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In the San Francisco Bay area, California, the Double-crested Cormorant Phalacrocorax auritus population has recovered from significant declines to reach breeding population sizes comparable to those from the late 19th century, when only one colony offshore at the South Farallon Islands (SFI) was known. The recent replacement of the bridge hosting one of the current largest colonies prompted a comprehensive assessment of Bay Area breeding population trends through 2017. Since the early 1970s, the Bay Area population has expanded from < 50 pairs at one site, SFI, to nearly 3 500 pairs at > 20 colonies, with breeding documented at 31 different locations. However, missing counts at many colonies before 2003 prevented calculation of precise, long-term growth rates. Expansion has been facilitated by cormorant adaptations to the urbanized estuary, including nesting on bridges, electrical towers, non-native trees, and managed pond levees. Breeding colonies that formed by 1984 on the San Francisco-Oakland Bay Bridge (SFOBB) and Richmond-San Rafael Bridge (RSRB) grew quickly, and in several different years totaled more than 1 000 nests. From 2003-2017, when construction of a new east span of the SFOBB (and demolition of the old span) was underway and when substantial maintenance of the RSRB occurred, the colonies on the bridges declined by 71 % and the overall Bay Area population declined by 39 %. The decline was likely due to reduced prey availability, although construction disturbance may have driven some birds from the bridges to colonies outside the region. On the Outer Coast, the colony at Hog Island was formed in 2001 and has become the largest in the study area since 2011. Nesting on artificial platforms installed on the new SFOBB east span in 2017 occurred only after demolition of the old span was complete, despite social attractions being in place since 2011.
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RAUZON, M.J., ELLIOTT, M.L., CAPITOLO, P.J., TARJAN, L.M., McCHESNEY, G.J., KELLY, J.P. & CARTER, H.R. 2019. Changes in abundance and distribution of nesting Double-crested Cormorants Phalacrocorax auritus in the San Francisco Bay area, 1975-2017. Marine Ornithology 47: 127-138.