Carotenoid-rich mouth colors influence the conspicuousness of nestling birds

When allocating investment among offspring, parents might maximize their fitness by biasing investment toward offspring with the best direct fitness prospects. The observed preferences of avian parents for carotenoid-rich mouth colors that advertise good condition has been interpreted as support for this hypothesis. However, because these condition-dependent visual signals might also make offspring more visually conspicuous, active parental preferences for carotenoid-rich traits are difficult to distinguish from passive responses to differences in detectability among offspring. Here, we used a visual model to examine how mouth colors influence the visual conspicuousness of nestling house sparrows (Passer domesticus) to parents under a suite of realistic ambient light conditions. We found little evidence that mouths rich in carotenoids provided more conspicuous targets to parents than mouths poor in carotenoids. While other features of mouth color may have evolved to increase conspicuousness, our results suggest that carotenoid-based coloration is not a product of detectability pressures and rather may serve as a signal of nestling quality. © Springer-Verlag 2009.