A Test of Parental Preferences for Offspring Sex and Ornamentation in Eastern Bluebirds (Sialia sialis)

Avian parents protect their fledglings from predators. Given parents have limited resources, they face trade-offs when simultaneously rearing multiple offspring. Parents might favor particularly valuable offspring. Offspring characteristics influence their future reproductive value and may influence parental favoritism. Because sons exhibit greater variability in reproductive potential than daughters, parents in high-quality environments should preferentially defend sons. Eastern Bluebirds exhibit bright blue plumage and brighter individuals gain higher reproductive success, thus parents should favor the more-ornamented sons. I tested parental favoritism for fledglings by simultaneously threatening two offspring with mock predators and recording parental defense behaviors. Given the choice of protecting sons versus daughters, fathers protected sons when mated to high quality-mates, but protected daughters when prior parental investment was high. When given the choice of protecting more versus less colorful sons, fathers favored brighter over duller sons. Mothers didn’t discriminate between sons and daughters or between brighter and duller sons. These differences in parental responses to offspring characteristics may occur because, fathers assume primary responsibility for fledgling care and plumage coloration in adults may mediate male-male interactions. Further, data suggest that ornamental plumage coloration in juvenile bluebirds functions as an honest signal of offspring quality to elicit greater parental care.