Better red than dead: carotenoid-based mouth coloration reveals infection in barn swallow nestlings

Nestling birds solicit food from their parents by displaying their open bri ghtly coloured gapes. Carotenoids affect gape colour, but also play a centr al role in immunostimulation. Therefore, we hypothesize that, by differenti ally allocating resources to nestlings with more brightly coloured gapes, p arents favour healthy offspring which are able to allocate carotenoids to g ape coloration without compromising their immune defence. We demonstrated t hat, in the barn swallow Hirundo rustica, (i) parents differentially alloca te food to nestlings with an experimentally brighter red gape, (ii) nestlin gs challenged with a novel antigen (sheep red blood cells, SRBCs) have less bright gape colour than their control siblings, (iii) nestlings challenged with SRBCs but also provided with the principal circulating carotenoid (lu tein) have more brightly coloured red gapes than their challenged but unsup plemented siblings and (iv) the gape colour of nestlings challenged with SR BCs and provisioned with lutein exceeds that of siblings that were unchalle nged. This suggests that parents may favour nestlings with superior health by preferentially feeding offspring with the brightest gapes.