The Bay Miwok are a cultural and linguistic group of Miwok, a Native American people in Northern California who live in Contra Costa County. They joined the Franciscan mission system during the early nineteenth century, suffered a devastating population decline, and lost their language as they intermarried with other native California ethnic groups and learned the Spanish language.
The Bay Miwok were not recognized by modern anthropologists or linguists until the mid-twentieth century. In fact, Alfred L. Kroeber, father of California anthropology, who knew of one of their constituent local groups, the Saklan (Saclan), from nineteenth-century manuscript sources, presumed that they spoke an Ohlone (a.k.a. Costanoan) language.
In 1955 linguist Madison Beeler recognized an 1821 vocabulary taken from a Saclan man at Mission San Francisco as representative of a Miwok language. The language was named "Bay Miwok" and its territorial extent was rediscovered during the 1960s (see Landholding Groups or Local Tribes section below).
The original Bay Miwok people's world view was a form of Shamanism. As they were centrally located along an arc of Miwok-speaking groups across Central California, the Bay Miwok probably shared the Kuksu religion ceremonial motifs common to both the Coast Miwok to the west and Plains Miwok to the east. The Kuksu religion (dubbed the Kuksu Cult by early historians) included a cycle of elaborate dancing ceremonies, each with its own group of actors and distinctive feather-decorated regalia, an all-male society that met in subterranean dance rooms, puberty rites of passage, shamanic intervention with the spirit world, and, in some areas, an annual mourning ceremony. Varying forms of the Kuksu Cult were shared with other indigenous ethnic groups of Central California, such as their neighbors the northern Ohlone, Maidu, Patwin, Pomo, and Wappo. However Kroeber observed less "specialized cosmogony" in the Miwok, which he termed one of the "southern Kuksu-dancing groups", in comparison to the Maidu and other northern California tribes.
The specific myths, legends, tales, and histories of the Bay Miwok are not well documented. C. Hart Merriam published a creation story, The Birth of Wek-Wek and the Creation of Man, centered on Mt. Diablo, that was told by a Hool-poom'-ne Miwok, perhaps a descendent of the Julpun Bay Miwok of Marsh Creek, eastern Contra Costa County.
One might suspect that the full corpus of Bay Miwok mythology and sacred narrative shared the motifs that the linguistically related and better-documented ethnographic Coast Miwok and Sierra Miwok held in common. All Miwok peoples believed in animal and human spirits, and saw the animal spirits as their ancestors. Coyote was seen as the representation of their creator god. The Sierra and Plains Miwok, as well as the Bay Miwok, believed this world began at Mount Diablo, following a flood.
The names and general territorial areas of seven Bay Miwok-speaking land-holding groups have been inferred through indirect methods, based for the most part on information in the ecclesiastical records of missions San Francisco and San Jose. In a 1961 Ph.D. dissertation, James Bennyhoff used data from the Alphonse Pinart transcripts of the mission records to identify four more East Bay local territorial groups, in addition to the Saclan, as members of this unique Miwok language group. "The major clues to the linguistic affiliation of these river mouth tribelets are provided by the personal names of female neophytes recorded in the baptismal registers ... Ompin, Chupcan, Julpun, and Wolwon [Volvon-ed.] are linked together by the use of a distinctive constellation of endings which appear in female personal names," he wrote. Milliken subsequently used the same technique, applied to the original mission records, to identify two additional local tribes—Jalquin and Tatcan—as Bay Miwok speakers. Milliken then inferred and mapped the relative locations of all seven groups, using clues from historic diaries together with mission register information regarding intermarriage patterns among East Bay local tribes. The locations of the seven Bay Miwok local tribes are generally as follows:
Another group, the Yrgin of present-day City of Hayward and Castro Valley, had Chochenyo Ohlone signature female name endings, rather than Bay Miwok name endings. Yet they were so highly intermarried with the Jalquin that it seems possible that they and the Jalquin formed a single bilingual local tribe.
Documentation of Miwok peoples dates back as early as 1579 by a priest on a ship under the command of Francis Drake. Identification and references to the Bay Miwok tribes exists from California Mission records as early as 1794.
Spanish-American Franciscans set up Catholic missions in the Bay Area in the 1770s, but did not reach the Bay Miwok territory until 1794. Beginning in 1794, the Bay Miwoks began to migrate to the Franciscan missions, most to Mission San Francisco de Asís (of San Francisco), but some others to Mission San José (in present-day Fremont). All but the Ompin and Julpun in the northeast were at the missions by the end of 1806; the latter two groups moved to Mission San José during the 1810-1812 period. The first baptisms and emigration to the missions of each tribe were:
Missionary linguist Felipe Arroyo de la Cuesta obtained the only extant Bay Miwok vocabulary during a visit to Mission San Francisco in 1821.
Estimates for the precontact populations of most native groups in California have varied substantially. (See Population of Native California.) Alfred L. Kroeber  put the 1770 population of the Plains and Sierra Miwok (but excluding the Bay Miwok, about whom he was not aware) at 9,000. Sherburne Cook carried out a more specific analysis of contact-period population in Alameda and Contra Costa counties west of the San Joaquin Valley, without regard to the Ohlone-Bay Miwok language boundary; he suggested a total population of 2,248. Richard Levy estimated 19,500 people for all five Eastern Miwok groups as a whole (Bay, Plains, Northern Sierra, Central Sierra, and Southern Sierra) prior to Spanish contact, and 1,700 specifically for the Bay Miwok.
A total of 859 Bay Miwok speakers were baptized at the Franciscan missions (479 at Mission San Francisco and 380 at Mission San Jose), most between 1794 and 1812. By the end of 1823, only 52 of the Mission San Francisco Bay Miwoks were still alive, along with 11 of their Mission-born children. No comparable data are available for Mission San Jose that year, but by 1840 only 20 Bay Miwok people were alive there. Late nineteenth century survivors from both missions intermarried with people from other language groups. Descendants are alive today (see Present Day section below).
Some descendents of the Bay Miwok from the Mission San Francisco and Mission San Jose, are members of the Muwekma Ohlone Tribe of the San Francisco Bay Area. (Mission records have assisted in substantiating native genealogical persistence.) The Muwekma Ohlone Tribe states that : "all of the known surviving Native American lineages aboriginal to the San Francisco Bay region who trace their ancestry through the Missions Dolores, Santa Clara and San Jose" and who descend from members of the historic Federally Recognized Verona Band of Alameda County.