Black-throated Green Warbler (Dendroica virens), Gilbert Water Ranch, Maricopa County

This Black-throated Green Warbler was present at the Gilbert Water Ranch from 23-24 October.  Prior to 1990 there were about 30 records for Arizona, but this represents maybe only the 4th documented record for the state since 1990 (see the photos of the bird on this site from Boyce Thompson Arboretum in Nov 2006 and the Arizona Bird Records Committee Reports elsewhere on this site for further details).  This pattern of decreased occurrence since 1990 seems to fit several other northerly breeding eastern warblers that are much less common as vagrants to the state than previously. 
 
Identification of Black-throated Green warbler in this plumage is fairly straightforward.  It is a bird with a bright yellow green face, darker ear patch, bright green unstreaked back, prominent white wingbars and mostly grayish white underparts with blackish streaks down the side and with at most only a faint yellow wash below.  The two most likely confusing species are Townsend’s and Hermit Warbler.  Hermit usually has plainer face, grayer back and lacks streaking on the sides.  Towsend’s has a more contrasting face, often streaking on the back and usually a much more yellow upper breast.  Either a very dull immature Townsend’s or a Hermit X Townsend’s Hybrid are likely to cause the most confusion.  In these and all cases the wash of yellow across the vent just behind the legs is diagnostic for Black-throated Green.
 
This bird was first identified and reported by Michael Moore on 24 Oct, but had been luckily photographed the day before by Harold Bond as well as later on 24 Oct by John Yerger. 

24 October 2007, Photo by John Yerger