In a June 1997 issue of discover magazine there is an interesting article...although I doubt it qualifies as peer reviewed, I am cutting and pating the relavant section.
Chandler’s paleontological housekeeping has brought him a vastly improved picture of the ecosystem in which Titanis lived. Two million years ago, Florida was an open savanna dotted with oak trees, where little rodents sneaked through the grass while ten-foot-tall ground sloths fed on leaves alongside mastodons, tortoises, three-toed horses, goat-size antelopes, giant armadillos, and jaguars. Chandler has also managed to flesh out our picture of North America’s terror birds. It has not yet been his luck to find a full skeleton of Titanis laid out from beak to tail, but he has unearthed shoulder bones, parts of its skull, snapped foot bones, vertebrae, claws, and thighbones--enough so he can safely say Titanis was among the biggest terror birds that ever lived. And Chandler has identified the first bones of a terror bird’s wing. Like other paleontologists, he’d assumed Titanis’s wings were vestigial, but the fossils have proved this assumption deliciously wrong.
Among those fossils is a three-inch fragment of the shoulder end of the bird’s humerus, the long bone of the upper arm. The bone was far bigger than Chandler expected and indicated that Titanis had a three-foot- long wing. That was still far too short to let the bird fly (condors, which weigh only 22 pounds, need wings five feet long), but it was significantly bigger than an ostrich wing. More important, the humerus of Titanis was a solid mass of hard tissue; most birds have thin-walled, hollow wing bones, which can remain stiff during flight without weighing the animal down. It doesn’t look like a bone of a bird, says Chandler. It’s got tremendous strength; it could be twisted and it wouldn’t break the way a normal bird bone would.
Another bone in Chandler’s collection came from Titanis’s hand. The dinosaurian ancestors of birds had three long fingers, which fused with the wrist to transform the arm into a long, jointed shaft that could support a sheet of feathers. Much of the wrist and portions of the fingers united into a single bone called the carpometacarpus, and the second digit enlarged to act as the tip of the feather-bearing shaft. Meanwhile the first digit, stiff and shrunken, became covered by a tuft of feathers known as the bastard wing, which helps keep a bird stable as it lands. The third digit became useless.
The joints of normal bird wings are so well-sculpted that the wings can fold up snugly against the body. Where the carpometacarpus of Titanis made contact with the forearm, however, the surfaces were flat. That, Chandler realized, meant the wrist couldn’t bend--the bird had to hold its hand out in front of its body. Equally bizarre was its first finger. On most birds, the joint of this finger is stiff, since the bastard wing needs only to be raised and lowered slightly to change the airflow around the wing. But on Titanis, Chandler found, the digit contacted the carpometacarpus at a ball joint shaped like the base of our thumb. It would have a ball joint only if it were doing something with it, explains Chandler.
These two bones and their wayward shapes have forced Chandler to conclude that Titanis had wings that would be better called arms. It held them out in front of its body, palms facing inward, and on each hand was a giant movable claw and two smaller fixed claws. Titanis would stalk mammals in the tall grass, then attack at high speed, and strike with its giant beak, possibly taking a quick swipe at a victim’s spine to paralyze the prey as lions do. And they would use their arms, Chandler suggests, to keep the prey from goring them with their horns or kicking them with their feet. As much as an antelope might struggle, the terror bird’s sturdy arm bones could resist its force. They could manipulate prey with their hands and impale them with their claws. Chandler further speculates that Titanis’s arms were probably bare--feathers would get matted down with blood and be a likely haven for infection.
The picture Chandler draws of Titanis is surprisingly similar to that of predatory dinosaurs like Velociraptor, which lived many millions of years earlier. Given that birds are the descendants of Velociraptor-like dinosaurs, it might seem that the arms and armaments of Titanis were the result of some drastic atavism--the sudden resurgence of a genetic program hidden for millions of years. It is true that silenced genes can find their voice again after thousands of generations. On rare occasions they can produce whales with the crude cartilage remnants of hind legs or horses with separate toes. If the mutations that produce them crop up often enough in a population of animals, atavisms can even help establish entirely new species. But this was not the case with Titanis.
I will look for some other articles but for now I offer this as one part of evidence for such things(it also reminded me of the more proper scientific term atavism as oppossed to re/de evolution). Considering the artilces I recalled dealt with species other than Titanis, it may be that this was something that happened in either related sub-species,or something common to all,or just in a few cases.