Migratory birds can carry parasites thousands of miles, and therefore their parasite communities may provide an informative means of ascertaining the breeding and wintering locations of their hosts. However, avian parasites (particularly haemosporidians) are rarely used for this purpose because their usefulness is conditional upon knowing where and when birds acquired them during their annual cycles. In this study, we evaluated haemosporidian assemblages among non-resident black-capped chickadees (Poecile atricapillus), which occasionally "irrupt" south from parts of their northern distributional range to spend winters in the mid-Atlantic and Midwest regions of the United States. Specifically, we compared haemosporidian prevalence and lineage diversity from irruptive black-capped chickadees to those of resident Carolina chickadees (Poecile carolinensis) and black-capped × Carolina hybrids at 4 southeastern Pennsylvania locations, where most irruptive black-capped chickadees could be genetically and phenotypically identified among local birds. We hoped to (1) gauge whether certain haemosporidian lineages could help identify irruptive individuals among populations of otherwise sedentary black-capped chickadees and (2) roughly estimate where these irruptive birds hatched or bred. Irruptive birds had higher rates of haemosporidian infection than residents, and irruptive birds exclusively harbored a single Plasmodium lineage (BT7). Literature review suggests that lineage BT7 is widespread across the Northern hemisphere but is unlikely to be transmitted between chickadees in our study location. Thus, the presence of lineages such as BT7 may be useful for assessing the composition of seasonally sympatric bird populations with different migratory strategies or at least for identifying non-resident individuals within these populations.
Rice et al. (2026) studied this question.