...include the maximum size a bird can be and fly in Earth's gravity.
Scientists have confirmed that Pelagornis sandersi most definitely flew. Probably a lot. It's essentially a very big albatross.
A very big albatross.
The extinct bird has a wingspan between 20 and 24 feet, twice the size of the current record-holder, the Royal Albatross, but very similar in build. Scientists aren't sure how it took off (one story says it couldn't take off from a standstill...but neither can the Royal Albatross, which has to run into the wind for a considerable distance to build lift - presumably Pelagornis had runways in its colonies as well).
The entire family died out 25 million years ago - and we're not sure why on that either. But if there could be an albatross that big, does this mean there was once such a thing as a roc?