Background <p>The most common environmental citizen science program type, often called “contributory” citizen science, invites volunteers to help with data collection. But is that all their volunteers do? Previous research has tended to consider the knowledge practices of citizen scientists from the perspective of program organizers, or has paid attention to knowledge as simply content (that is, the input, output, and outcomes of programs). There is a need to better understand knowledge practices as experienced by citizen scientists. This exploratory and ethnographic study investigated the knowledge practices of biodiversity monitoring volunteers without making assumptions about what they should do, what they actually do, or their levels of expertise. We conducted semi-structured interviews and participant observation of two citizen science programs in Australia, one recording frog presence and another one monitoring endangered beach birds’ nesting success rates and threats.</p> Results <p>We found that aside from collecting data, volunteers in these contributory biodiversity citizen science programs engaged in a range of knowledge practices. These practices involved not only producing, but also expanding, sharing, and using knowledge in multiple ways, such as asking additional research questions and analyzing data, acquiring and disseminating knowledge of biodiversity and environmental policy, and using citizen science data for advocacy, habitat work, and other conservation efforts. Volunteers carried out some of these practices as part of environmental collectives, not only as individuals, and their participation in and through collectives was often key to their engagement in citizen science programs.</p> Conclusions and implications <p>Recognizing that volunteer practices go beyond what they are officially requested to do is important for being able to acknowledge, value, and support volunteers’ contributions. Citizen science programs may benefit from adopting a less pre-defined understanding of participation in citizen science, instead recognizing that modes of participation evolve and are co-created with participants. Appreciating and making visible the breadth of volunteer knowledge practices could be important for instrumental reasons, such as collecting more and better data, and increasing volunteer engagement and retention. Recognizing the agency of participants in determining what participation is for them can also help in advancing the democratization of science and decision-making.</p>