A general size range would be 1/2 to 1 inch for cockatiels. However, the ideal perches will vary in size – the natural branch type that many shops carry are great. In the wild, a cockatiel would experience all sizes of branches to land on. There is no such thing as a perch that is too thin or wide – as long as you offer a variety. This exercises the foot and makes the feet stronger. Check a pet shop or online for the perches that attach to the side of the cage and generally only go halfway across the cage. Again, look for the natural wood type. If all of the perches are perfectly round and the exact same size, this is what can eventually cause foot disorders or even arthritis as the bird gets older. Buy a variety of the natural branches and introduce them gradually – only one at a time until your bird is comfortable using it.
Thank you for asking Lafeber,