Warbler Guy, which warbler species are the most likely to mate together to produce hybrid (intergrade) individuals?

Whoa, Jennifer (in Houston): This is not an easy question.

And the answer requires me to put on my professor's cap.

So here goes: 

 Brewster's Warbler (right, above)

Lawrence's Warbler (right, above)

In general, Golden-winged X Blue-winged Warbler intergrades and Hermit X Townsend intergrades are two cases worth mentioning.

In the former, genetic shuffling may result in individuals called Brewster’s Warbler that express field marks from both the Golden-winged and Blue-winged. These hybrids show yellow underparts and a dark throat and head area behind the eye. In most cases, a Brewster’s results from the pairing of a “pure” Golden-winged and “pure” Blue-winged, though, in some cases, a Brewster’s is produced from the crossing of a Golden-winged and Brewster’s.

The Lawrence’s Warbler as a hybrid is less likely when a Golden-winged and Blue-winged mate. That’s because genetic traits involving an understanding of dominant and recessive traits come into play. 

For example, the white underparts of the Golden-winged are considered a dominant trait. So is the reduced head pattern of the Blue-winged. When these two dominant traits are seen in an individual, we call it a Brewster’s expressing the two dominant traits from its Golden-winged/Blue-winged parents.

Recessive traits include the yellow color of the Blue-winged’s underparts and the bold head pattern (throat and head area) of the Golden-winged. Newborns produced from these two types of warblers result in a Lawrence’s. Lawrence’s may also occur from the pairing of two Golden-wingeds expressing recessive genes as their underpart colors or, more rarely, the mating of two Brewster’s Warblers.

Got it? If you’re still with me and understand this explanation, then consider yourself a genius and an ardent warbler fan.

Other warbler species that often create hybrids are crosses between Hermit and Townsend’s Warblers, especially in Washington state. Here the breeding ranges of these two species overlap. In many cases, hybrids between Hermit and Townsend’s express the face pattern of Hermit and the underpart colors of Townsend’s, with the back of hybrid individuals more green than a pure Hermit. In addition, the black on the rear crown of male hybrids extends farther forward than on a pure Townsend’s.