Introduction Citizen science efforts involve the general public in various aspects of the scientific process (Bonney et al. 2009a). However, there are different degrees to which volunteers can participate (Shirk et al. 2012). Citizens hire scientists to answer local questions in contractual projects, whereas volunteers collect or analyze data for professional scientists in contributory ones. In collaborative projects, volunteers and project managers work together on certain parts of the project, whereas co-created projects involve collaboration between volunteers and project managers throughout the entire project (from asking scientific questions to sharing results). Finally, in collegial projects, citizen scientists perform research independent of professional scientists. While many papers have anecdotally noted that most projects tend to be top-down, contributory projects (e.g., Pocock et al. 2017), few have documented this through research (Bela et al. 2016; Groulx et al. 2017). Volunteers have many diverse motives for participating in citizen science. Studies have investigated motivation for participation in individual projects (Raddick et al. 2013; Domroese and Johnson 2017) and across multiple projects (Alender 2016; Geoghegan et al. 2016). Both types of studies indicate participant motives that include contributing to science, to conservation efforts, or to the community; connecting with nature or with a specific place; socializing; furthering a career; exercising; having fun; and learning. The quality of overall volunteer participation in citizen science is defined by how well project outcomes align with volunteer needs and motives (Shirk et al. 2012). Participating in citizen science programs can result in various outcomes, but one that has received attention in the literature is learning. Multiple citizen science stakeholders, including volunteers, community members, and scientists, can learn about scientific inquiry and environmental issues when engaging in citizen science projects (NASEM 2018). This paper focuses on learning outcomes of volunteers specifically. Participating in citizen science can increase volunteer scientific literacy (Bonney et al. 2009b). Knowledge gains, engagement in inquiry-based reasoning, and changed conservation attitudes and environmental behaviors are all documented learning outcomes in citizen science (NRC 2000; Trumbull et al. 2000; Jordan et al. 2011; Toomey and Domroese 2013). Scientific literacy has important benefits for both individuals and society (Laugksch 2000). Here, we define the highest level of literacy as the ability to make an evidence-based decision that alters one's behaviors (UNESCO 1978; Balgopal and Wallace 2009). Individuals empowered to engage in scientifically informed behaviors can make decisions in their personal lives that also result in economic or