BACKGROUND:
First fall female Blackburnian warblers are the dullest of all plumages of that species. Warbler ID resources mention how that plumage can be confused in the field with fall female and first fall male Cerulean warblers. I thus felt it instructive to construct this collage on that very issue. Dunn in his Warbler Peterson Field Guide mentions that many late season Ceruleans may be in fact mis-identified first fall female Blackburnian warblers. Also while the Cerulean warbler is a very early fall migrant, it is also possible to run into first fall female Blackburnian warblers during their entire migration.
THE PROBLEM:
1 illustrates the problem with limited views in the field which are compromised by distance, obstruction, and lighting much of the time. Is 1 a first fall female Blackburnian warbler or a fall Cerulean warbler? They both have paired wing bars, a distinct light stripe above the eye (supercilium) and a darker auricular area, and may both have a lemon yellow or ochre type wash on the throat and upper chest areas, and some darker side striping.
The view in 1 alone is therefore not diagnostic to species. It is either a first fall female Blackburnian, or a fall female or first fall male Cerulean. Either 2 or 3 is another view of 1 and both are diagnostic to species. One of them is a Cerulean, and the other a Blackburnian.
THE DIFFERENCES:
I believe the easiest way to separate the two is a look at the length of the undertail that protrudes past the undertail coverts, or said another way the length of the tail beyond the meaty part of the bird. There is a marked difference in this characteristic between 2 and 3 above. 2 with a very shot undertail extension is a Cerulean, and 3 is the Blackburnian with the much longer undertail extension. Since many views are distant, and from well below the canopy this characteristic is the easiest to see in the field. Even a poor photograph can be useful when blown up.
Above 1 and 3 are the same bird.
In fall both species can be seen foraging quite low sometimes allowing decent views of the dorsal aspect. When the dorsal aspect (top or backside of the bird) is seen well the species have marked differences. in 4 you can see the pale central forehead spot on the Blackburnian, and the pale pair of longitudinal stripes on the back. Ceruleans will have neither of these dorsal findings as per 5 which shows a fall female and first fall male side by side. The male is the brighter/bluer bird in 5 and it has some evidence of dark streaking on the back, and dark streaking on the lateral crown. Finally compare the length of the undertail between the fall Cerulean female in 6 and the first fall female Blackburnian in 3.
WHAT YOU CAN'T RELY ON:
Some warbler sources mention the 'rounding off' of the auricular as a way to differentiate the two. This is helpful only if clearly present as many first fall female Blackburnians may not show this in the field. The bright lemon yellow color of the throat and chest on some Ceruleans can also not be used to identify a bird as a Blackburnian as shown in 2. Finally given close looks and good lighting it is fairly easy to separate these species based on coloring alone. Ceruleans are either green, or shades of blue dorsally, and first fall female Blackburnians are shades of brown
and whitish. Side streaking is generally sharper and browner in first fall female Blackburnians. However with many situations such as 1 and 2 it is difficult to discern true colors.
Tags: Warbler Geek Series first fall female Blackburnian warbler fall female and first fall male Cerulean warbler kennesaw mountain
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