Environmental and genetic variation in T-cell-mediated immune response of fledgling American kestrels

We investigated genetic and environmental components of variance in avian T -cell-mediated immune response (CMI) through a cross-fostering experiment c onducted on wild American kestrels (Falco sparverius). CMI was evaluated in vivo by an experimental challenge with phytohaemagglutinin, a T-cell mitog en, injected intradermally in fledglings. Additionally, we assessed two mea sures of nutritional condition (body mass and circulating plasma proteins) which could influence the variance components of CMI. A two-way nested ANOV A indicated that CMI of fledgling kestrels was explained more by the nest w here the bird was reared (33% of the explained variance) than by the nest o f origin (12%). Body mass was explained equally by familial and environment al components, while plasma proteins were only related to the rearing envir onment. CMI of fledglings was not related to their circulating plasma prote ins, but was positively correlated with their body mass. Fledgling body mas s seemed to be influenced by pre-hatching or post-hatching maternal effects prior to manipulation since resemblance in body mass of sibships at the ag e of manipulation was high (h(2)less than or equal to 0.58), and body mass at this age predicted body mass at fledging. Therefore, pre-manipulation pa rental effects on body mass, such as investment in egg size, could have inf lated the familial effects on body mass of fledglings and then on its corre lated CMI. When controlling for body mass, most of the variation in CMI of fledglings was explained by the nest where the bird was reared (36.6%), whi le the variance explained by the nest of origin (4%) was not significant. T his means that environmental influences are major determinants of offspring CMI. The low proportion of variance explained by the familial component ma y have been due to the high correlation of CMI to fitness.