Relationship between energetic condition and indicators of immune function in thrushes during spring migration
Evidence suggests that the ability of an animal to maintain its immune system and (or) mount an immune response depends on its nutritional health and energetic condition. Migration is a period within an animal's annual cycle when energetic condition varies, especially after a long, nonstop flight over a large ecological barrier. Our objective was to determine if measures of immune function in migrating Wood Thrush (Hylocichla mustelina (J.F. Gmelin, 1789)), Swainson's Thrush (Catharus ustulatus (Nuttall, 1840)). Gray-cheeked Thrush (Catharus minimus (Lafresnaye, 1848)), and Veery (Catharus fuscescens (Stephens, 1817)) were related to the energetic condition of the birds at a stopover site during spring migration. We present data on total leukocyte, lymphocyte, an heterophil counts, heterophil/lymphocyte ratio. serum immunoglobulin gamma G (IgG) concentration, and immune response to phytohemagglutinin. Thrushes arriving at the stopover site in poor energetic condition had low leukocyte and lymphocyte counts. Heterophil/lymphocyte ratio, heterophil count, and IgG concentration were not related to energetic condition. Furthermore, immune response to phytohemagglutinin was positively related to change in mass and days spent in captivity, suggesting that immune function may improve during stopover. We suggest that migrating thrushes arriving at a stopover site in poor energetic condition may also be in poor immunological condition and may have increased Susceptibility to disease or parasite infection.