Aeron Scales
The brown-headed cowbird is an unusual bird. Instead of raising its own young, it lays its eggs in the nests of other birds. Cowbirds usually choose smaller passerines (songbirds), but they are not picky—they have been recorded parasitizing at least 220 species’ nests, including unsuitable host species such as sandpipers, owls, and hawks.[1] This is not a mutually-beneficial relationship. The cowbird usually destroys at least one of the host’s eggs to make room for its own, and the cowbird’s fast-growing chicks often starve their smaller nestmates.[2] The cowbirds also often retaliate against hosts who eject the cowbird eggs by destroying the host’s entire clutch.[3]
Through these racketeering behaviors, the cowbird has contributed to the endangerment of several songbird species that have already otherwise been made vulnerable due to deforestation and development.[4] The cowbird is more insidious than other invasive birds, like the bluebird-murdering house sparrow, because the cowbird is actually native to North America.[5] Although the federal government has carved out an exception for cowbirds (along with other common agricultural pest species), as a native migratory bird, the cowbird is protected under the Migratory Bird Treaty Act of 1918, which generally prohibits “tak[ing] (including killing, capturing, selling, trading, and transport) of protected migratory bird species without prior authorization by the Department of Interior U.S. Fish and Wildlife Service.”[6]
The cowbird is not always immediately recognized as a threat to vulnerable species. For example, when the southwestern willow flycatcher was being considered for endangered status in 1995, one of the arguments against its inclusion on the endangered species list was that the cowbird and the flycatcher are “positively associated” and “can coexist.”[7] The Fish and Wildlife Service determined that the cowbird did pose a substantial threat to the flycatcher, concluding that, “cowbirds and [the southeast willow flycatcher] are positively associated because cowbirds require, and therefore associate with, prospective hosts.”[8]
Cowbirds, which historically followed bison in the Great Plains—feeding on insects disturbed by the bison’s hooves—now follow cattle across the country.[9] In the past, many of the now-vulnerable songbirds nested deep inside old-growth forests and were thus largely shielded from cowbird parasitism.[10] However, the amount of interior forest across the United States has been dramatically reduced since European settlement and the accompanying land clearing and habitat fragmentation.[11]
Cowbird trapping programs have been implemented to protect four at-risk songbird species. These programs are still necessary to protect the endangered least Bell’s vireo and southwestern willow flycatcher, and the nearly-endangered black-capped vireo.[12]The greatest success story is the Kirtland’s warbler, a particularly vulnerable species whose nests were heavily parasitized by cowbirds. The warbler was removed from the endangered species list in 2019 after its population increased from “a low of 167 males in 1987 to over 2300 males.”[13]
Unfortunately, however, the cowbird is not the only threat to the Kirtland’s warbler. Cowbird trapping was able to save the warbler from extinction, but their population did not increase for almost 20 years after trapping began. Part of what made the warbler so vulnerable to cowbird parasitism was their extremely specialized breeding strategy—the Kirtland’s warbler nests exclusively in young jack pine forests in central Michigan, an environment that is only created naturally through “large, intense wildfires.”[14] It wasn’t until a prescribed burn in 1980 escaped control, that enough of this habitat was created to help recoup the warbler’s breeding habitat.[15] Additionally, the warbler may soon be at risk in its wintering habitat, the Bahamas, due to climate change causing sea levels to rise.[16]
Other species negatively impacted by the brown-headed cowbird share similar stories—habitat loss from deforestation is the more direct cause of the least Bell’s vireo and southwestern willow flycatcher’s endangered status, and federal conservation funding is lost once a species escapes its endangered species status.[17] Cowbird trapping is an expensive stopgap to treat a symptom of a larger issue. Critical habitats for birds, such as interior forests and wetlands, must be protected. Some states and the United States Department of Agriculture have implemented programs that incentivize landowners to preserve and maintain forests on private land,[18] and the Department of the Interior recently proposed a new conservation rule that would protect mature and old-growth forests on federal land.[19] In an era in which environmental protections can depend on election results, it is crucial for the continued survival of vulnerable species that legislators and the courts be vigilant and aware of the complex threats posed to birds (and other wildlife) by land development and climate change.[20]
[1] See Lawrence D. Igl & Douglas H. Johnson, Brown-headed Cowbird, Molothrus ater, Parasitism and Abundance in the Northern Great Plains, 121 Canadian Field-Naturalist 239, 245 (2007) (a study examining brown-headed cowbird nest-parasitism, including choice of hosts).
[2] See Lisa Petit, Brown-Headed Cowbirds: From Buffalo Birds to Modern Scourge, Smithsonian’s Nat’l Zoo & Conservation Biology Inst. (Jan. 1, 1996), https://nationalzoo.si.edu/migratory-birds/news/brown-headed-cowbirds-buffalo-birds-modern-scourge (an overview of cowbird brood parasitism).
[3] See Jeffrey P. Hoover & Scott K. Robinson, Retaliatory Mafia Behavior by a Parasitic Cowbird Favors Host Acceptance of Parasitic Eggs, 104 Proc. of the Nat’l Acad. of Sci. of the U.S. 4479 (2007) (explaining that cowbird destruction of “ejector” nests may be why so many host species accept cowbird eggs and nestlings, which “differ dramatically in appearance from their own”).
[4] “Although brown-headed cowbirds were historically restricted to prairie ecosystems, forest clearing and agricultural development of Michigan’s Lower Peninsula in the late 1800s facilitated the brown-headed cowbird’s range expansion into Kirtland’s warbler nesting areas such that brown-headed cowbirds were common within the Kirtland’s warbler’s breeding range by the early 1900s.” Removing the Kirtland’s Warbler From the Federal List of Endangered and Threatened Wildlife, 84 Fed. Reg. 54436, 54450-54451 (Oct. 9, 2019) (to be codified at 50 C.F.R. § 17) (internal citations omitted).
[5] See Kenn Kaufman, Brown-headed Cowbird, Audubon, https://www.audubon.org/field-guide/bird/brown-headed-cowbird.
[6] 16 U.S.C. 703-712 (the Migratory Bird Treaty Act of 1918); List of Birds Protected by the Migratory Bird Treaty Act, 50 C.F.R. § 10.13; Depredation Order for Blackbirds, Cowbirds, Crows, Grackles, and Magpies, 50 CFR § 21.150.
[7] Final Rule Determining Endangered Status for the Southwestern Willow Flycatcher, 60 Fed. Reg. 10694, 10700 (to be codified at 50 CFR § 17.11).
[8] Id.
[9] Kaufman, supra note 5.
[10] See Nick M. Haddad et al., Habitat Fragmentation and its Lasting Impact on Earth’s Ecosystems, Sci. Advances, Mar. 2015, at 1.
[11] Id.
[12] The black-capped vireo has been removed from the endangered species list, although cowbird trapping continues to protect its population. Brian D. Peer et. al, Management of the Brown-headed Cowbird: Implications for Endangered Species and Agricultural Damage Mitigation, 14 Human-Wildlife Interactions 461, 461 (2020).
[13] Kirtland’s Warbler Conservation & Management, Marra Lab (2019), https://marralab.com/conservation-adaptive-management/.
[14] Id.
[15] Id.
[16] Id.
[17] Id.; see Kenn Kaufman, Bell’s Vireo, Audubon, https://www.audubon.org/field-guide/bird/bells-vireo; Southwestern Willow Flycatcher, Am. Bird Conservancy, https://abcbirds.org/bird/southwestern-willow-flycatcher/.
[18] See, e.g. NRCS, U.S.D.A., Conservation Programs & Practices for Forest Interior Wildlife Habitat,
https://www.nrcs.usda.gov/sites/default/files/2022-10/Forested%20Habitats.pdf.
[19] Department of Interior moves to protect mature and old-growth trees and forests from logging, Earthjustice, March 30, 2023, https://earthjustice.org/press/2023/department-of-interior-moves-to-protect-mature-and-old-growth-trees-and-forests-from-logging; but see Nat’l Audubon Soc’y, Supreme Court Decision Threatens Waterways that Birds (and People) Need, Audubon.org, May 25, 2023, https://www.audubon.org/news/supreme-court-decision-threatens-waterways-birds-and-people-need (explaining a recent Supreme Court ruling that dramatically lessens protections on wetlands).
[20] In January of 2021, the Trump administration finalized a rule that allowed the “incidental take” by corporations of birds protected under the Migratory Bird Treaty Act (such as by pollution or habitat destruction). The rule was reversed in September of 2021 by the Biden administration. See Andy McGlashen, Trump to Birds: Drop Dead, Audubon (Jan. 05, 2021), https://www.audubon.org/news/trump-birds-drop-dead; Andy McGlashen, Biden Administration Restores Migratory Bird Treaty Act Protections, Audubon (Sept. 29, 2021), https://www.audubon.org/news/biden-administration-restores-migratory-bird-treaty-act-protections.