Late pleistocene passerine birds from Sonora, Mexico

Abstract

Songbirds (Passeriformes) have a very limited fossil record in spite of making up more than one-half of the world's 10,000 living species of birds. From the late Pleistocene fossil site of Térapa in east-central Sonora, Mexico, the identifiable fossils of songbirds consist exclusively of species of Icteridae. The seven extant species (Red-winged Blackbird Agelaius phoeniceus, Yellow-headed Blackbird Xanthocephalus xanthocephalus, Brewer's Blackbird Euphagus cyanocephalus, Great-tailed Grackle Quiscalus mexicanus, Bronzed Cowbird Molothrus aeneus, Brown-headed Cowbird M. ater, and Orchard Oriole Icterus spurius) still occur in Sonora, either as residents or migrants. The eighth fossil icterid from Térapa is an extinct genus and species of cowbird, Pandanaris convexa. The genus Pandanaris was known formerly only from late Pleistocene sites in southern California ( P. convexa A. H. Miller) and Florida ( P. floridana Brodkorb). We synonymize P. floridana with P. convexa because their supposed differences either cannot be substantiated or can be attributed to sexual dimorphism in size within a single species. Among the few Pleistocene sites from Mexico or the United States with well-studied and substantial assemblages of fossil passerines, Térapa is unique in its dominance of icterids (which may be common elsewhere but not dominant) and in its absence of corvids. Today, tropical “foothills thornscrub” is the vegetation type at Térapa except for riparian forest along the río Moctezuma. The icterid fossils are compatible with previous evidence (from plant, invertebrate, and other vertebrate fossils) that tropical/subtropical marsh, savanna, and riparian forest existed at Térapa during the late Pleistocene. Just as the two extant species of cowbirds associate today with large grazing mammals, the extinct Pandanaris convexa may have foraged in association with Pleistocene mammals whose extinction led to the demise of P. convexa as well.