Capsule: Previous observations in Barcelona and other cities showed that approximately 40% of the food ingested by adult Monk Parakeets Myiopsitta monachus is of anthropogenic origin. Here we show that this type of food source is also used for feeding nestlings. Aims: To quantify the proportion of anthropogenic food within the diet of nestling Monk Parakeets, a non-native species in Barcelona. Methods We analysed stable isotopes of 13C and 15N in feathers of Monk Parakeet nestlings collected in the Barcelona city area. We also sampled potential food sources ingested by Monk Parakeets to reconstruct the nestling diet using Bayesian mixing models with MixSIAR. Results: Almost 30% of the nestlings’ diet was composed of anthropogenic food. Conclusions: Since food availability is a major factor regulating population growth, we propose educating the general public to reduce the food supply for the species and ultimately limit its population growth.