The Trump administration made it clear this week that it is sapping the strength of a century-old law to protect birds, issuing guidance that the law would not be used as it has been to hold people or companies accountable for killing the animals.

In an opinion issued Wednesday to federal wildlife police who enforce the rule, the Interior Department said “the take [killing] of birds resulting from an activity is not prohibited by the Migratory Bird Treaty Act when the underlying purpose of that activity is not to take birds.” For example, the guidance said, a person who destroys a structure such as a barn knowing that it is full of baby owls in nests is not liable for killing them. “All that is relevant is that the landowner undertook an action that did not have the killing of barn owls as its purpose,” the opinion said.