Apparent absence of blood parasites in Winter Wrens in British Columbia

ABSTRACT.   To investigate the possible effect of elevation on the prevalence of hematozoa infection, we collected blood smears from a population of Winter Wrens ( Troglogytes troglodytes ) in the Coast Mountains of southwestern British Columbia, Canada. We sampled 119 Winter Wrens, including 88 at low elevation (0–500 m) and 31 at high elevation (900–1100 m) sites. In addition, five other species, including Swainson's Thrush ( Catharus ustulatus ; N = 12), Dark-eyed Junco ( Junco hyemalis ; N = 11), Hermit Thrush ( Catharus guttatus ; N = 8), Varied Thrush ( Ixoreus naevius ; N = 4), and American Robin ( Turdus migratorius ; N = 4), were sampled. No Winter Wrens were infected by blood parasites. Among the other species, Haemoproteus infection was detected in one Varied Thrush (25%) and five Swainson's Thrushes (42%). Thus, despite the occurrence of infection in sympatrically breeding species, blood parasites are apparently absent or occur at extremely low prevalence in Winter Wrens in our study area. The presence of hematozoa in European populations of Winter Wrens, combined with an abundance of vector species in our study area, suggest population-level resistance to infection. Further study is needed to determine the specific mechanisms involved in the apparent lack of infection in our study population.