Depending on the boldness and agressiveness of the predator. Perhaps if tempered like a Sarcophilus or a mustelid, they could defeat those apparently slow prey. I suppose they were not so brave, because they are not any more related to these as they are to the more timid Didelphis.
I _really_ wouldn't use phylogenetic bracketing for temper. It changes too quickly to contain any phylogenetic signal. :-)
> We can't rule out big cursorial hunters simply eating the adults at > night, either. Dire wolves and sabertooths would have been all too > eager to take advantage of any big stubborn bird that had bedded > down in the open and was adapted for fight rather than flight.
The reason this fails is that no extinction of phorusrhacids is known to coincide with the Great American Interchange.
Something similar to the lateral shaking of dogs can be observed in this gull, admittedly in not so great frequency, so it may not be so unlikely to occur in other predatory birds:
It's widespread in predatory birds. Even quetzals do it (the adults are rather herbivorous, but they feed their young with large lizards).