Skin and flange colour, but not ectoparasites, predict condition and survival in starling nestlings

Parents are expected to strategically partition their limited resources among the current and future progeny in order to maximize their fitness. Since an equal investment in offspring of different reproductive value entails fitness costs, natural selection has promoted the evolution of reliable signals of offspring condition, allowing parents to invest in their progeny accordingly. In birds, mouth and skin colouration are hypothesized to be honest signals of offspring condition, because they are affected by diverse factors. Among these, ectoparasite load has been shown to affect nestling condition, but its influence on visual components of begging is poorly known. We experimentally investigated whether nest ectoparasite removal affected flange and skin reflectance of first- and second-brood European starling (Sturnus vulgaris) nestlings. We also tested whether high reflectance in visual components of begging mirrored other aspects of nestling condition, such as morphological (high stature) and physiological (high haematocrit and immune response) traits, and pre-fledging mortality. Ectoparasite removal did not affect visual components of begging in first or second broods. However, larger nestlings from both broods displayed higher ultraviolet (UV) reflectance of skin and higher flange reflectance in the visible-wavelength region (but lower flange UV reflectance) than their siblings. A higher skin UV reflectance relative to siblings also positively predicted pre-fledging survival within-brood. Therefore, visual components of begging did not mirror ectoparasite infestation in this species. However, they provide parents with reliable information about individual quality, thus affecting resource allocation and promoting survival of the most valuable offspring during the entire breeding season. In species with parental care, natural selection favours the evolution of reliable signals of offspring quality, thus allowing parents to invest in their progeny accordingly. We experimentally show that skin and beak flange colour does not mirror ectoparasite infection in European starling nestlings. However, begging visual signals predict nestling body size and survival until fledging. A seasonal variation in the strength of the association between begging visual signals and nestling condition is also shown, indicating that change ecological conditions can affect the association between different condition-dependent traits.