Juveniles are bolder than adults in black-headed gulls (Chroicocephalus ridibundus): a field test of the experience hypothesis

The experience hypothesis predicts that juvenile birds (including immature and subadult individuals) are more exploratory and bolder than their adults. Alternatively, the habituation hypothesis suggests that with urbanization, antipredator strategies are relaxed in certain animals that coexist with humans, resulting in losses of some wild behaviors during development. Flight initiation distance (FID) is widely used to assess trade-offs in bird responses to threats; however, age differences in FID have mostly been studied in passerines. In this study, the FIDs of adults and juveniles were determined in a wintering population of black-headed gulls (Chroicocephalus ridibundus) in China, to test the two hypotheses for avian responses to human activities. The findings indicate that juveniles exhibit shorter FIDs than adults when approached by observer wearing black, white, red, and green, thereby supporting the experience hypothesis. Additionally, results from four sites with a black-clothing control group further support this conclusion. These results contribute new experimental evidence regarding age-related differences in cognition and behavior among birds.