If you see a bird sitting on the perch, you will notice that the toes grip the perch. The leg continues towards the rear of the bird, where it bends forwards at a right angle like the human knee. This initially looks as if the bird's knee is bending backwards, i.e. the other way round than the human knee.

What you can see, however, is not the bird's knee. The long toes give the deceptive impression that they are already the whole foot. In reality, however, the foot continues further back. The bent joint that you can see is therefore the ankle. 

The knee of the bird sits a little further up and is usually concealed in the plumage.