A judge for the US District Court for the Southern District of New York struck down a Trump administration policy that she said upended almost 50 years of protections for birds. In her decision published Tuesday, Judge Valerie Caproni began by referencing “To Kill A Mockingbird,” stating: “It is not only a sin to kill a mockingbird, it is also a crime.”

In December 2017, the US Department of the Interior (DOI) Principal Deputy Solicitor issued a memo renouncing almost 50 years of the DOI’s interpretation of “takings” and “killings” under the Migratory Bird Treaty Act of 1918 (MBTA). Under the MBTA, it is illegal “at any time, by any means or in any manner, to hunt, take, capture, kill, attempt to take, capture or kill” certain migratory birds.

From the 1970s until 2017, the DOI interpreted the MBTA to prohibit incidental takes and kills, imposing liability regardless of whether the activities targeted or were intended to take or kill birds. In January 2017, DOI’s solicitor official issued a memo reaffirming the DOI’s interpretation that the MBTA prohibited incidental take.

However, in December 2017, the new Principal Deputy Solicitor issued a new memo that permanently replaced the January memo. This memo changed the interpretation so that the MBTA did not prohibit incidental take.

Following the issuance of the memo, environmental interest groups and states brought three separate suits, which were consolidated, to vacate the memo and subsequent guidance issued. They argued that the new interpretation of the MBTA was “arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law” in violation of the Administrative Procedure Act (APA). The plaintiffs moved for summary judgment, and the DOI cross-moved.

On Tuesday, the judge found that DOI was only entitled to Skidmore deference, under which the court considered the “agency’s expertise, the care it took in reaching its conclusions, the formality with which it promulgates its interpretations, the consistency of its views over time, and the ultimate persuasiveness of its arguments.” The judge stated that these factors disfavored deference, because the December 2017 memo was a “recent and sudden departure from long-held agency positions backed by over forty years of consistent enforcement practices.” Also, the memo was an informal pronouncement lacking any protective rulemaking procedures.

The judge granted summary judgment to the plaintiffs. The December 2017 memo was vacated and remanded to the DOI.