| Summary: | The Rusty Blackbird (Euphagus carolinus) has declined precipitously over the past several decades, and stressors on both the breeding and wintering grounds are suspected causes. Over 3 years, we collected blood samples from breeding birds in Alaska and Maine and from wintering birds in Mississippi and Arkansas to determine the prevalence of hematozoan infections at different times of the year. The prevalence of hematozoa (percent of birds infected) in Alaska was 44% of 43 birds, lower than previously reported from Newfoundland and Ontario (83% of 23 birds). Blood parasites were found among 67% of 12 Rusty Blackbirds sampled in Maine, not different from the prevalence in Newfoundland and Ontario. Leucocytozoon was the most common parasite; few breeding birds were infected with Plasmodium or Trypanosoma sp. During the winter in Mississippi and Arkansas, the overall prevalence of hematozoa was 49% over three years. In winter as in summer, Leucocytozoon was the most commonly encountered parasite (n = 62 birds), followed by Haemoproteus (n = 5), microfilaria (n = 2), Trypanosoma (n = 2), and Plasmodium (n = 1). The prevalence of hematozoa among wintering Rusty Blackbirds was much higher than expected because winter is generally a time when there are few transmissions and in most birds blood parasites are absent from the peripheral circulation. This high prevalence might indicate a nonseasonal relapse of hematozoan infections among wintering birds; possibly due to high levels of stress, which are known to lower the immune response and trigger nonseasonal relapses.
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