A chicken is not a mammal. A chicken is a bird. Though a domesticated chicken doesn't fly very well, its body is made for flight.
The chicken has a short, strong body with a broad, flat breastbone to support its flight muscles. Its legs are strong, both to push off at takeoff and to absorb the shock when landing. Its wings correspond to the hands in humans but lack the finger bones. The tail is short and made to support the tail feathers.
Also, unlike most mammals, chickens lay eggs. When the chicks hatch, the mother doesn't nurse them but leads them out to find food.