Recent trends in Tricolored Blackbird colony size: analysis of the 2008 through 2017 Triennial Statewide Surveys

Tricolored blackbird (Agelaius tricolor) is a colonial breeder, largely restricted to grasslands, wetlands, and agricultural habitats of California. Tricolored blackbird abundance declined considerably during the 20th century. Recent trends have been less clear, however, hindering efforts to evaluate the conservation needs of the species. We assessed trends in tricolored blackbird colony size using the 2008, 2011, 2014, and 2017 Triennial Tricolored Blackbird Statewide Survey, a community-science effort involving hundreds of volunteer observers. After accounting for variation in observer characteristics and survey effort, we found a clear, statistically significant decline in colony size of approximately 5% per year, which translated to a decrease in colony size of approximately 40% between 2008 and 2017. To the extent that colony size is proportional to total abundance, this decline translates to a species half-life of roughly 15 years. We conclude that tricolored blackbird is in considerable need of protection and recovery efforts.