Size and Weight:
If you crossed a duck, a flamingo and a goose, you might wind up with something like the prehistoric bird Presbyornis, which has caused much head-scratching among paleontologists until only recently. (For the record, this bird was once thought to be related to flamingos, then it was classified as an early duck, then as a cross between a duck and a shorebird, and finally as, well, a kind of duck again.)
Why all the confusion? Well, the beak and head of Presbyornis were similar to those of a duck, but it also had long, flamingo-like legs. It's thought that this bird nested in vast flocks along the lakeshores of North and South America and Asia, and filtered tasty food (probably including small plants as well as crustaceans) with its broad beak.