The speaker of the poem is one of the mythical sirens, immortalized by Homer in The Odyssey. The sirens are bird/women who sing a beautiful song that lures men to their death. In Homer's version, the sirens lure sailors to jump overboard, which is why Odysseus has his men put beeswax in their ears and tie him to a post.
It appears in the beginning that this siren is speaking directly to the reader. We know this because of the use of the pronoun "you" -
Shall I tell you the secret
and if I do, will you get me
out of this bird suit?...
I will tell the secret to you,
to you, only to you.
She suggests to the reader that she will reveal the nature of the song to him. She will reveal the true secret. The "secret" is that the siren doesn't like her job, and that the song is actually a cry for help.
What is revealed in the last stanza is that her audience has been male readers - the siren only lures men to their death in the tradition. In this, the siren's "call for help" has been a lure, as she reveals:
it is a boring song
but it works every time.
In other words, in pretending to reveal the secret, she has lured another man to his death.