Hiding high in the canopy, the Warbling Vireo is more often heard than seen. Its song reminds me of a Purple Finch singing at half speed. This is the far-western subspecies V. g. swainsoni characterized by its small size and relatively unswollen bill compared to populations further east. These plain, unassuming birds have been declining in California for the last two decades. Formerly a fairly common breeder, they are especially vulnerable to brood parasitism by Brown-headed Cowbirds which have expanded into California in response to agricultural changes. Warbling Vireos are entirely migratory with most birds leaving the US for wintering grounds centered primarily in western Mexico.
