One of three birds present. Rare in Northern California especially in winter. Present since 19 January. California's first Black Skimmer was found on 8 September 1962 at the mouth of the Santa Ana River in Orange County but they soon colonized Southern California and have spread north to the San Francisco Bay Area where they are an uncommon breeder. The Black Skimmer is one of North America's most distinctive and unusual birds. The lower mandible is longer than the upper. It uses it to slice the shallow water and when it touches a fish, the upper mandible snaps shut to catch it. They are somewhat crepuscular and their eyes close down into slits like that of a cat. Among birds, I believe this is unique to skimmers. They also bark like a dog. The North American subspecies R. n. niger is more coastal than the South American races. It is characterized by having a mostly white tail with dark center, white underwing coverts and broad white tips to secondaries. In winter plumage, this species has a white hind-collar as seen here. Canon PowerShot SX60 HS.
References:
Gochfeld, M. and J. Burger (1994). Black Skimmer (Rynchops niger), version 2.0. In The Birds of North America (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.108
Lefevre, K. L. (2018). Black Skimmer (Rynchops niger), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/nb.blkski.01
Zusi, R.L. & Garcia, E.F.J. (2020). Black Skimmer (Rynchops niger). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/54056 on 5 February 2020).