Individual quality mediates trade-offs between reproductive effort and immune function in tree swallows

1. Individual variation in the trade-off between self-maintenance and offspring quality was assessed in tree swallows, Tachycineta bicolor, by manipulating reproductive effort while simultaneously immunochallenging breeding females. 2. An experimental manipulation of parental effort was conducted by creating broods of, on average, three, five and eight nestlings. Breeding females were immunochallenged to mount a humoral immune response to sheep red blood cells (SRBC) and a cell-mediated response to phytohaemagglutinin (PHA). The consequences of adult decisions on offspring quality were assessed through immune responses to PHA, growth rates and residual body mass of nestlings. 3. Clutch initiation date, a strong measure of individual quality in tree swallows, was linked with immune responses, with earlier-nesting, presumably higher quality, females mounting stronger immune responses than did later-nesting birds. Increased reproductive effort led to decreased parental immune responses. There was a significant interaction between individual quality and reproductive effort treatment, with lower-quality individuals showing greater depression of humoral immune response to SRBC while raising enlarged broods, suggesting individual-level variation in trade-offs. 4. Breeding females raising enlarged broods tended to raise offspring of similar quality to control females, with only growth rate decreasing with increasing brood size, but not residual nestling body mass or nestling immunocompetence. This suggests that females are maintaining offspring quality at the cost of their own immune system maintenance.