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Tricolored blackbird (Agelaius tricolor) is a colonial breeder, largely restricted to grasslands, wetlands, and agricultural areas of California, USA. Tricolored blackbird abundance declined considerably during the twentieth 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 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 decrease in average colony size of approximately 5% per year, which translated to a decrease in average colony size of approximately 40% between 2008 and 2017. This decrease in colony size matches the overall decline in abundance reported in another recent study and reinforces the conclusion that tricolored blackbird is in need of protection and recovery efforts.