Giant knotweed (Fallopia sachalinensis) flowered this week along the creek — its tall stems and creamy panicles now joining two other invasive species already established at the margins. and Bohemian knotweed are present here in heavy stands, and the Bohemian is particularly significant — it is a hybrid of the other two, evidence that cross-pollination is actively occurring in the watershed.
This matters because hybrid vigor often exceeds either parent species. The offspring spread faster, colonize wider ranges, and resist management more stubbornly. Summer floods will carry seeds and rhizome fragments downstream to uncolonized banks, establishing new beachheads. What we see here is not stalled invasion, but accelerating divergence.
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Watch one feed and its way of making a living is plain. Golden-wings probe curled, hanging dead leaves in the understory, often dangling upside down like a chickadee to reach inside for caterpillars and spiders. That dead-leaf specialty is why the bird turns up low in shrubby edges and tangles rather than in the canopy, and it is a useful habit to know in fall, when a quiet bird working a leaf cluster is more likely to catch the eye than one that is singing.
What the species needs to breed is young, shrubby ground: the regrowth after a clearcut or a burn, an old field filling in with saplings, a shrub swamp, a utility right-of-way. That is a habitat that does not last. Left alone, a shrubland grows into a pole-stage forest in a decade or two and the warblers leave. Golden-wings once bred across the Appalachians, Pennsylvania included, and their retreat from this part of the range has two well-documented causes: young forest that has matured or been developed, and the Blue-winged Warbler, which moves into the same openings, competes for them, and hybridizes with the Golden-wings that remain. Roughly seventy percent of the world's breeding Golden-wings are now in the upper Great Lakes states and Manitoba. The species is on the federal list of Birds of Conservation Concern, tracked but not listed as endangered.
A bird here at the end of August is on its way from wherever it nested to wintering grounds in Central America and northern South America, feeding in whatever cover the route offers. Sewickley Heights Borough Park, 1,005 acres of borough-owned woods and edges open to the public, is one such stopping place, and the next few weeks are when the rest of the warblers will be passing through it.
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