This small far-western subspecies is characterized by its small, unswollen bill compared to populations further east. These 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. Canon PowerShot SX60 HS.
References:
Brewer, D. (2018). Warbling Vireo (Vireo gilvus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/61263 on 18 October 2018).
Gardali, T. and G. Ballard (2000). Warbling Vireo (Vireo gilvus), version 2.0. In The Birds of North America (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.551