Humoral immunocompetence correlates with date of egg-laying and reflects work load in female tree swallows

Because quality differences between individuals affect fitness, much resear ch has attempted, with limited success, to relate physiological condition ( e.g., body reserves), to differences in life history between individuals. R ecently, it has been suggested that immunocompetence may reflect condition, and it thus may mediate variation in individual quality and reproductive p erformance and, ultimately, fitness. We measured humoral immunocompetence ( HIC) by immunizing female tree swallows with a harmless antigen and measure d the specific antibody responses in a novel enzyme-linked immunosorbent as say developed for passerine birds. HIC was strongly correlated with egg-lay ing date, an important determinant of reproductive success in female tree s wallows. We also investigated the effect of increased workload on HIC by ma nipulating female flight costs by clipping flight feathers. Clipped females had lower HIC than nonclipped females. These data suggest that HIC is a me asure that may reflect phenotypic quality and also appears to be sensitive to increased workload in female tree swallows.