Seven species of geographically variable birds have been reported as part of the resident avifauna of San Esteban Island in the Gulf of California, Sonora. Two of these, the Curvebilled Thrasher and the Blue-gray Gnatcatcher, may not actually have breeding populations there, at least at the present time. Of the seven, only two, the Verdin and the doubtfully resident thrasher, can be used in a determination of the relationships of the avifauna, and both show affinities to that of Sonora. The Ash-throated Flycatcher and the House Finch are represented by the same form on San Esteban and in both Sonora and Baja California at the latitude of the island. Neither the Ladder-backed Woodpecker nor the endemic Black-throated Sparrow shows greater similarity to the Baja California or Sonoran avifauna. The slight balance of evidence indicates that the avifauna of San Esteban Island is best considered with that of Sonora.
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Relationships of the avifauna of San Esteban Island, Sonora