Tricolored Blackbird Breeding Status in 2015 in the Foothill Grasslands of the Sierra Nevada, California

Abstract

In 2015, we continued studies of the Tricolored Blackbird (Agelaius tricolor) initiated in 2014 (Airola et al. 2015) in the grassland area on the east side of the Central Valley and lower foothills of the Sierra Nevada (“foothill grasslands”). We expanded studies from the “central foothills” (from Placer County south to Stanislaus County) to similar adjacent grassland areas (“northern and southern foothills”) to describe the breeding biology of Tricolored Blackbirds throughout the Sierra foothill region. Through geographic information systems (GIS) analysis, we identified 1,269 km² of suitable Tricolored Blackbird habitat in the study region and estimated that we surveyed 28% of all suitable habitat and 34% of that in central foothill region. We found a total nesting population of 55,270 birds at 26 active colonies, all but one of which were in the central foothills. Compared to 2014, the number of breeding birds increased by 12,261 (29%) in 2015, which was mainly attributable to increased survey coverage. The low proportion of suitable habitat surveyed indicates that the total nesting population in the foothills may be substantially higher than the number we detected. More intensive surveys are needed throughout foothill areas, especially in areas lacking public access. All but one of 26 active colonies (i.e., those that progressed to or beyond nest-building) produced young. Low rainfall occurred during the 2013-14 and 2014-15 seasons (51% and 71% of annual average, respectively), and may have especially reduced nesting habitat and use in the southern foothills. Only 11 (42%) of 26 colonies active in 2015 were at sites occupied in 2014. As in 2014, Tricolored Blackbirds in this region nested primarily in non-native Himalayan blackberry (Rubus armeniacus).  Two colony sites disturbed after the 2014 nesting season were not reoccupied in 2015.  Half of a large 7,000-bird colony was destroyed when young were in nests, eliminating an estimated 1,386 to 2,345 nests and an equal number of potential fledglings. Habitat disturbance we observed shows a need for ongoing outreach and coordination with landowners whose lands support active colonies. More intensive surveys are needed, especially in the southern foothills if annual rainfall increases and nesting habitat conditions improve.  Developing nesting habitat in the southern foothills also is a conservation priority.  The annual dynamics of nesting colony locations poses challenges for traditional acquisition-based conservation strategies and reinforces the need to work actively and sensitively with private landowners in the region.