If asked to name a bird that we considered “our” bird, most Minnesotans would claim the Common Loon, the state bird. But the bright and unassuming Golden-winged Warbler would be a better choice. Over half of the global population of Golden-wings nest in the state, and as a species that has suffered a precipitous 66% decline in population since 1966, this little bird needs our help.
The Golden—winged Warbler is small and restless, looking superficially like a chickadee with a yellow cap, until you observe the bill, which is slender, the bill of an insect eater. In Minnesota, they breed in shrubby, tangled thickets in the northern half of the state, including east central Minnesota.
They have a sibling species, the Blue-winged Warbler, that is so closely related that they share 97 % of their mitochondrial DNA. This close genetic affinity means the two species hybridize easily, producing individuals that look nothing like either parent. (see photo) However, the characteristic songs for each aren’t mash-ups and the hybrids sing either Golden-winged songs or Blue-winged songs.
It is this hybridization with Blue-winged Warblers that threatens the Golden-wings, as the former push ever further into historical Golden-wing habitat. However, mtDNA analysis has also found that some “perfect” Golden-winged Warblers carry Blue-Winged mtDNA, and are what biologists call “cryptic hybrids.” This fluidity between species has led some researchers to consider the two species a gene complex, rather than wholly separate species. The overlap in genetics has led researchers to postulate that the two have been separate only one-two million years, since the last Ice Age. The ice sheet, they propose, divided one population of southeastern North America (the Golden-wing precursors) from another on the western side, nearer the Mississippi valley (the Blue-wings.) When the ice retreated, the two species no longer had a barrier, and speciation had been imperfect, allowing viable hybrids.
Recently, researchers attached tiny radio transmitters to Golden-winged nestlings in two Minnesota national wildlife refuges at Tamarack, by Detroit Lakes, and at Rice Lake, by MacGregor. Both of these refuges are in the northern hardwood forest transition zone. The researchers found that when the babies left the nest, mortality was high, a whopping 48%, mostly by mammals and snakes. But where the fledglings went after leaving the nest made a difference. Those young birds spending their earliest days in mature forest, or sapling-dominated clear-cuts, had higher survival rates than those that remained in a shrub-dominated wetlands, where the birds tend to built their nests.
In other words, the warblers use shrubby wetlands for nesting, but mature woods for bringing their young to maturity. Nests built on an edge between wetlands and forest was the best location for breeding success. There’s much we don’t know about this bird experiencing such rapid decline. Little is known about their overwintering grounds in Central and South America, though bird counts pick up more birds in Costa Rica, Nicaragua and Honduras, than in Colombia. They seem to fly a loop, more westward going north, and more easterly headed south, like many warblers. They will continue to attract active research to stabilize their numbers.
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