For the majority of bird carers, putting a few sticks in for the birds to sit on is all it takes. Unfortunately, a lot of people don't realise that there is a lot of bird psychology linked to perches, as well as bird health.
With raptors, you have birds with a stocky, heavy body that would spend quite a bit of time on the wing, and have large spaces for manoeuvring when coming in for a landing. In captivity they tend to sit a lot more, get fatter than they would in the wild, and they can drop on their perches with a lot of force. This makes them very susceptible to a complaint called bumble foot. It can start out as pressure sores, or a little cut, or puncture. Because the underside of the foot is continually being whacked when the bird lands, and is under pressure while the bird sits, the wound gets progressively worse. Once a wound is open, it will get infected- raptors use their feet to hold on to meat, which is a good source of the bacteria that live in wounds.
Bumble foot is difficult to treat. This is because the birds don't have a lot of 'meat' in their feet, so they can't afford to loose much from an infection. Bird feet are also not that well supplied with blood, compared to the muscles in the body. This means that antibiotics do not circulate into the feet through the blood that well. Bits of infection are walled off by the immune response, and the infection can persist in pockets. A raptor with red swollen pads is cause for grave concern and immediate treatment- and perch renovation.
Prevention of bumble foot is much better than the possibility of a cure. 'Stage 3' type bumble foot is the most advanced stage. The only way to treat this is with invasive surgery. Really well designed housing and excellent nutrition help greatly with healing. Lotagen can be applied directly to the feet, or antibiotics in a DMSO solvent. There may be a long recovery. Complete recovery isn't that likely, and in many cases where an infection is advanced, the bacteria enter and start to dissolve bone. At this point the only choice is euthanasia, as a one footed raptor is guaranteed to get bumble foot in captivity.
Raptor perches can be made of natural tree branches. Red gum (Marri) is a good choice, as it is easy to get from tree prunings, and the bark is 'soft'. The corky texture of the bark is good for absorbing the impact of landing, and it has antibacterial properties. Smooth bark or bare wood should be avoided unless it has good texture, and is placed in a situation where the birds won't land hard or sit there constantly. Rough rocks are good ground perches, as are stumps if they are slanted to drain water. An excellent alternative to natural materials is astroturf. This is the fake grass carpet used on indoor sports grounds and on back verandahs. There are different types and qualities. The short leaf variety is easier to clean, but is not as good as the long leaf for absorbing impact. For extra padding, you can use foam, pipe insulation or another layer of turf, under the outside layer of astroturf. Duct or cloth tape can be used to attach it to the perch underneath, or you can use very tight electrical ties. If you can slide something under the wire tie to cut it, it isn't tight enough. It has to be tight enough that it won't catch a bird's talon. Sisal rope wound tightly around a perch is also a good surface, though again, you have to make sure it is tight enough the birds can't get their talons stuck and rip a nail off. Coconut fibre doormat is another good surface, and is used on stumps or shelf type perches.
The actual design and how the perches are put up is extremely important. PVC water pipe is great for making perches, as it flexes and sways (like a tree) when the birds land. It does not rot, and is easy to rig so that it can be taken down to replace the covering on the perch. Long branches bolted on to an upright near one end will also bounce or flex a bit when landed on. Bungee cord, or ockie straps can be used on one or both ends of perches. Make sure the vertical cords are not going to get in the birds way when they fly though, or they may damage feathers. Feathers can also get damaged if perches are attached directly to a wall, or across a corner. They rub their feathers when turning, or hit the wall when opening their wings to fly.
Perch psychology is something that is often overlooked. I can't tell anyone how to set up their flight pen without seeing it, because every pen and every bird is different. A pen that is set up correctly for kestrels is not going to be appropriate for eagles without changing the perches, or vice versa. This is partly because of the size range- the eagles won't be able to sit on the smaller diameter perches, and the kestrels need those for the most comfort. Eagles also need bigger open spaces to move gracefully, so cut back any foliage. If your birds have difficulty moving around, or if they are too exposed, they will not feel comfortable.
Another factor in perch psychology is how many birds share the aviary. Raptors are not particularly social, and won't often share a perch. Always ensure that there are enough high perches to go around. A long high perch with a dip in the middle or overhanging foliage may count as two. A perch that is lower than the others will not be favoured, and any bird pushed into that spot will likely challenge others for their taller perches. I have heard some people say that this promotes activity- I say it promotes stress. Stress will inhibit recovery and damage health. If the birds aren't moving much because all the other perches are claimed, your aviary is too crowded. If you put too many birds in a pen with too few perches, someone may get territorial enough to reduce their numbers for you!
A ladder perch can be a ladder with astroturf covered rungs, or a set of perches that partially flighted birds can use to get to higher perches. A couple of roofed areas is a good idea so the birds can choose to move out of the rain or hail without crowding eachother. Owls and some diurnal raptors also like shelter boxes to hide in.