Haemosporidian parasite communities are broadly similar in Boulder County, CO between two common songbirds –– the Black-capped Chickadee (Poecile atricapillus) and Mountain Chickadee (Poecile gambeli). However, Mountain Chickadees appear more likely to be infected with Plasmodium and potentially experience higher infection burdens with Leucocytozoon in contrast to Black-capped Chickadees. We found that elevation change (and associated ecology) drives the distributions of these parasite genera. For Boulder County chickadees, environmental factors play a more important role in structuring haemosporidian communities than host evolutionary differences. However, evolutionary differences are likely key to shaping the probability of infection, infection burden, and whether an infection remains detectable over time. We found that for recaptured birds, their infection status (i.e., presence or absence of detectable parasite infection) tends to remain consistent across capture periods. We sampled 234 chickadees between 2017–2021 across a ~1500-meter elevation gradient from low elevation (i.e., the city of Boulder) to comparatively high elevation (i.e., the CU Boulder Mountain Research Station). It is unknown whether long-term haemosporidian abundance trends have changed over time in our sampling region. However, we ask whether potentially disparate patterns of Plasmodium susceptibility and Leucocytozoon infection burden could be playing a role in the negative population trends of Mountain Chickadees.