Genetic, environmental and maternal effects on magpie nestling-fitness traits under different nutritional conditions: a new experimental approach

Rearing full siblings under different environmental conditions allows partitioning of the total phenotypic variance of a trait into its genetic and environmental components. This, in natural bird populations, is usually achieved by cross-fostering experimental designs. We estimated genetic and environmental components of nestling-fitness traits using an alternative experimental approach in a magpie (Pica pica) population. Two broods of full siblings were reared under contrasting environmental conditions of first and replacement clutches. With this approach, potential maternal effects related to differences in clutch size and egg size could also be partially evaluated. In addition, the nutritional condition of half of the nestlings within each nest was manipulated by providing a calorie-rich paste enriched with micronutrients. Our results are only indicative because of very low sample sizes. In food-supplemented nestlings, the heritability estimates of tarsus length, body mass and T-cell-mediated immune response tended to be higher compared with control nestlings. No causal conclusions could be drawn for changes in heritability estimates of body mass and T-cell-mediated immune response; for tarsus length, the results suggest a lower potential to adapt to poor nutritional conditions. Furthermore, we found some indication that maternal effects related to clutch/egg size inflated causal estimates of phenotypic variance in tarsus length.