In this task, you're given a question, along with a context passage which has extra information available on certain terms mentioned in it. Your job is to determine which sentence(s) from the passage can be used to search for further information needed to answer the question.
Q: Question: Approximately how many California least tern remain? Passage:The San Francisco Estuary is a major stop on the Pacific flyway for migrating waterfowl. Yet little is known about the flow of carbon in or out of the estuary via birds. Millions of waterfowl annually use the bay shallows as a refuge. Most of the birds are dabbling ducks that feed on submerged aquatic vegetation. Diving ducks (such as scaups) feed on epibenthic organisms like C. amurensis, representing a possible flow of carbon from that otherwise dead end. Two endangered species of birds are found here: the California least tern and the California clapper rail. Exposed bay muds provide important feeding areas for shorebirds, but underlying layers of bay mud pose geological hazards for structures near many parts of the bay perimeter. Piscivorous birds such as cormorants and pelicans also inhabit the estuary, but their trophic impact remains poorly studied.

A:
Two endangered species of birds are found here: the California least tern and the California clapper rail