Avian immune function and the energetic demands of long-distance migration
The immune system is the defense for vertebrates against pathogenic microorganisms. Yet studies show animals reduce or alter immune function when participating in energetically demanding activities. Bird migration is a period of exceptional energetic demand and an ideal context within which to examine the effect of prolonged, strenuous exercise on the avian immune system. In this dissertation, I investigate the trade-offs between the energetic demands of migration and immune function by addressing several central questions: (1) What is the relationship between immunocompetence and energetic condition? (2) Do migrants suppress immune function during migration? (3) Do migrants reallocate resources to their immune system during stopover, following periods of immunosuppression? Data was collected at a study site located in Johnson Bayou, Louisiana, on the northern coast of the Gulf of Mexico. Focal species include Swainson's Thrush ( Catharus ustulatus ; SWTH), Wood Thrush ( Hylocichla mustelina ; WOTH), Veery (Catharus fuscescens ; VEER), Gray-cheeked Thrush (Catharus minimus ; GCTH), and Ovenbird (Seiurus aurocapillus ; OVEN). The parameters of health and immune function presented in this dissertation include (a) absolute and differential leukocyte counts, (b) hematocrit, (c) sedimentation rate, (d) immunoglobulin gamma levels, and (e) cell-mediated immune response. In a study examining the condition-dependence of immune function in birds during spring migration, I found total leukocyte count and hematocrit positively correlated with body condition. Other parameters of immune function did not correlate with energetic condition. In a descriptive study, comparing the immune status of birds captured during spring with conspecifics captured during the breeding and fall migration season, I find birds in migratory condition are immunocompromised compared with breeding birds. A controlled experiment was designed to further test whether birds suppress immune function when in migratory disposition. My results show Swainson's Thrushes exhibiting migratory activity had reduced cell-mediated immune response compared to control (nonmigrating) birds. Having shown birds suppress immune function during migration, I then investigated whether birds renew immune function during stopover. Both leukocyte count and cell-mediated immune response improve during stopover. Furthermore, my results suggest that both period of rest and replenishment of depleted fat stores are important for renewing immune function during stopover. Given the high costs of migration and the importance of immune function, trade-offs between the two will have consequences for a bird's survival and reproductive success.