Only B.I.G. could give you a definite answer, but here are some ideas. Techibeetle wrote, "2:52 am—Whoa. I’m picking up one sound and one sound only from the headquarters: a high, piercing whistle. It’s kind of beautiful, so why am I suddenly so afraid?"  

This makes me think it has something to do with playing the Power as an instrument—something you can only do if you have a bird’s beak, coincidentally. So they must either have a bird working for them, or they must've kidnapped one and forced it to play. 

I never tried to play the instrument, first of all because I'm not really a flute girl, but second of all because I never wanted to risk summoning a mythical beast.

I'm guessing it's more complicated than that, and I'm guessing not all Powers are activated the same way.  

Here's Techibeetle's diary, in case you don't have the link:

http://www.cricketmagkids.com/chatterbox/kyngdom-wing/node/207254