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

Because quality differences between individuals affect fitness,much research has attempted, with limited success, to relatephysiological condition (e.g., body reserves), to differencesin life history between individuals. Recently, it has beensuggested that immunocompetence may reflect condition, andit thus may mediate variation in individual quality and reproductiveperformance and, ultimately, fitness. We measured humoral immunocompetence(HIC) by immunizing female tree swallows with a harmless antigenand measured the specific antibody responses in a novel enzyme-linkedimmunosorbent assay developed for passerine birds. HIC was stronglycorrelated with egg-laying date, an important determinant ofreproductive success in female tree swallows. We also investigatedthe effect of increased workload on HIC by manipulating femaleflight costs by clipping flight feathers. Clipped females hadlower HIC than nonclipped females. These data suggest that HICis a measure that may reflect phenotypic quality and also appearsto be sensitive to increased workload in female tree swallows.