It is well known that proper perching surfaces are crucial for the health of captive birds. Raptor enthusiasts often describe this as "padding" the perch. This is unfortunate, because it lends itself to a gross misunderstanding of the true nature of the problem. Fortunately, the methods used to pad a perch do often provide for the actual needs of the raptor's feet.
Those who keep other sorts of perching birds in captivity generally refer to the need for a variety of shapes, textures and sizes in the perching surfaces to keep their feet healthy. This much more accurately portrays what is needed to keep raptor feet in prime health. As with other birds, constantly placing pressure on the same points of the foot creates sores which can progress to serious threats to the raptors health. In the wild, raptors are continuously exposed to a variety of diameters, shapes, and textures in the surfaces they are standing on. It takes some forethought to replicate this kind of environment in a captive situation.
Another important function of the perching surface is to help wear away the talons and beak at just the right rate to prevent overgrowth. There are also other factors besides the perch that keep the talons and beak in proper shape, but it is also prudent to keep this function in mind as a secondary benefit to the perching surface chosen.
The most natural perching surface to provide for the perching raptors is a carefully selected tree branch with the bark left on. The bark will strop the beak and talons as the bird scrapes them on the perch, and the branches imperfect shape as well as the bark texture will ensure that the hawk is varying the portions of its feet that are in contact with the perch. However, branches have a couple of very serious disadvantages. First, is that it is difficult to use them on a hawk that is tied down without creating a danger of the leash fouling. Secondly, they rapidly wear out and require replacement. Thirdly, its almost impossible to incorporate a "bark on branch" perching surface into most modern styles of metal perch.
Astroturf has been a very popular perching surface, as has rope wrapping. Both surfaces are an acceptable alternative to natural bark, but the perching surface they produce is not as variable as natural bark. They also tend to foul the leash where they come in contact with the metal structure of the perch because it is difficult to get them to taper smoothly. Both surfaces also wear out and need regular replacement. Additionally, rope wrapping has been known on rare occasions to trap a talon between the strands, which can lead to a talon being ripped out. Rope perching surfaces, in particular, are excellent for helping to keep the talons and beak trimmed by providing a rough surface that wears the talons and beak down.
I have spent several years developing what I believe to be the ultimate replacement for a natural bark perch. I call it synthetic bark. It is made of a natural cord substrate, that is wrapped in several successive layers. The imperfections in the cord, along with the variations in spacing and tension with each successive layer of wrapping combine to create a variation in the perching surface. I also consciously build some areas of the perching surface to thicker diameters than others in order to intentionally enhance the variable dimensions of the perching surface. These strands are all bonded together to form a durable solid surface. This completely alleviates the danger of a talon becoming trapped between the strands of the cordage. It also creates a continual wear layer. As the perch is worn away, the properties remain consistent. I have not had the opportunity to age test the wear on synthetic bark, but I would expect it to last for several decades. I have several perches that have seen daily use for 17 years and they are as good now as the day they were made - including the perch in the first image on this page. I believe in this method so much I will not accept any other perching surface for my metal perches.