The evolution of honest animal communication is mostly understood through the handicap principle, which is intrinsically dependent on the concept of individual quality: low-quality individuals are prevented from producing high-quality signals because if they did so, they would pay greater production costs than high-quality individuals. Here, an alternative explanation is tested for the black bib size of male house sparrows Passer domesticus, an honest signal of quality whose expression is...

Many theoretical models on the evolution of nestling begging assume this behavior is costly, so that only nestlings in real need of food would profit from giving intensive signals to parents. However, evidence accumulated for the last 2 decades is either contradictory (growth costs) or scant (immunological cost). Here, the existence of both costs is experimentally tested in pied flycatcher nestlings.