Blood parasite prevalence based on microscopic examination of stained blood smears was determined in adults of 11 passerine bird species sampled during their breeding season (May and June 1997–98) in interior Alaska (USA). These species included primarily Nearctic migratory species such as the dark-eyed junco (Junco hyemalis) and neotropical migratory species such as the blackpoll warbler (Dendroica striata), alder flycatcher (Empidonax alnorum), Swainson's thrush (Catharus ustulatus), northern waterthrush (Seiurus noveboracensis), and bank swallow (Riparia riparia) as well as one long-distance palearctic migrant, the arctic warbler (Phylloscopus borealis). The more prevalent parasites were Leucocytozoon dubreuili (73% of the sampled turdinids), L. fringillinarum (42% of the sampled fringillids and parulids), and Trypanosoma avium (39% of the sampled hosts). Other parasites (H. fallisi: 18% of the sampled turdinids; Haemoproteus paruli: 14% of the sampled parulids; H. fringillae: 5% of the sampled fringillids; microfilariae: 4% of the sampled hosts) were observed less frequently. Plasmodium vaughani was found only in two yellow warblers (Dendroica petechia). Overall parasite prevalence varied between 0% in the alder flycatcher to >80% in Swainson's thrush, arctic warbler, and Townsend's warbler (Dendroica townsendi). Prevalence of various hematozoa also was bird species-dependent. No relationship was observed between prevalence and either foraging (aerial versus trees/shrubs) or nesting habits (ground versus arboreal) or general location of the wintering area of the different species examined. Prevalence also was unrelated to average dates of arrival on breeding grounds and, therefore, to potential duration of exposure to local insect vectors before capture. Differences in blood parasite prevalence among species breeding in a same region and in the same type of habitat may result from differences in host specificity such as immunological resistance to infection or blood meal preference by potential vectors and/or in behavioral adjustments/physiological traits that alter exposure to vectors.
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