Dietary protein selection in a free-ranging urban population of common myna birds

Anthropogenic environments can offer rich sources of energy to urban wildlife, but little is known about how they impact on nutritional balance and food selection. Common mynas (Sturnus tristis) provide a powerful model system for testing the nutritional constraints and priorities of an invasive species that has successfully adapted to urban environments. Here, we use behavioral observations, field-based feeding trials, videography, and the right-angled mixture triangle model (RMT) to examine the macronutrient preferences of these invasive birds. Our behavioral observations showed that mynas consumed insects (40.6%), worms (33.2%), human discards (17.6%), and plants (8.6%). Our feeding trials using nutritionally defined foods showed that mynas had a clear preference for food dishes containing only high-protein (HP) pellets over high-lipid (HL) or high-carbohydrate (HC) pellets. In addition, mixed feeders were also presented in 2 combinations: 1) contained equal proportions of HP and HC pellets and 2) equal proportions of HP and HL pellets. HP pellets were selectively consumed from both mixed feeders, this involving an increase in feeding time. Overall, the RMT showed that mynas consumed a higher proportion of protein from the feeders than in their natural diet. Furthermore, the majority of our observations of birds feeding at the dishes containing HP foods ended in intraspecific aggression, suggesting that protein is a contestable resource. These results suggest that mynas at our urban study site are deficient in protein relative to fats and carbohydrates.