The Palila (Loxioides bailleui) is a small bird native to Hawaii that was listed as endangered in 1967. In 1998, the U.S. District Court for the District of Hawaii, pursuant to a stipulation submitted by the parties, issued an order requiring the State of Hawaii to conduct semi-annual "aerial sightings" for ungulates (e.g., pigs, deer, sheep, goats, cattle) in the Palila’s critical habitat area. Further, if any ungulates are sighted, the order requires the State to "commence aerial shooting" of the ungulates.
In 2012, the County of Hawaii, who was not a party to the stipulation or the underlying Endangered Species Act (ESA) litigation, enacted an ordinance prohibiting "any person to engage in the eradication of any animal for any reason while being transported by helicopter, airplane, or any other similar means." Afraid that compliance with the 1998 order would result in a violation of local law, the parties to the underlying ESA litigation filed a motion with the district court seeking both a declaration that the County ordinance was preempted by federal law, and an injunction prohibiting the County from enforcing the ordinance with respect to activities conducted pursuant to the 1998 order. In opposing the motion, the County not only refused to agree that it would not enforce the ordinance, but also indicated that aerial eradication efforts could potentially be prosecuted under State law.
The district court held (pdf) that, under the Supremacy Clause to the U.S. Constitution, the County of Hawaii was preempted from prosecuting any violation of the 2012 ordinance or State law if the violation occurred as a result of any action to enforce the specific terms of the 1998 order. The district court expressly noted that, while the County of Hawaii was not a party to the original ESA litigation, this did not prohibit the court from issuing an injunction in order to protect its prior decision.