Data from: Using citizen-science observations to understand long-term trends in common and Pacific loon populations in urbanized Southcentral Alaska

For over 30 years, the Alaska Loon Watch (1985−1999) and the Alaska Loon and Grebe Watch (2000−2015) engaged citizen scientist participants to record over 10,000 observations of common Gavia immer and Pacific loons Gavia pacifia at 346 lakes in five sub-regions of Southcentral Alaska. We used generalized linear mixed models to estimate long-term trends in adult loon counts and chick survival and examined environmental variables associated with loon abundance. Adult common loon counts increased in all five sub-regions by 0.6-3.6% per year, while Pacific loons decreased 3% in the Anchorage sub-region, but otherwise had trends not distinguishable from zero. Lake area was positively associated with common loon abundance and negatively associated with Pacific loon abundance. We also noted an inverse relationship between common loon and Pacific loon presence, consistent with the premise of interspecific competition. We did not find strong relationships between loon presence and predictor variables indicative of human disturbance or lakeshore development. Estimates of chick survival over time also revealed no clear pattern, although common loon chicks showed a decline in survival over the study period in one sub-region. Citizen science programs provide agencies with a cost-effective tool to collect data over large spatial and temporal extents, which may not be feasible otherwise. However, there are ramifications of common data deficiencies associated with casual or unstructured observations, which can violate the assumptions required for rigorous statistical analysis. A carefully pre-defined sampling protocol avoids sampling bias, eliminates stringent assumptions, and ensures higher information content of citizen-science data