He answered that it was the bells that hung on the top of their cages.

Then he said to us, Will you have me make these monk-hawks whom you see bardocuculated with a bag such as you use to still brandy, sing like any woodlarks?
Pray do, said we.

He then gave half-a-dozen pulls to a little rope, which caused a diminutive bell to give so many ting-tangs; and presently a parcel of monk-hawks ran to him as if the devil had drove 'em, and fell a-singing like mad.
Pray, master, cried Panurge, if I also rang this bell could I make those other birds yonder, with red-herring-coloured feathers, sing?
Ay, marry would you, returned Aedituus.

With this Panurge hanged himself (by the hands, I mean) at the bell-rope's end, and no sooner made it speak but those smoked birds hied them thither and began to lift up their voices and make a sort of untowardly hoarse noise, which I grudge to call singing.
Aedituus indeed told us that they fed on nothing but fish, like the herns and cormorants of the world, and that they were a fifth kind of cucullati newly stamped.
He added that he had been told by Robert Valbringue, who lately passed that way in his return from Africa, that a sixth kind was to fly hither out of hand, which he called capus-hawks, more grum, vinegar-faced, brain-sick, froward, and loathsome than any kind whatsoever in the whole island.
Africa, said Pantagruel, still uses to produce some new and monstrous thing..