Date: 6/25/18 3:46 pm From: Kimball Garrett <kgarrett...> [LACoBirds] <LACoBirds-noreply...> Subject: [LACoBirds] Big Rock Creek Cassin's/Plumbeous Vireos
Maybe it's just me, but I seems that Cassin's vs. Plumbeous Vireo identification is an underappreciated issue, particularly in late spring and summer when Cassin's are at their dullest state of plumage wear. Both species have been found in summer along Big Rock Creek on the north side of the San Gabriel Mtns., though Cassin's (albeit being somewhat uncommon and local) is the species found in most of the San Gabriels. On Sunday 24 June I had a grayish individual a short ways above Big Rock Campground. It did a lot of singing, but although I'm aware of some average differences, I can't confidently distinguish the songs of the two species. The bird looked gray and white to me in the field, with a touch of pale yellow-olive on the flanks, but my (poor) photo seemed to reveal more yellowish than I thought had been present. Of course it's hard to know how much of that color is actual pigment and how much represents reflection of green leaves. And my impression that the back and wings were gray and lacked any olive color might have been based on less than ideal viewing conditions.
So my plea is for anybody birding that area to try to pay special attention to any "solitary" vireos present. Some characters to pay attention to include:
Bill size: averages larger in Plumbeous than Cassin's
Sides of the breast, upper sides: Always gray in Plumbeous; some olive here in virtually all Cassin's
Cheek/throat contrast: Averages sharper in Plumbeous
Rear sides, flanks, vent: Any yellowish-olive color should be very limited in Plumbeous (especially in worn birds in summer?), more extensive in Cassin's. But the key here is ascertaining the real color, not apparent color resulting from reflection of green foliage
Back color: Gray with scarcely any hint of olive in Plumbeous, always at least a hint of olive (and usually a strong olive wash) in Cassin's. Seems simple, but getting good dorsal views of a vireo in good lighting is challenging.
Song: Good voice recordings are always helpful
Some additional notes about this vireo are in my eBird checklist for Big Rock Creek here: https://ebird.org/view/checklist/S46780279 The exact coordinates where the vireo was present were 34° 22' 56.53" N 117° 46' 19.18" W
I don't know if the road leading uphill from Big Rock Campground was open all the way to the Angeles Crest Hwy. at Vincent Gap, but it was fine for at least a ways beyond the campground. There is beautiful mixed pine/oak habitat along this road, looking very "Arizona-like" in some respects (whether or not that translates into Arizona-like birds....).