the.com/iron bird

means A non-flying test rig built with an aircraft's real hydraulic, electrical, and mechanical systems, used to verify they all work together before anything leaves the ground.

from Aerospace engineering slang, most likely from the rig's bare-bones look: a steel skeleton or framework strung with tubes, wires, and actuators, standing in for a fuselage, with no skin, wings, or paint to soften it. It's 'bird' the way pilots and engineers casually call any aircraft a bird, crossed with 'iron' for its stripped, skeletal, unglamorous hardware — a plane's nervous system laid bare on a test stand.