Feet

Now before you start thinking that I have a foot fetish, I am talking about the feet of flying birds. Most people will think that the foot has four toes of which three are pointing forward and one is pointed rearward. That is called anisodactyl.


You will be correct except that there are a lot of exceptions. And I do not mean webbed feet. Take the case of the kingfisher. Look carefully at its both its outermost toes.  


Here is another picture of a similar foot.


Both birds' outermost toes are not able to splay outwards like most birds. This type of foot structure is called syndactyl.

There are cases where the foot consists of two forward facing toes and two rearward facing toes (zygodactyl or heterodactyl). Notice the parakeet's foot in the centre. 


You can see the same foot structure in the picture below.


Woodpeckers are supposed to have the ability to change the front toes to face rearward when needed.

Update: 

It turned out that I was incomplete about the toes arrangement of birds. There are birds that have less than 4 toes. Those walking birds are left out of the conversation. For birds whose feet are splayed in the X shape, it turned out that there are two kinds. Zygodactyl where toes one and four are facing rearward. Heterodactyl where toes one and two are facing rearward. Trogons are the sole survivors with this trait.

There is also Pamprodacty. That is when both toes one and four can turn forward or rearward.  

I was also incorrect in saying it is the feet of the birds as it is their toes that are being discussed. The backward knees are actually the ankles of the birds.