According to the speaker of the poem, the actual song is a cry for help.  The "siren" that is speaking tells us a secret that this song, the one that lures men overboard, is actually a desperate cry for someone to come and rescue her from her "birdsuit." 

Shall I tell you the secret
and if I do, will you get me
out of this bird suit?
I don’t enjoy it here
squatting on this island
looking picturesque and mythical
with these two feathery maniacs,
I don’t enjoy singing
this trio, fatal and valuable.

I will tell the secret to you,
to you, only to you.
Come closer. This song

is a cry for help: Help me!

Atwood, a well-known feminist, is attacking men for their stupidity in this poem.  She comments that although her job is "boring", it "works every time."  There are no unique men, they are all victims of the siren.  As the siren is symbolic of female beauty and sexuality, she is chastising men's inability to overcome their lust.

It was Homer way back in the Greek days who created the myth of the Siren Song which is the same myth used in the poem. According to Homer's Iliad, the Siren Song is irresistible. It is inaudible to anyone but to the intended victim, and the sound is so harmonious and hypnotic that it drives sailors to their own death. It is an accepted concept among the part human/part bird mythology as well as the merman/mermaid lore, and it is universally known as one of the main siren's qualities.