Citizen science is when ordinary people help with scientific research, e.g. within biodiversity, molecular biology and astronomy. At Artdatabanken, which is an organization that observes biodiversity, citizen science is used to enable data gathering of species in Sweden. Previous research has investigated how to maintain long-term engagement by observing motivation for citizen scientists regardless of skill-levels. In this study, the motivations of experienced citizen scientists are investigated. Semi-structured interviews were conducted with citizen scientists at Artdatabanken, and thematic analysis was used to find themes in the answers.  The participants were open both to increase contributions and to report other species. The results showed mainly extrinsic motivations, which could be due to a lack of reflection of intrinsic motivation. Three main themes were found in the interviews. The first theme, Efficiency, was connected to usability-issues, mainly focusing on time-efficiency and simplifying the reporting tool. The acknowledgement-theme involved motivation to receive recognition for the contributions. The third theme, Knowledge and Understanding, was connected to motivation for increased knowledge regarding species and data. Some suggestions for implementation are presented to make the motivations feasible for design implementations of citizen science systems.