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Saturday, June 27, 2015

Dangerous Liaisons: Lawrence's Warbler and Brewster's Warbler

[Port Huron SGA. May/June 2015]

Specialty species. Target species. Endemic species. Where would Birding be without the fundamental concept of a "species"? But, what exactly is, or is not, a species? In this post we consider the status of two uncommon warblers -- Brewster's and Lawrence's -- observed recently in the avian-rich tracts of Port Huron SGA (State Game Area).

Liaisons between different species are not unknown in the Animal kingdom -- in the US, Indigo and Lazuli Buntings, Rose-breasted and Black-headed Grosbeaks, Black Ducks and Mallards, among others, will sometimes hybridize where their ranges overlap. 

According to the American Genetic Association (in an article by Richard G. Harrison and Erica L. Larson), a species can be defined as:

... populations that are diagnosably distinct, reproductively isolated, cohesive, or exclusive groups of organisms. 

We start with Golden-winged Warbler, seen here from an image taken at Magee Marsh in 2013:

Golden-winged Warbler seen at Magee Marsh
The Golden-winged Warbler has a grey back, white undersides and a distinctive facial pattern that includes a black mask and chin; the crown and wing-bars are yellow.

The Blue-winged Warbler, on the other hand:

Blue-winged Warbler seen at Port Huron SGA

... shows the same grey back, but white wing-bars, black eyeline and a yellow body. Both warblers have distinct but similar buzzy songs.

The expansion in the Blue-winged Warbler's range Northward (possibly due to rising temperatures), permeates the invisible geographic boundary between the two species and this range encroachment dilutes one of the important criteria in the definition of a species -- namely, that of "reproductively isolated" populations. Indeed, both Blue-wingeds and Golden-wingeds will interbreed and the resulting liaison does produce viable offspring.

When this happens, a species may come under the threat of genetic extinction -- indeed, this is precisely the case with the Golden-winged Warbler. This species has suffered a precipitous decline in its population and is currently classified as "Near Threatened". Concomitant with the decline in the number of Golden-winged's has been a rise in Brewster's and Lawrence's Warblers. Both of these warblers are the product of this inter-species liaison and are increasingly being reported in e-Bird:

Brewster's Warbler
Brewster's Warbler shows the back of a Blue-winged -- grey with white wing-bars -- but the golden crown and white undersides of a Golden-winged. The mask of the latter is absent; replaced instead, with the black eyeline of the Blue-winged's.

Lawrence's Warbler seen at Port Huron SGA
Lawrence's Warbler, on the other hand, shows the black facial markings of the Golden-winged, the yellow body of the Blue-winged and two wing-bars -- each of which could belong to either species.

In an excellent article, the genetics behind these hybrids are explained. And, it comes to light that the Lawrence's Warbler hybrid is the rarer of the two -- requiring the presence of recessive genes from both parents; while Brewster's Warbler are much more common.

Brewster's Warbler
Both hybrids are attractive in their own right, but the Lawrence's is truly spectacular:

Lawrence's Warbler
So, while both Brewster's and Lawrence's Warblers are special, intriguing and genetically distinct, one thing they are not, however, are species in their own right. 

1 comment:

Bob Pelkey said...

An intriguing story indeed, Hemant. It was additionally interesting to read in your link to Braddock Bay Bird Observatory that ""no Lawrence’s Warbler will have a “pure” Blue-winged or Golden-winged parent."" Your photography continues to capture nature sweetly.