Comparing our Blue-winged, Golden-winged and Brewster’s Warblers (Warning: Genetics Included!)

This season we have banded several Blue-winged, Golden-winged and Brewster’s warblers.  Blue-winged and Golden-winged warblers are considered to be separate species, and they inter-breed to produce two hybrids – Brewster’s and Lawrence’s.  Interestingly, despite the fact that these hybrids are usually fertile, Brewster’s and Lawrence’s warblers do not qualify as unique species.  Conversely, it has been suggested that Blue-winged and Golden-winged Warblers are actually the same species, but DNA evidence argues against this hypothesis.

A male Blue-winged Warbler

A male Golden-winged Warbler.

Brewster’s and Lawrence’s warblers can vary extensively in appearance.  Brewster’s Warblers typically have the thin eyestripe of their Blue-winged parent, but they often have the white belly and gray back of their Golden-winged parent.  They may show the white wing bars of their Blue-winged parent or the yellow wing bars of their Golden-winged parent.  Lawrence’s Warblers typically have the larger mask of their Golden-winged parent but the bright yellow bellies of their Blue-winged parent.

We can explain the differences by employing Mendelian genetics; what follows is somewhat simplified.  We can think of a single gene controlling body color; in this case, white is dominant (WW) and yellow is recessive (ww). Apparently, a single gene actually does control the presence of a throat patch: an absent patch is dominant (PP) and an existing black patch is recessive (pp).  “Pure” (homozygous) Blue-winged Warblers are therefore wwPP, while “pure” Golden-winged Warblers are WWpp.  That is, we can think of them as each exhibiting one dominant trait.

A first generation Brewster’s Warbler is a result of a pure Blue-winged Warbler and a pure Golden-winged Warbler, and has the genotype WwPp.  Therefore, it exhibits both dominant traits:  a white body color and no throat patch.

If two first-generation Brewster’s Warblers cross (and hypothetically produce a perfectly distributed brood with 16 chicks in all 9 possible genetic combinations!), the result should be:

• 9 offspring exhibiting both dominant traits (i.e. Brewster’s Warblers, although they would come in 4 different genetic combinations:  WWPP, WWPp, WwPP, and WwPp)
• 3 offspring exhibiting the dominant throat patch trait (i.e. Blue-winged Warblers, in two different genetic combinations:  wwPP and wwPp)
• 3 offspring exhibiting the dominant body color trait (i.e. Golden-winged Warblers, in two different genetic combinations:  WWpp and Wwpp)
• 1 offspring exhibiting neither dominant trait (i.e. Lawrence’s Warbler:  wwpp)

When a first generation Brewster’s Warbler back-crosses with a pure Blue-winged Warbler, 50% of the offspring will be Brewster’s, 25% will be pure Blue-winged Warblers, and 25% will be “impure” (heterozygous) Blue-winged Warblers.  Similar comments apply to a first-generation Brewster’s crossing with a pure Golden-winged Warbler.  Furthermore, “impure” Blue and Golden-winged can cross, as well as back-cross with their hybrids.  Given the two traits being discussed, however, there are only the nine possible genetic outcomes listed above.  Of course . . . in reality there are more than two traits for plumage coloration, yielding many more possible combinations!

The above explains why Brewster’s are more common than Lawrence’s Warblers:  the latter requires the recessive version of both genes, and that will occur less frequently.  Furthermore, notice that no Lawrence’s Warbler will have a “pure” Blue-winged or Golden-winged parent.  They are all the result of crossing between heterozygous Blue-winged and Golden-winged Warblers, Brewster’s Warblers, and other Lawrence’s Warblers.

The two Brewster’s Warblers we captured this spring are quite different in appearance:

This Brewster’s Warbler looks identical to a Blue-winged Warbler except for its yellow wing bars. Notice, however, that this bird only exhibits one of the dominant traits – the absence of a throat patch.  According to the above two-gene analysis, we ought to call this bird a Blue-winged Warbler.  We did band it as a Brewster’s however, based on the yellow wing bars and on two guidebooks.  Photo by Rick Lightbody.

This Brewster’s Warbler is “typical” in that it has no throat patch and light underparts.

For more information, check out the following:

Hybrids of Golden-winged and Blue-winged Warblers: More than Meets the Eye  (from Birding, May/June 2005)

The Genetics of the Golden-winged x Blue-winged Warbler Complex (the original Parkes article from 1951)