Combining datasets helps identify biodiversity threats–if we can account for their different observation biases. eBird is a citizen science project with variable geographic coverage and observer effort. The Breeding Bird Survey (BBS) is a structured survey repeated each year. Together, they offer a massive resource on bird distribution and abundance. To combine them, we have to account for variation in habitat, time of day, observer effort and expertise (many eBirders are novices), and even the tendency of different species to be observed together.

The differences can be substantial. BBS observers count more species per observation effort, especially species that are common and detected by sound. eBirders count more “popular” and wetland species. Species typically identified by sound are reported more at sunrise than late morning. This knowledge of their biases is now being used to synthesize data from both sources.

Scher, C.L. and J.S. Clark. 2023. Species traits and observer behaviors that bias data assimilation and how to accommodate them. Ecological Applications, in press.