Skin and flange colour, but not ectoparasites, predict condition and survival in starling nestlings
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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.
KeywordsBegging behaviour Ectoparasites Flange colour Parent-offspring communication Skin colour Seasonal variation
We are grateful to L. Zangari and G. Viscardi who greatly helped during field work. Three anonymous referees also provided very useful comments to previous versions of the manuscript.
Compliance with ethical standards
This research was undertaken (capture and experimental treatments) under the combined prescriptions of Art. 4 (1) and Art. 7 (5) of the Italian law 157/1992, which regulates studies on wild bird species. This law also regulates taking, keeping and manipulating procedures to be followed for the study of wild birds. Standard procedures for capturing and handling nestlings in the nest-box were used. The duration of handling nestlings was kept to a minimum to minimize stress.
SP and AP were partially supported by a grant from the Fondazione CARIPARO (Progetto di eccellenza 2007). AR was funded by the Postdoctoral Fellowship Program of the Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Conflict of interest
The authors declare that they have no conflict of interest.
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