Birds nesting in knotweed - now published

May 2025 - Now published in the Wilson Journal of Ornithology: Bird nesting habitat in knotweed (Polygonum cuspidatum s.l.) in North America and beyond

This paper was a collaboration between Erik Kiviat (Executive Director, Hudsonia), Christopher Whelen (Adjunct Research Associate Professor, Dept of Biological Sciences, University of Illinois at Chicago), and Michelle Haviland (Intern, Hudsonia)

Abstract

Data are scarce on bird habitat provided by long-present, abundant, nonnative plants. Knotweed (Polygonum cuspidatum s.l.) is a robust, shrub-like herb 2–3 m tall that can produce a large aboveground biomass and dense cover. Knotweed is native to eastern Asia and was introduced in the 1800s to Europe and North America, and it has become widely invasive on both continents. We report observations from 2003–2023 on 50 knotweed stands with 64 nests of nine species of birds in the northeastern USA. We also collected 70 unpublished volunteer observations of 19 species nesting in knotweed across the northern USA and southern Canada. We summarize literature about many additional nests in nonnative knotweed in North America and Europe, and in native knotweed in Japan. Dense foliage conceals nests during the breeding season, thus many nests were found after leaf fall. Species found nesting in or under knotweed in North America comprise two nonpasserines and 16 passerines. Red-winged Blackbird (Agelaius phoeniceus) and Gray Catbird (Dumetella carolinensis) were the most frequently identified. Minimum height of a knotweed plant with a nest was 2.2 m, and area of a knotweed stand with a nest ranged from 2 m2 to 10,000 m2. Knotweed is often managed (e.g., by herbicide or cutting). We recommend that such treatments be instituted either (1) in August or later, after the breeding season of most songbirds, or (2) in winter or early spring to remove the overwinter structure of standing dead canes that is important for nest attachment in order to discourage nest construction in the knotweed, followed by treatment of the new shoots in May. Our study indicates the potential importance of a nonnative plant for nesting birds as well as the utility of surveying for nests during leaf-off season. Our data show a previously unrecognized, taxonomically and geographically widespread behavior of birds using a nonnative weed, and suggest that other large, abundant, nonnative plants could also have nuanced habitat relationships with birds.

Kiviat, E., C.J. Whelan, and M.L. Haviland. 2025. Bird nesting habitat in knotweed (Polygonum cuspidatum s.l.) in North America and beyond. The Wilson Journal of Ornithology 1–16. https://doi.org/10.1080/15594491.2025.2494359