Leukocyte profiles vary with breeding latitude and infection status in a seasonally sympatric songbird

Abstract

Abstract Billions of animals migrate annually in pursuit of food, safety, and reproduction. Long-distance migration can be energetically expensive, which can force tradeoffs with investment in other physiological systems (e.g., suppressing immunity). Understanding the physiological impacts of migration is important to predict when and where such animals may be vulnerable to parasites and how changes to migration might affect infectious disease risks. To isolate relationships between migration and physiology from energetic tradeoffs and hormonal shifts associated with spring reproduction, we assessed differences in leukocyte profiles between seasonally sympatric resident and recently arrived fall migrant dark-eyed juncos (Junco hyemalis) in the Appalachian Mountains. When examining heterophil:lymphocyte (HL) ratios, which can elevate for long durations after even mild stressors, we found weak associations with migratory strategy (resident or migrant subspecies). In contrast, feather δ2H values showed that HL ratios were highest for juncos breeding at more northern latitudes, and this relationship was strongest for birds that arrived at the overwintering site infected with haemosporidian parasites (Plasmodium and Haemoproteus spp.). These patterns were more pronounced and better indicated hematological responses to stressors when using multivariate analyses. Our findings suggest that short- and long-distance migration may be more energetically costly in the presence of infection and highlight how approximating breeding latitude and using multivariate analyses can help understand host physiology.