Monday, June 14, 2010

Blue-winged and Golden-winged Warbler

Today I banded a Blue-winged Warbler, a southern bird that has expanded its range since the 1800s into central Minnesota and the northeastern United States.  Ornithologists hypothesize that human cutting of eastern forests have facilitated this warbler's northward expansion--the bird likes shrubby habitat.
As Blue-winged Warblers move north, they replace or hybridize with Golden-winged Warblers.  The reasons for the displacement of Golden-wings by Blue-wings are not understood. This photo below of a Golden-winged Warbler is from Northfield last spring--I did not see one this year. Both species are also declining due to the destruction of scrubby habitat (for housing developments and farmland) in the US and to tropical deforestation in their wintering areas.
The genetics of these species' hybridization are fascinating. Golden-winged Warblers are homozygous dominant for white underparts (WW) and homozygous recessive for black throats (pp).  Blue-winged Warblers are the opposite: they are homozygous recessive for underpart color (ww), which gives them yellow underparts. Blue-winged Warblers are also homozygous dominant for throat color (PP)—this gives them plain (i.e., not black) throats.  (I hope you recall from high school or college that homozygous genes are the same—one from mom, and the other from pop—and  dominant genes usually mask recessive genes, which are only read when not masked by a dominant one.)

When Golden-winged and Blue-winged hybridize, the young get one gene from each parent, and thus carry WwPp genes.  They are heterozygous for both genes and have white underparts (because of the dominant W) and plain throats (due to the dominant P).  The recessive genes are just along for the ride. Because ornithologists did not know it was a hybrid, originally this hybrid was named Brewster’s Warbler.


Now things get complicated.  Watch what happens when two Brewster’s Warblers (WwPp birds) hybridize.  Each sperm or egg gets one of each gene, so might randomly contain WP, or Wp or wP or wp genes.  You can make a Punnet square, which shows what happens when these genes get back together when the sperm fertilizes the egg (remember that recessives gene is masked by dominant ones):


WP
Wp
wP
wp

WP

WWPP

Brewster’s Warbler

WWPp

Brewster’s Warbler

WwPp

Brewster’s Warbler

WPpP

Brewster’s Warbler

Wp

WWPp

Brewster’s Warbler

Wwpp

Golden-winged Warbler

WwPp

Brewster’s Warbler

Wwpp

Golden-winged Warbler

wP

WwPP

Brewster’s Warbler

WwPp

Brewster’s Warbler
wwPP
Blue-winged Warbler

WwPp

Blue-winged Warbler

wp

WwPp

Brewster’s Warbler

Wwpp

Golden-winged Warbler

WwPp

Blue-winged Warbler
?

So far, out of the same nest, on the average you are getting 9 Brewster’s Warblers, 3 Golden-winged Warblers, and 3 Blue-winged Warblers!  What about that 16th bird?  It’s genes are wwpp, which gives us a warbler we have yet to see.  A Lawrence’s Warbler has yellow underparts and a black throat.  Again ornithologists first thought it was a valid species.  Brewster’s Warblers are uncommon (I have only seen two in my career and I have never banded one), and only 1/16th of their young are Lawrence’s Warblers (I have never seen a Lawrence’s Warbler—I almost did once, but that is another story).

This discussion is inspired by Richard Pough’s Audubon Land Bird Guide (1949).  The genetics of these crosses is actually a bit more complicated, but this discussion will suffice as an introduction.

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