Original description: Passerculus sanctorum Ridgway 1883
Taxonomic history in AOU/AOS Check-list
AOU 3 (1910): San Benito Sparrow, Passerculus rostratus sanctorum
AOU 5 (1957): Savannah Sparrow, Passerculus sandwichensis sanctorum
AOU 6 (1983): Savannah Sparrow, Passerculus sandwichensis [rostratus group]
AOU 7 (1998): Savannah Sparrow, Passerculus sandwichensis [rostratus group]
IUCN Conservation Status: near-threatened
Because of its extremely restricted geographic range, totaling approximately 1,300 acres, and its necessarily small population, the San Benito Sparrow is listed as threatened in Mexico. In a study of 251 nests from all three islands, eight were destroyed by gulls or Common Ravens, making natural predation an important cause of nest failure. The islands have no native mammals, but in 2006, a peromyscus mouse was inadvertently introduced to San Benito Oeste; there is no evidence that the mice prey on the eggs or young of the San Benito Sparrow, but if their population increases, they might come into competition with the birds for seeds and other food.
Habitat: The San Benitos, described by one visiting ornithologist as “a vast heap of broken lava, pumice, tufa, ashes and obsidian,” with “but little vegetation,” are home to nine endemic plants, two endemic beetles, and three other endemic birds. In the absence of salt marsh or anything at all like it, the sparrows place their nests in low bushes, especially Lycium, or even in the abandoned burrows of the islands’ breeding alcids and storm-petrels.
Behavior: San Benito Sparrows forage in low vegetation and on the ground, often in the open on rocky and sandy surfaces, where they walk and run on strong gray-brown legs and feet. Like many birds resident on island without mammalian predators, they can be trusting of humans; in the village on one of the islands, San Benito Sparrows “readily interact with human inhabitants … accepting food scraps from them.”
Voice: The vocalizations of this species appear to be undescribed in any detail. Presumably, the calls are thin and short, as in other Passerculus sparrows, and the males probably have a buzzing song delivered from a low perch.
Detailed description and measurements drawn from standard reference works
Adult: Proportionally short, narrow, noticeably notched tail edged narrowly white in fresh plumage. Upper tail coverts, rump, and back medium-pale grayish brown, the feathers with diffuse dark brown centers and low-contrast gray edges, producing obvious but not conspicuous pattern of streaks. Primaries dull gray-brown, secondaries slightly warmer brown. Tertials with black centers and broad pale brown edges. Greater and median coverts warm brown with large black teardrops. Nape brown-gray with fine dark brown streaking. Underparts dull white, tinged brown on flanks and breast sides. Breast and flank feathers with rather narrow, well-defined brown shaft streaks, creating irregular, widely spaced streaks on underparts. Throat white, separated from off-white jaw stripe by narrow, often incomplete or speckled brown lateral throat stripe. Crown dull brown with fine black streaks; usually no discernible hint of central crown stripe. Ear coverts brown-gray, bordered below by darker brown whisker and above by brown eye line, neither strongly contrasting. Supercilium paler grayish white, with no bright yellow above lore. Long, straight-edged bill swollen at base; mostly pink-brown below, mostly dark above. Thick, short tarsi gray-brown with faint pink tinge.
Juvenile: Probably buffier above and below, with less well organized streaking.
Length 125-129 mm (4.9-5.1 inches)
Wing chord 67-71 mm (2.6-2.8 inches)
Tail 49-51 mm (1.9-2.0 inches)
W:T 1.39
