Saturday, October 31, 2015

Yellow-throated Warbler

The Yellow-throated Warbler (Setophaga dominica) is an annual vagrant to Nova Scotia in small numbers. Most are reported in autumn, mainly during October (McLaren, 2012). One to four have been reported during fall migration in the last five years. This warbler breeds in the eastern US; the population nearest to our province is in Delaware, however its breeding range is expanding northward (Alderfer, 2014).

On the morning of Oct 25, 2015, Paul Gould and I found a Yellow-throated Warbler in Arcadia, Yarmouth County. It was only my second sighting of this species, the first being a yard bird during the fallout of April 2012.

Figure 1. Yellow-throated Warbler in Arcadia, Yarmouth County, Oct 25, 2015. Photo by Alix d'Entremont.

McLaren (2012) describes two subspecies that can be expected in Nova Scotia; the western S.d. albilora and the eastern S.d. dominica. Alderfer (2014), Pyle (1997) and Sibley (2014) all describe the geographic variation in this species. Alderfer (2014) lists three subspecies, the two already mentioned above and an additional S.d. stoddardi that is found nesting in the Florida panhandle and an adjacent portion of Alabama.

Recent research (McKay 2008, 2009) suggests recent expansion of this species and shows that the morphological differences between the subspecies are clinal. Sibley, in his blog post regarding identifiable subspecies, explains that Coastal (eastern) and Interior (western) forms differ slightly in plumage and bill length, but extensive variation makes identification of many birds uncertain. Some believe that this species should be treated as monotypic (Alderfer, 2014). The continued inclusion of subspecific field marks in recent publications (Alderfer 2014, Sibley 2014) indicate that, even if morphological differences are clinal, it is still useful to investigate these varying features of the Yellow-throated Warbler. These differing characteristics might still provide information on the likely origin of a vagrant bird, but possibly only for individuals on the extreme ends of the continuum. It is still interesting to study the features of vagrant Yellow-throated Warblers that arrive to our province. Patterns of occurance will be statistically stronger when backed by more information. For the remainder of this article, I will use the terms western birds for albilora and eastern birds for dominica - this will better represent recent understanding of the variation within this species as a cline.

Western birds show white above the lores (rarely yellow) and often a small white spot at the base of the chin. Eastern birds show yellow above the lores (rarely white) and rarely shows a small patch of white on the chin (Alderfer, 2014). Bill length increases from west to east, although birds breeding along the Gulf Coast and on the Delmarva Peninsula have markedly longer bills (McKay et al., 2012).

This bird shows a patch of white feathers on the chin (Fig. 2), especially when viewed from below. The supraloral area seems to show a very slight hint of yellowish, but is predominantly white. Bill length is difficult to assess without measurements in the hand, but this bird appears somewhat short billed (see Fig. 3 as well).

Figure 2. Yellow-throated Warbler in Arcadia, Yarmouth County, October 25, 2015. Photo by Alix d'Entremont.

Figure 3 compares this bird to one with slightly more yellow in the supraloral area and another with still more yellow. All three of these birds show some amount of white on the very upper chin.

Figure 3. Yellow-throated Warblers. Photos by Alix d'Entremont (left and middle) and Ronnie d'Entremont (right).

Individuals breeding in the western portion of the range show more white in the outer tail feathers (McKay et al., 2012). Dunn and Garrett (1997) state that the white on the outer rectrices of western birds appears to meet the white undertail coverts. Figure 4 seems to show extensive white under the rectrices - similar to that of a western bird.

Figure 4. Yellow-throated Warbler in Arcadia, Yarmouth County, October 25, 2015. Photo by Alix d'Entremont. 

McKay (2008) concluded that there was a strong west-to-east clinal change in bill length and proportion of yellow in the lores. He goes on the say that a discriminant function analysis failed to correctly assign most individuals, especially those collected near the subspecies border. The analysis did however correctly group individuals from the extreme east or west part of the range into subspecies. This provides evidence that in actuality only the extreme individuals in the continuum can be confidently assigned to a geographic area.

This Yellow-throated Warbler shows mostly white above the lores, only slight white on the chin, what looks like a relatively small bill and extensive white on the underside of the outer tail feathers. Due to the lack of a strong delineation between currently accepted subspecies, I believe that we can assume that this bird isn't from the far east, but might be from the intermediate zone. A quick Google Image search produces photos of birds with an entirely white supraloral area and a wide white patch on the upper chin. These are likely a good representation of the appearance of the birds from the far west.

It will be interesting to see if the recommendation of McKay (2008) to eliminate the subpecies S.d. albilora and S.d. stoddardi will widely be accepted by the birding and ornithological communities.


Alderfer, J., J.L. Dunn. 2014. (Ed). Complete Birds of North America, 2nd Edition. National Geographic Society. Washington DC, USA.

Dunn, J. and K. Garrett. 1997. A Field Guide to Warblers of North America. Peterson Field Guide
Series. Houghton Mifflin Co., New York.

McKay, B. D. 2008. Phenotypic variation is clinal in the Yellow-throated Warbler. Condor 110(3):569-574.

McKay, B. D. 2009. Evolutionary history suggests rapid differentiation in the Yellow-throated Warbler Dendroica dominica. Journal of Avian Biology 40(2):181-190.

McKay, Bailey and George A. Hall. 2012. Yellow-throated Warbler (Setophaga dominica), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:

McLaren, I.A. 2012. All the Birds of Nova Scotia: status & critical identification. Gaspereau Press Ltd, Kentville, N.S., Canada

Pyle, P., S.N.G. Howell, R.P. Yunick, and D.F. Desante. 1997. Identification guide to North American Birds, Part 1, Columbidae to Ploceidae. Slate Creek Press, Bolinas, California.

Sibley, D.A. 2014. The Sibley Guide to Birds 2nd Ed. Alfred A. Knopf, New York, N.Y.

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