As a follow-up on Copyright of children's folklore songs, artist A wants to use the melody of "Five little ducks". The song is not on the list of public domain songs. A sound recording of the melody and the score are on Wikipedia; the lyrics and an animated video are on a US government website. Even so, Artist A cannot find author or date information.
How can artist A assess whether a melody such as this one is copyrighted or in the public domain?