Though the Americans With Disabilities Act defines service animals as trained dogs or miniature horses, airlines are bound by the more liberal Air Carrier Access Act of 1986, which allows free travel for “any animal” that is trained to assist a person with a disability or that provides emotional support. Airlines can require passengers with creatures in the second category to produce a letter from a physician or mental health professional, but the documents are easily forged or obtained from websites that provide cursory, questionnaire-style “exams.”

The result, airline officials complain, has been a surge in poorly trained animals that has turned some flights into airborne menageries, with dogs blocking beverage carts, cats urinating on seats and ducks wandering the aisles.

“It’s created a real issue on our planes,” said Taylor Garland, a spokeswoman for the Association of Flight Attendants, which applauded Delta’s changes. Garland said one union member was asked to administer oxygen to a dog that, according to its owner, was having anxiety midflight. Others have been bitten. “The aircraft cabin is a unique space, and … we need to recognize the limitations that exist when you’re flying in the air in a metal tube.”