An interesting phenomenon is happening in the abandoned farm fields of Hastings County; the hybridization of two colorful North American Wood-warblers.
Area of Observation
In the spring and early summer of 2015 I made several trips around southern Hastings County to locate and count hybrid Warblers known as Brewster's and Lawrence's. On the direction of fellow birdwatchers, I visited two small known areas of hybridization in southern Hastings county. The first area is west of the village of Ivanhoe and the second area is north of the community of Read. When viewed on a larger map, these two areas form part of a line across North America, known as the “Hybrid Line”, which is slowly moving northwards as the breeding range of the Blue-winged Warbler expands and overtakes that of the Golden-winged.
The Hybrids
Brewster's is the more commonly encountered hybrid of the two, as it inherits the dominant characteristics. Lawrence's inherits the more recessive characteristics.
As most hybrids show slight variations in plumage, and as I encountered some individuals more than once, I gave them very complicated scientific names, those of characters from the children's television program Sesame Street.
Elmo (photo 1) is a typical Brewster's Warbler, showing a black eye line and mostly white breast and belly with a varying degree of yellowish tinge.
Photo #1
https://www.flickr.com/photos/94355719@N02/17832894609/in/photostream/
Oscar the Grouch (photo 2) is another Brewster's, but his face shows an unusual partial mask.
Photo #2
https://www.flickr.com/photos/94355719@N02/18279769406/in/photostream/
Big Bird (photo 3) has the recessive black throat and mask of a Lawrence's Warbler, and he is perhaps a variation of a Lawrence's Warbler, as his wing bars show mostly yellow rather than white.
Photo #3
https://www.flickr.com/photos/94355719@N02/18808204250/
Songs
From mid-May to late June the males declare their territories to females by singing from exposed perches, making detection much easier than outside the breeding season.
GWWA and BWWA have distinct buzzing songs. Hybrids sing either of these songs, or sometimes a clumsy combination of both. Some hybrids sing with a “didididi-zee”, much like the secondary songs which GWWA males sing later in the breeding season. I did observe at least one GWWA and one BWWA alternating between both song types.
Any Golden-winged or Blue-winged song heard in these areas can not reliably be assigned to species.
The Numbers
In my limited time I counted a total of 8 Brewster's (7 males, 1 female) and 1 Lawrence's.
When comparing these single year sightings in Hastings County to all documented sightings of the Kingston region prior to 1988, (an area to the east of Hastings County), the numbers are almost equal; 9 Brewster's and 2 Lawrence's.
The hybrids seem to be in localized pockets in Hastings County which were probably chosen by their BWWA parents several generations before. In other areas of Hastings County the GWWA population appears to be free of hybrids and their BWWA parents, notably in the north and north east of the hybrid areas.
When viewed on ebird.org's species map across Ontario, the “Hybrid Line” takes roughly the same line as the Canadian Shield. Perhaps in a way “shielding” the remaining GWWA from BWWA encroachment.
References
Weir, Ron The Birds of the Kingston Region, 1988
eBird.org