Tricolored blackbird survey methods

Abstract

Surveys for the tricolored blackbird (Agelaius tricolor), a California-listed threatened species, are needed to provide essential information about its status and distribution to support conservation efforts. Surveying for the species, however, poses challenges because of its colonial nesting habits, large flock sizes, and frequent interannual movements among colony sites. We present standardized approaches for tricolored blackbird surveys to accommodate a variety of goals, including determining potential for occurrence, detecting colonies, estimating population numbers, and assessing nesting success. Pre-survey research should assess overlap with the species’ geographic range and include database searches for recent and historical occurrences. Site surveys should assess availability of sufficient suitable foraging habitat (grasslands, irrigated pasture, shallow wetlands, agricultural fields) within 5 km of potential nesting habitat and nearby drinking water. Confirming presence or absence of nesting at a historical colony site, or at an area within the range of the tricolored blackbird that provides suitable nesting and foraging habitat, requires at least three surveys conducted about three weeks apart during the breeding season, generally April, May, and June in most of the species’ range. Colonies can be located by searching for foraging flocks and following them to colonies and by checking for nesting activity at suitable nesting substrates, especially those used in the past. Surveyors can estimate numbers from counts of foraging flocks departing and arriving at colonies, average density and area occupied by the nesting colony, and post-nesting density transects. Colony nesting success (i.e., whether a colony successfully fledged some young), is easily determined and is often the only feasible metric to attain on reproduction. Determining reproductive success (the average number of young fledged per occupied nest) may be feasible under ideal and intensive surveys but is impractical at many colonies due to accessibility limitations.