Nest size predicts the effect of food supplementation to magpie nestlings on their immunocompetence: an experimental test of nest size indicating parental ability

Post-mating sexually selected signals are expected to indicate parental quality. The good parent model assumes that expression of the sexual character positively reflects parental ability, resulting in a potential link between the exaggeration of the character and nestling-fitness traits. We tested this prediction in a population of a monogamous passerine, the magpie (Pica pica), for which nest size is known to act as a post-mating sexually selected signal. We provided a food supplement to half of the magpie nestlings in each nest, keeping the other half as control nestlings. We found that food-supplemented nestlings experienced a significantly higher T-cell-mediated immune response and a tendency to an increased condition index. In accordance with the good parent model, we found that nest size was positively related to T-cell mediated immune response for control magpie, whereas this relationship was nonexistent in food-supplemented nestlings. In addition, the difference in T-cell mediated immune response between food-supplemented and control nestlings of the same nest was principally explained by nest size. Based on our results, we discuss that magpie pairs with large nests provided their nestlings with higher quality food as compared to pairs with smaller nests, nest size thereby being an indicator of parental ability. To our knowledge, this is the first study showing a link between a post-mating sexually selected signal and nestling immunocompetence, a trait closely related to fitness in birds.