In times where governments are increasingly unable to solve problems on their own, it is important to understand what motivates citizens to engage in co-production. Drawing on identity theory and on a sample of 966 citizens in Zurich, Switzerland, we analyze how public service motivation (PSM) is related to engagement in a digital co-production platform enabling citizens to support public service provision. As our dependent variable, effort intensity in platform use, is a behavioral outcome, we provide insights into the actual behavioral implications of PSM, going beyond selfreports. Using negative binomial regressions, we found a positive relationship between engagement and the PSM dimension "commitment to the public interest", and a negative one with the dimension "compassion". Additionally, social identification with the city moderates the relationship with engagement for certain PSM dimensions. In sum, our study contributes to a better understanding of what motivations drive citizens to collaborate with public organizations, which is an ongoing trend in many countries around the world.