The birds in my novel THE FLOCK are Terror birds, also known as Phorusrhacids were an extinct form of the modern type we know as ratites. These are almost all large birds that are flightless. Most of them today are much smaller than the Terror birds and are herbivorous rather than carnivorous. Why the herbivores thrived while the flesh-eaters died out is a matter for paleontologists to puzzle. About the closest we have to the Terror birds these days would be the Cassowary which is considered the most dangerous living bird, having killed a total of two human beings in the past few hundred years. I've read that they do occasionally chase down and consume insects and small reptiles, thus making them omnivorous rather than strict herbivores. Like most of today's amazing creatures, they're near extinction.
Recently, I read that the eggshell of the extinct Aepyornis, the heaviest bird known to have ever lived, gave up its DNA. This is, I suppose, something of a genetic coup, but I don't know if it means that the species can be resurrected. From what I've read on the subject, I rather doubt it. Still, it's interesting to know that the DNA of these critters can be recovered and studied, if not actually used to recreate the lost animals. Which is a shame, of course, because unlike the Terror birds who exist only in my novel, ratites such as Aepyornis and the moas of New Zealand were killed off by humans and not by the normal means of Natural Selection.