When they drink water afterwards, the bread soaks it up and causes the duck to sink and drown. Honest.

And, of course, the bread inside the drowned duck will then soak up further water, try to expand despite being contained inside the duck and finally cause it to explode on the bottom of the lake. Small-scale tsunamis ensue.

So remember as a general rule: eat all edible things yourself, because otherwise birds go boom. Fun to watch, maybe, but if anyone saw you, you'll be in deep trouble for creating explosive devices without a license.