From the 2nd source I linked to (BBC)...
Dr Silviu Petrovan noticed the unusual behaviour of a group of mallards while he was out bird watching with friends near a national park in southwest Romania.
He saw the adult female duck grab the grey wagtail in her beak, and repeatedly submerge it in the water, before eventually eating it.
A second bird - a fledgling black redstart - then landed in the water, where it was chased by juvenile mallard ducks.
“The poor bird landed on the water and was screaming and trying to navigate itself out of danger,” said Dr Petrovan. “Then it was almost instantaneously attacked by the mallards.”
The bird eventually disappeared - assumed to be drowned or consumed.
The scientists could find no record of mallard bird predation in the scientific literature, which suggests such behaviour is both “very rare” and newly-learned.
“The mallard was massively struggling to eat that wagtail, presumably because it couldn’t actually tear it to pieces because the bill is flattened - it’s not designed for ripping prey apart,” said Dr Petrovan.
“Digesting bones and feathers - that’s not something that mallards have really evolved to do.”
http://www.bbc.com/news/science-environment-40445379
Damn climate change.