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Variation in innate immune function during incubation, chick-rearing and moult in Little Penguins (Eudyptula minor)

Abstract Physiologically demanding and energetically expensive life-events, such as incubation, chick-rearing and moult, may cause a trade-off with costly immune defences and, in turn, decrease immune function. We examined innate immunity in Little Penguins (Eudyptula minor) during the physiologically demanding periods of incubation, chick-guarding, chick-rearing and moult using a bacterial-killing assay and white blood-cell counts. The capacity of individuals to kill Escherichia coli varied significantly in relation to life-stage, with bacterial killing significantly higher during incubation compared with late chick-rearing, the most energetically expensive period in the annual cycle of Little Penguins. The elevated bacterial-killing capacity during nesting stages could be particularly beneficial for protection from the presumed high pathogen (bacterial) loads in nests. However, this relationship remains to be tested. Counts of white blood cells varied during breeding but were significantly higher in chicks compared to adults during guarding and post-guarding stages, and significantly higher at the end of moult compared to all other stages, which could indicate inflammation from infection, or dehydration. These results suggest that immune function may be adversely affected during energetically demanding life-stages but may also offer protection from bacteria encountered in the nesting environment.