lower leg bone, in birds consisting of the tibia fused at the distal end together with the proximal (upper) tarsal bones (ankle bones in humans), between the knee joint and the intratarsal (ankle) bone; the longest leg bone in most birds, including eagles; parallel with the much narrower fibula, which fuses to it about two-thirds of the way down toward the intratarsal joint (ankle); photo of tibiotarsus with fibula on lateral side © Idaho Virtual Museum as licensed by Creative Commons