On August 22, 2025, the United States Court of Appeals for the D.C. Circuit upheld the decision of the U.S. Fish and Wildlife Service (Service) to deny a petition to remove the southwestern willow flycatcher (Empidonax traillii extimus) from the list of species protected by the Endangered Species Act (ESA). The Service listed the southwestern willow flycatcher as an endangered species in 1995. The southwestern willow flycatcher is a small migratory bird that winters in Central and South America with a range in the United States that includes southern California, southern Nevada, southern Utah, southern Colorado, Arizona, New Mexico, and western Texas.
The southwestern willow flycatcher is listed as a subspecies under the ESA, which authorizes the Service to list distinct population segments (or DPSs) and subspecies in addition to species. The New Mexico Cattle Growers’ Association (New Mexico Cattle Growers) petitioned to delist the southwestern willow flycatcher in 2015 on the grounds it is not a valid subspecies of the willow flycatcher species. After initially determining that the petitioned action may be warranted, the Service initiated a 12-month review and ultimately determined in a finding issued in 2017 that the petitioned action is not warranted and that the best available scientific information supports classification of the southwestern willow flycatcher as a subspecies.
The New Mexico Cattle Growers challenged the Service’s determination in federal district court on the grounds it violated the Administrative Procedure Act. The district court ruled in favor of the Service, and the New Mexico Cattle Growers appealed. The singular issue presented on appeal was whether the Service failed to use a “non-arbitrary” standard for evaluating the appropriateness of the subspecies classification for the southwestern willow flycatcher. More specifically, the New Mexico Cattle Growers argued that the non-clinal geographic variation standard used by the Service to determine whether the southwestern willow flycatcher is a subspecies is “impermissibly indeterminate.”
Non-clinal geographic variation refers to abrupt changes in species traits over short geographic distances. One way to think about the dispute between the New Mexico Cattle Growers and the Service is as a disagreement about whether available data regarding changes in the songs, feathers, and DNA of flycatchers across a defined geographic range indicated gradual variation or step changes. Another way to think about the dispute is a disagreement about whether the test for differentiating subspecies—in this instance, distinguishing between gradual variation and step changes—is itself problematic because there is no bright line between the former and the latter.
On appeal New Mexico Cattle Growers argued the test itself was arbitrary, and the D.C. Circuit was steadfast in rejecting the argument. The panel held that the Service’s decision must be reasonable and reasonably explained, and it concluded that the agency had met this burden after having given careful consideration to the pertinent data and analyses including the research relied on by the New Mexico Cattle Growers to argue there was no step change justifying the subspecies classification. The decision is an important reminder of the deference federal courts continue to show to agency technical determinations even after the Supreme Court’s decision in Loper Bright holding federal courts need not defer to agency legal determinations.
