WASHINGTON — As the state of Virginia prepared for a major bridge and tunnel expansion in the tidewaters of the Chesapeake Bay last year, engineers understood that the nesting grounds of 25,000 gulls, black skimmers, royal terns and other seabirds were about to be plowed under.

To compensate, they considered developing an artificial island as a haven. Then in June 2018, the Trump administration stepped in. While the federal government “appreciates” the state’s efforts, new rules in Washington had eliminated criminal penalties for “incidental” migratory bird deaths that came in the course of normal business, administration officials advised. Such conservation measures were now “purely voluntary.”

The state ended its island planning.

The island is one of dozens of bird-preservation efforts that have fallen away in the wake of the policy change in 2017 that was billed merely as a technical clarification to a century-old law protecting migratory birds. Across the country birds have been killed and nests destroyed by oil spills, construction crews and chemical contamination, all with no response from the federal government, according to emails, memos and other documents viewed by The New York Times.

Not only has the administration stopped investigating most bird deaths, the documents show, it has discouraged local governments and businesses from taking precautionary measures to protect birds.