Quoting from:
Answer
Pet and performing birds originating in the United States may be reimported without being quarantined if they are accompanied by a United States veterinary health certificate and, if necessary, a permit from the U.S. Fish and Wildlife Service - with the exception of coming from a country where Highly Pathogenic Avian Bird Flu is evident, in which case they are subject to quarantine. This health certificate must be obtained prior to departure from the United States and must include a leg band or tattoo number
First off, I am not a lawyer. I can't tell you what to do, and anything that you do is at your own risk.
With that said, I see a couple of options:
1. Anonymously call US customs from another number. Don't identify yourself, don't give the species of bird, and if asked you could say you were calling on behalf of your elderly mother or something. Tell them the situation, and ask for advice. You may or may not get good advice. The person you are talking to may or may not know what they're talking about. There is a minor risk of you having legal or other trouble from this. However, there is also a chance that you will get a straightforward answer.
2. Call the vet and ask if they will issue you a health certificate based on the bird's last visit. Not sure how well this will work.