Citizen science has proven to be a valuable tool for biodiversity conservation. However, to maximize the conservation benefits of citizen science programs, researchers and practitioners would gain from a better understanding of project volunteers and what drives them to participate. We examined the diverse motivations of volunteers (n = 3041) participating in Audubon's Christmas Bird Count, one of the world's oldest ecological monitoring citizen (or community) science projects. Principal axis factor analysis along a 16-item scale revealed six distinct intrinsic and extrinsic motivational constructs: science and conservation, outdoor recreation and discovery, commitment and tradition, social interaction, classic birding, and personal accomplishment. Most participants reported multiple motivations, but 40% indicated contribution to science and conservation was their primary reason for initially engaging with the project. As project participation continued, science and conservation-related motives became even more important (with 55% listing as primary continuing motivation). Regression analyses showed motivational orientations varied by socio-demographic attributes and levels/type of project participation. For example, social interaction and tradition were more important to aspiring project leaders than casual observers. Results highlight insights into deepening project engagement and recruiting and retaining citizen scientists. Adapted and applied across different contexts, our instrument and motivational constructs could help to facilitate volunteer management and enhance citizen science's capacity to advance biodiversity conservation goals.