Hybridization of Golden-winged Warbler-A species in trouble
· Golden-winged warbler is a species in need for conservation because it has shown a rapid decline in numbers in the last 35 years, and because of its limited range. Some figures that have been noted are a decline by 71% in the U.S. and a 94% decline in their northeastern range (Confer and Barker, 2002). If this trend continues, this species could be headed for extinction.
· Two factors are correlated with this decline.
· A loss of breeding habitat as a process called succession, converts shrub land into secondary forest, especially in the southern part of their range.
· Northward expansion and the resultant zone of overlap with the Blue-winged Warbler. When the Blue-wings come in close contact with the Golden-winged, a species closely related, the two species frequently hybridize with each other. The Blue-winged will successively replace the Golden-winged, because it can tolerate a wider range of successional habitats (Johns).
· When Blue-winged expand northward, the initial result is a population of mostly Golden-winged with a few Blue-winged and “Brewster’s” hybrids mixed in. As the number of Blue-winged and hybrids increase, eventually the population consists of Blue-winged with a small number of Golden-winged and a few “Lawrence’s” warbler. After a period of 50 years or less, Golden-winged disappear altogether with a few hybrids present. Now the population consists of all Blue-winged with an occasional “Lawrence’s” present (Dunn and Garrett, 1997).
· Brewster’s warbler is a hybrid resulting from a pairing of a Golden-winged and a Blue-winged warbler. There is a high degree of variation that exists in these hybrids reflecting the interaction between dominant and recessive genes. These interactions are thought to be classic Mendelian genetics, but there is still much to learn about the gene interactions, and it is not fully understood yet what is going on. The variable plumages of these hybrids, suggest there may be two genes involved in determining plumage variations between Blue-winged and Golden-winged warblers. By studying these unusual hybrids, it may help scientists understand the genetic basis of hybridization between these two species. Because these hybrids sing hybrid songs, scientists may be able to explore the role that heredity plays in the formation of song (Robertson, 2001). Golden-winged and Blue-winged warblers yield offspring that have dominant traits, the “Brewster’s” phenotype (heterozygous). This phenotype can also result from a second generation hybrid pairing between a Golden-winged and a Brewster’s warbler.
· The white under parts of the Golden-winged and lack of facial pattern or black eye-line of the Blue-winged are dominant traits.
· The yellow under parts of the Blue-winged and the bold head pattern, throat and cheek patch of the Golden-winged are recessive traits.
· The rarer “Lawrence’s” warbler which is the phenotype with the recessive traits, are the result of second generation backcrosses (homozygous for both gene pairs). Subsequent pairings between a Brewster’s and a Golden-winged or Blue-winged heterozygous can produce a Lawrence’s phenotype. A rare mating of two Brewster’s warblers could also produce this phenotype (a 1 in 16 chance) (Dunn & Garrett, 1997).
· As an area is taken over by Blue-winged warblers, the numbers of hybrids increase. These hybrids only pair successively about half the time, with the parental succeeding 90% of the time. When one of these hybrids mates, it is usually with one of the parental types (Cornell, 2000). There have been gradations of color and pattern mixtures between these hybrids, suggesting that the inheritance of color is more complicated than simple Mendelian genetics, perhaps involving multiple but tightly-linked loci, incomplete dominance, or modifying genes (Robertson, 2001).
· In 1997 Frank Gill, an ornithologist, did a study using mitochondrial DNA (mtDNA) to show that Golden-wing populations can quickly lose their genetic integrity when they hybridize with Blue-wings. Since mtDNA is inherited from the female, when a female Blue-wing mates with a male Golden-winged, the resulting Brewster’s inherits the mother’s Blue-winged mtDNA, and passes her blue-winged DNA to her offspring. Even birds that look like pure Golden-wings could have Blue-winged mtDNA. In one of his study populations, in the initial stages of hybridizations, Gill found that 27% of these birds had Blue-winged mtDNA, and four years later this increased to 70%. In another one of his study populations, 98% had blue-winged mtDNA, including birds that had the Golden-winged phenotype.(Confer and Barker 2002)
· For any management program to be successful in conserving the Golden-winged warbler, four concerns need to be addressed.
1. Maintain or create sufficient amounts of appropriate habitat.
2. The management program needs to assess the impact of Blue-winged warblers and might require further study. Needed control measures might be needed, and populations that are suspected to be changing should be monitored every several years.
3. The effect of nest parasitism by Brown-headed cowbirds may need to be controlled.
4. The effect of loss of winter habitat needs to be assessed and studied and corrective efforts may be needed (Confer, 1998)
For more information about Golden-winged Warbler and their natural history go to http://birds.cornell.edu/gowap/. This site has information about their suitable habitat, song types, species account, and behavior and displays. Cornell Lab or Ornithology is in charge of this project.
http://birds.cornell.edu/publications/birdscope/Spring1999/golden_wingd99132.html. This article is from the Cornell Laboratory of Ornithology (Barker, Confer, Rosenberg, Birdscope, Volume 13, Number 2: 7&16) and goes into why Golden-winged warbler populations are declining. The two main reasons are loss of habitat, and the interbreeding with the Blue-winged warbler. The North American Breeding Bird Survey relies almost totally on distinctive songs of each species to identify birds. Since the hybrids sing the same songs as the parental, there needs to be an atlas that visually identifies the birds counted in areas where the two species ranges overlap, so the frequency of hybridization can be better understood.
http://birds.cornell.edu/publications/birdscope/Winter2002/A_Tale_of_Two_Species_Inscribed%20in%20DNA.html. A Tale of Two Species, Inscribed in DNA. This article is also from Cornell Lab of Ornithology (Confer & Barker, Birdscope, winter 2002/Volume 16 number 1). This article describes a survey done in 1998 done by John Confer on how hybridization is affecting Golden-winged populations in Sterling Forest State Park, NY. Confer hypothesizes that Golden-winged persist in the region because they are able to breed in the wetlands there, which Blue-wings seem to avoid, making a refuge for them. Next season the Golden-winged Warbler Atlas Project team hopes to collect more atlas data and DNA analysis, so they can protect Golden-wings where they have the best chance of sustaining healthy populations.
http://elibrary.unm.edu/sora/index.php. This link takes you to the next four articles and to the Sora (Searchable Ornithological Research Archive). The articles are in PDF form and to see them, put in the name of the author, title and year.
The Auk-Hybridization in Birds by Frank B. Gill (Vol 115 No. 2 April 1998). In this article Gill looks at hybridization in warblers that are not closely related and the paradox that genomic compatibility and the potential for hybridization can occur among strikingly different birds, striking down the ideology that hybridization only occurs in closely related species. He does talk about the Golden-winged/Blue-winged hybrids, and how scientists look at the dynamics of ecological competition, genetic invasion, and cytonuclear disequilibria to try and understand the genetic and social architectures of these birds.
The Auk-Historical Aspects of Hybridization Between Blue-winged and Golden-winged warblers by Frank B. Gill (Vol 97 No 1 January 1980). Gill summarized information on the historical populations and changing abundance and hybridization in Blue-winged and Golden-winged in southern Connecticut. In 1850 both warblers were rare in the state, but around 1880 Golden-wings increased in some localities in the Connecticut River valley. Blue-wings increased in this area from 1880-1900 and spread throughout the valley in 1920. Ultimately, the Blue-winged replaced the Golden-winged in this area due to hybridization. The population increase of these warblers can also be tied to the abandonment of small New England farms after the Erie Canal opened and farms in the Great Lakes region had a distinct competitive advantage.
Barker, Sara E., Confer, John L., Rosenberg, Kenneth V. (1999). Golden- winged Warblers: A species in Decline. Birdscope vol.13 number 2. 9/9/2004. http://birds.cornell.edu/Publications/birdscope/Spring1999/golden_wingd99132.html
Canterbury, Ronald A. (2003). Encounters with Brewster’s Warblers. Birdscope vol. 17 number 2. 11/18/2004. http://birds.cornell.edu/Publications/birdscope/Spring 2003/Brewsters.html
Confer, John L. and Barker, Sara E. (2002) Birdscope vol. 16 number 1. 9/16/2004. http://birds.cornell.edu/publications/birdscope/Winter2002/A_Tale_of_Two_Species_Inscribed%20in%20DNA.html
Cornell Laboratory of Ornithology, Golden-winged Warbler (2000) 9/16/2004. http://birds.cornell.edu/BOW/GOWWAR/
Dunn, Jon and Garrett, Kimball (1997) Warblers. New York, NY Houghton Mifflin.
Johns, Mark, Wildlife Profile-Golden-winged warbler 9/16/2004.