Western Meadowlark (Sturnella neglecta)

Western Meadowlarks are generally ground-dwelling creatures, hanging out in the grass of the non-swampy parts of the refuge, but males will alight on higher perches in spring & summer and sing to defend their nesting territory, and this will likely be your best chance of getting a close look at one here. They're widespread in California but are found nowhere else unless you count southern Canada, the continental US from the west coast to the Great Lakes, and most of Mexico.