The T-cell-mediated immune response and return rate of fledgling American kestrels are positively correlated with parental clutch size

Life-history theory predicts that parents face a trade-off between the numb er and viability of the progeny they produce. We found evidence for an appa rent trade-off in a free-living population of American kestrels (Falco spar verius), as larger clutches produced more but lighter fledglings. However, while the body mass of fledglings has traditionally been used as a measure of survival prospect, offspring immunocompetence should also play an import ant role; We thus measured the T-cell-mediated immune response of fledgling kestrels in relation to brood traits and nest-rearing conditions through a cross-fostering experiment. The immune response was positively correlated with the body condition of fledglings, but was also higher in those hatched from five-egg than four-egg clutches. These results were not influenced by other brood traits, nor by current exposure to stressors and infectious ag ents, as measured by serological variables. Such ability to resist pathogen s may account for why the probability of offspring returning to the study a rea in subsequent years, when controlling for brood size, was higher for fi ve-egg than four-egg clutches. These results suggest an optimal clutch size through maternal effects on offspring immunocompetence rather than a trade -off between the number and quality of the offspring.