In fact, besides birds, distal fibular reduction also occurred independently within at least three other lineages of Ornithodira: Alvarezsauridae (Chiappe et al. 2002), Oviraptorosauria (Vickers-Rich et al. 2002), and Pterosauria (Dalla Vecchia 2003; Bonaparte et al. 2010; Fig. 7).
In pterosaurs, the tibia is much longer and more slender than the femur, and the fibula is considerably reduced as in birds.
The fibula is reduced and adheres extensively to the tibia, usually reaching only 2/3 of its length. Only penguins have full length fibula.
Terrestrial flightless birds show adaptations to weight-bearing by legs alone.http://www.sciencealert.com/scientists-have-grown-dinosaur-legs-on-a-chicken-for-the-first-time
To figure out how this evolution occurred, researchers in Chile have manipulated the genes of regular chickens so they develop tubular, dinosaur-like [tetrapod-like] fibulas on their lower legs - one of the two long, spine-like bones you’ll find in a drumstick.
While modern bird embryos still show signs of developing long, dinosaur-like [tetrapod-like] fibulae, as they grow, these bones become shorter, thinner, and also take on the splinter-like ends of the Pygostylian bones, and never make it far enough down to the leg to connect with the ankle.
The shank (zeugopod) of most tetrapods has two equally long bones—the medial (inner) tibia and the lateral (outer) fibula. In early theropod dinosaurs, which are bird ancestors, both bones were equally long, although the fibula is more narrow and in close contact to the tibia. This condition was still present in the basal bird Archaeopteryx (Ostrom 1973; Mayr et al. 2005). Within the Pygostylia, closer to modern birds, the fibula became shorter than the tibia and splinter-like toward its distal end, no longer reaching the ankle (O'Connor et al. 2011a). In modern birds, the fibula is typically about two-thirds the length of the tibia, but fibulo–tibial proportions show considerable evolutionary variation, with proportionally shorter or longer fibulae in different species (Owen 1866; Streicher and Muller 1992).
The reason why this happens though remains a bit of a mystery. Modern birds of different sizes and ecologies all show evidence of this fibula reduction. This suggests that it is what is called a ‘non-adaptive’ process, as it is highly unlikely that such a feature would play a part in such different roles.http://teachmeanatomy.info/lower-limb/bones/the-fibula
The fibula, along with the tibia, makes up the bones of the leg. The fibula is found laterally to the tibia, and is much thinner. As it does not articulate with the femur at the knee joint, its main function is to act as an attachment for muscles, and not as a weight bearer.
So when the fibula is reduced it is no longer acts so much as an attachment for muscles.
Below your knee you don’t have a chicken-like drumstick. You have two leg bones: the thicker shinbone (the tibia) and the thinner fibula. The knobs on the sides of your ankle are the lower ends of these bones. These paired bones are a great design, and the hind legs of dinosaurs and of most other tetrapods are similarly equipped. Though the fibula is much thinner than the tibia and supports comparatively little weight even among those of us who walk on two legs, it serves an important role in stabilizing the ankle and providing leverage for the muscles attached below the knee.
But a bird’s lower limbs are designed differently. A bird’s knee and the thighbone above it are hidden up inside the bird’s body. This hidden part of the bird’s leg helps prevent abdominal air sacs from collapsing and helps it breathe. Because it walks with its hips and legs at different angles from biped humans and theropod dinosaurs, a bird doesn’t need the stability and leverage afforded by a full-length fibula and its attachments. The fibula on a chicken is very thin and so short it doesn’t even reach the ankle, leaving the bird to support its weight on its tibias.