Do I draw a card if I enchant an opponent's Eidolon of Blossoms with Hypnotic Siren?
First gatherer note on Hypnotic Siren:


  If you Hypnotic Siren for its bestow cost, and you gain control of a creature with a constellation ability, that ability will trigger when Hypnotic Siren enters the battlefield. You'll controll [sic] that triggered ability.


Looks like the answer is "yes".