferi s.l. in ticks collected from birds compared to host-seeking ticks may be explained by the observed differences in sensitivity to host serum among the B. burgdorferi s.l. strains. During feeding, ticks take up host-derived molecules as complement and other blood components. It has been proposed that the genospecies B. afzelii is sensitive to avian complement, and that these spirochetes are eliminated in the tick midgut, whereas B. garinii survives such a blood meal and can be transmitted to the host . Comparison with present finding of genospecies composition in ticks feeding on birds support the notion of a genospecies specific association between birds and B. garinii, and an elimination of B. afzelii infections.
B. burgdorferi infection was detected in 3.8% of larvae carried by the avian species redwing (Turdus iliacus), blackbird (Turdus merula), chaffinch (Fringilla coelebs), whitethroat (Sylvia communis) and winter wren (Troglodytes troglodytes). Larval infection does not necessarily imply host reservoir competence, as infection may also arise from transovarial transmission or from co-infection . However, previous studies have demonstrated Turdus spp. as reservoir hosts for B. garinii and B. valaisiana[6, 20, 21], supporting the possibility of the bird as a source of infection. However, further studies are necessary to determine the potential reservoir capacity of chaffinch, whitethroat and winter wren.
As previously described, the birds were not thoroughly examined for ticks every day during autumn migration. Furthermore, our material includes migratory as well as resident bird species. These factors may have influenced findings in the present study, and future studies should attempt to avoid these potentially confounding factors.