Citizen science is a key approach in scientific research that involves crowdsourced data collection for environmental monitoring and conservation. Participants in citizen science can gain scientific knowledge, change their pro-environmental attitude and behavior. Taiwan Roadkill Observation Network (TaiRON) was founded in 2011 and has a community of approximately 20,000 members. To collect data for further biological research, participants adopt a simple approach: taking pictures of roadkill and uploading them to a database. While many studies have explored citizen science learning outcomes, this study examines how a program with a unique participant task -- photographing road kill -- has an impact on participants pro-environmental learning. The learning outcome with the largest change was environmental attitude, suggesting that the participants' feelings, awareness, and values regarding the environment changed after joining TaiRON. The reasons for the attitude changes could be attributed to the characteristics of TaiRON's work: participants collect data from animal carcasses, prompting them to reflect deeply on the value of life. The second key learning outcome was behavioural intention, which means participants were driven to action to mitigate roadkill occurrence. Our results provide an example of evaluating learning outcomes in citizen science and also contribute to promoting environmental literacy.