Garbage in may not equal garbage out: sex mediates effects of ‘junk food’ in a synanthropic species

Human influence on ecosystems is rapidly expanding, and one consequence is the increased availability of human food subsidies to wildlife. Human food subsidies like refuse and food scraps are widely hypothesized to be ‘junk food’ that is nutritionally incomplete; however, the impacts of ‘junk foods’ on the health and fitness of individual organisms remain unclear. In this study, we aimed to understand how human food consumption affects the body condition and fecundity of a generalist predator, the Steller’s jay (Cyanocitta stelleri). We used stable isotope analysis to quantify individual human food consumption (using δ13C as a proxy), estimated individual body condition based on body mass and feather growth bar width and assessed jay fecundity. Adults consumed more human food than juveniles on average, and we observed sex-specific responses to human food use where male body condition tended to increase, whereas female body condition tended to decline with human food consumption. However, fecundity was not strongly related. Thus, we found some evidence for the ‘junk food’ hypothesis in this system, which suggests that human foods may not be an equal replacement for natural foods from a nutritional perspective, especially for females. Human foods tend to be carbohydrate rich, but protein poor, which may benefit males because they are larger and limited overall by calorie intake. Females, particularly reproducing females, are more nutritionally limited and thus may experience fewer benefits from ‘junk food’. Our study advances knowledge of human–wildlife interactions by increasing the resolution of our understanding of the fitness benefits, or detriments, experienced by individuals that consume human foods.