Seasonal Differences in Immune Profiles and Body Conditions of Migratory and Permanent Resident Neotropical Flycatchers

The activation and maintenance of the immune system demand a cost of nutrition and energy, and this demand may result in a trade-off with other energetically demanding activities. To test for such a trade-off, in the cerrado of central Brazil, we compared the immune profile and body condition of the resident Plain-crested Elaenia (Elaenia cristata) in the rainy nonbreeding season with that in the dry breeding season. Then we compared the immune profiles and body conditions of the Plain-crested Elaenia and Lesser Elaenia (Elaenia chiriquensis), a migrant, during the breeding season. Finally, we compared the immune profiles of the Plain-crested Elaenia at four stages of molt. The Plain-crested Elaenia's immunosuppression profile was typical (decreased lymphocyte number, increased heterophil number, and H:L ratio) during the dry season but not at other seasons. Condition measured as a body-mass index did not vary significantly by season. The Plain-crested Elaenia's immunosuppression profile did not change within the breeding season, but the Lesser Elaenia's did. We found weak correlations between immune profile and body condition in both species. The immune profile of the Plain-crested Elaenia did not vary significantly by stage of molt, but it did from the wet to the dry, potentially energetically stringent season, possibly because of shortage of food. Variation in the immune profile of the Lesser Elaenia is apparently associated with migration. This study is the first to evaluate the immune profile of a wild passerine throughout the year and to compare congeneric species that differ in their migratory status.