Black-throated Green Warbler

On Saturday, 15 May 2015, I banded a gorgeous Black-throated Green Warbler in Northfield, Rice Co., Minnesota. At the time, I noticed the black post-auricular spot.  I also was aware of faint black streaks on the bird’s back. After researching the afore-mentioned spot, I slowly concluded that this warbler is probably a hybrid between a Black-throated  Green and Townsend’s warbler. Black-throated Green and Townsend’s warblers hybridize in the limited area where their ranges overlap in central British Columbia (Toews et al. 2011).

Here is why I thought this bird was odd:

1) The black post-auricular spot started my unease with my initial identification. Most field guides don’t show this mark. A few photos on the Web of birds identified as Black-throated Green Warblers do show similar dark spots. The stripes on the sides of this bird’s face, however, are darker than expected on a typical Black-throated Green Warbler.

2) In the field, I noticed the faint black streaks on this bird’s back. A Black-throated Green Warbler’s back should be completely unstreaked. Male Townsend’s Warbler’s have dark-streaked backs.

3) After reaching the conclusion that my bird might be a hybrid, I returned to my photographs and found the photo below. Although a pure Townsend’s Warbler should have a black crown, unlike the green one on a Back-throated Green, notice the black feathers along the upper edge of this bird’s yellow supercilliary stripe.

I should have stopped my research there. But, instead, I continued on to Stephenson and Whittle’s definitive book, The Warbler Guide. There I found photos of Black-throated Green Warblers showing all the field marks I have discussed. They write that the back has little or no streaking (the emphasis is mine). Several photos show black post-auricular spots, and a couple of them show black-lined, yellow supercilliary stripes. Well, “easy come, easy go” is all I have to say.  I just had hybrids on the brain after my apparent hybrid between an Audubon’s and Myrtle warbler.