Let's have a look of traditional characters of 马 and 鸟。马is 馬 and 鸟is 鳥. They are totally different. 马emphasizes long hairs on its neck but 鸟indicates the long tails bird.
The simple characters are changed depending on the calligraphy(cursive character) which is written very quickly and four legs are writen in one stroke.
I am sorry I can't show you the development of the character writing here. It's not easy in typing but handwriting. Maybe you can google some pictures. Good luck!