Evidence that fathers, but not mothers, respond to mate and offspring coloration by favouring high-quality offspring

Parents have limited resources and, when simultaneously rearing multiple offspring, are expected to favour particularly valuable offspring. Because offspring characteristics like sex and ornamentation influence future reproductive value, these traits may influence parental favouritism. Parents in high-quality nest environments should prefer sons because sons show greater variability in reproductive potential than daughters. Moreover, when ornamental plumage coloration is condition dependent, parents should favour brighter over duller sons. We tested parental favouritism for fledgling-aged eastern bluebirds, Sialia sialis, a species in which fledglings display condition-dependent UV-blue coloration on their wings and tails. We simultaneously threatened two offspring with mock predators and recorded which offspring the parents defended. In accordance with the predicted value of offspring, fathers protected brighter over duller sons and protected sons over daughters when mated to high-quality mates. Mothers did not discriminate between sons and daughters or between brighter and duller sons. These results suggest that ornamental plumage coloration in juvenile bluebirds functions as a signal in father–offspring interactions. Paternal and maternal responses of eastern bluebirds to offspring characteristics may differ because of differences in parental allocation strategies or because males assess offspring quality based on variation in plumage coloration, a trait that also functions in competitive interactions among adult males.