Interplay between energy reserves, parasitism, stress and immunity in migrating songbirds

Migrating animals likely face trade-offs in allocating resources to different physiological functions, with one potential trade-off between fat accumulation and immune defense. Here, we sampled three migratory songbird species during their fall migration through coastal Georgia and quantified body condition, fat loads and infection with blood parasites. We predicted that parasitized birds would show decreased fat scores, poorer body condition and increased stress levels. We also asked whether birds with greater immune defense would have higher or lower body condition measures and fat levels and examined the association between infection status and immune defense. Parasitized birds had elevated leukocyte counts but showed no evidence of greater stress, poorer body condition or lower fat levels. Although we found no relationship between leukocytes and body condition or fat scores, results from this study suggest that investment in immune defense might be an important cost of parasite infection for animals undertaking long-distance migrations.