Smaller than similar Black-headed Gull, this species has an oddly shaped blunt, all black bill as seen here. This uncommon, poorly known species is listed as threatened by IUCN. It is a scarce winter visitor to Japan. In China, loss of Common Seepweed essential for breeding is the key threat. Some of this loss was caused by introduced Eastern Smooth Cordgrass which turns mudflat into salt-marsh while making seepweed grow too thick for nesting Saunder's Gulls. The same cordgrass was introduced into San Francisco Bay where an expensive, ill-advised and poorly implemented effort to eradicate it resulted in a catastrophic 50% decline of endangered Ridgway's Rails. Canon PowerShot SX60.
References:
BirdLife International 2018. Saundersilarus saundersi. The IUCN Red List of Threatened Species 2018: e.T22694436A132551327. http://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T22694436A132551327.en. Downloaded on 09 May 2019.
Burger, J., Gochfeld, M., de Juana, E., Garcia, E.F.J. & Kirwan, G.M. (2019). Saunders's Gull (Saundersilarus saundersi). 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/53998 on 10 May 2019).
Morlan, J. 2017. California Ridgway’s Rails Imperiled by Spartina Removal. Sequoia Audubon Society Bulletin 68(1):1. (PDF)