Genetic and environmental effects on a condition-dependent trait: feather growth in Siberian jays

Condition, defined as the amount of 'internal resources' an individual can freely allocate, is often assumed to be environmentally determined and to reflect an individual's health and nutritional status. However, an additive genetic component of condition is possible if it 'captures' the genetic variance of many underlying traits as many fitness-related traits appear to do. Yet, the heritability of condition can be low if selection has eroded much of its additive genetic variance, or if the environmental influences are strong. Here, we tested whether feather growth rate - presumably a condition-dependent trait - has a heritable component, and whether variation in feather growth rate is related to variation in fitness. To this end, we utilized data from a long-term population study of Siberian jays (Perisoreus infaustus), and found that feather growth rate, measured as the width of feather growth bars (GB), differed between age-classes and sexes, but was only weakly related to variation in fitness as measured by annual and life-time reproductive success. As revealed by animal model analyses, GB width was significantly heritable (h(2) = 0.10 +/- 0.05), showing that this measure of condition is not solely environmentally determined, but reflects at least partly inherited genetic differences among individuals. Consequently, variation in feather growth rates as assessed with ptilochronological methods can provide information about heritable genetic differences in condition.