Brewster's Warbler (hybrid between Blue-winged and Golden-winged warblers) ©George West, Birchside Studios

The Golden-winged Warbler has declined precipitously during the last 35 years; populations have decreased by 71 percent in the United States and by 94 percent in the Northeast, according to the U.S. Breeding Bird Survey. These declines coincide with the loss of breeding habitat and the expansion of the Blue-winged Warbler into the golden-wing range.

Blue-winged and Golden-winged warblers rarely coexist in breeding habitat for long: regional declines and even local extirpation of golden-wings typically occur within 50 years after blue-wings arrive. The Sterling Forest region of southern New York, where golden-wings have coexisted with blue-wings for more than a century, is the only known exception to this pattern. Identification and analysis of the factors allowing this unique long-term coexistence may help inform management decisions in other regions where both species nest in the same habitats.

Where blue-wings and golden-wings overlap, competition and hybridization between the two species may ultimately contribute to golden-wing declines. After invasion by blue-wings, populations that once consisted exclusively of golden-wings may change to a mixture of both species and hybrids, then finally to entirely blue-wings.

A 1997 study by Frank Gill used mitochondrial DNA (mtDNA) to show that Golden-winged Warbler populations can quickly lose their genetic integrity when they hybridize with blue-wings. MtDNA is inherited maternally; when a female blue-wing mates with a male golden-wing, the resulting hybrid inherits the female blue-wing's mtDNA. A female hybrid passes on the blue-wing DNA sequence to her descendants. Thus, even birds that look like golden-wings could have blue-wing mtDNA, inherited from a female blue-wing many generations past.

One Pennsylvania population in the initial stages of hybridization was composed of golden-wings and hybrids, based on plumage. Using genetic techniques, Gill found that 27 percent of these birds had blue-wing mtDNA, a figure that increased to 70 percent just four years later. In an actively hybridizing population in the Delaware River valley, 98 percent had blue-wing mtDNA, including birds that were identified as golden-wings based on appearance.

Seeking DNA Samples We are seeking genetic samples of Blue-winged and Golden-winged warblers from banders across the range of these species. If you have been collecting blood or feather samples from Golden-winged or Blue-winged warblers for your research program or are mist netting these birds, please contact our staff so we can speak with you about collaborating on this genetic study. Call (607) 254-2465 or send email.

But how is hybridization affecting golden-wing populations in Sterling Forest, where golden-wings and blue-wings have persisted in the same habitat? In 1998 and 1999, John Confer and Shelagh Tupper from Ithaca College found that 10 percent of males at their study sites were hybrids, recognizable as distinctive "Brewster's" Warblers. However, to determine whether hybridization is affecting the genetic integrity of the population as a whole, DNA analysis was required. This summer, Confer collected blood samples from Golden-winged and Blue-winged warblers and passed them on to Irby Lovette, the Lab's new director of Evolutionary Biology.

Lovette's preliminary results show two distinct mitochondrial types, a golden-wing genetic sequence and a blue-wing sequence. Almost all of the birds in the preliminary sample had mtDNA that matched their phenotype, or physical features such as plumage.

These genetic results will need to be augmented by additional samples and more sophisticated laboratory techniques, but the preliminary findings are heartening because they suggest that the golden-wing population at Sterling Forest is holding its own against the influence of blue-wing hybridization, unlike in other golden-wing populations such as those studied by Gill.

Confer hypothesizes that golden-wings persist in the Sterling Forest region because they are able to nest in wetlands, a refuge from blue-wings. Both species breed in upland areas but golden-wings also nest in wetlands where blue-wings are rare.