A birdcage is the bearing housing that rides on a dirt car's rear axle tube, free to rotate around it, carrying the radius rods and the shock and spring mounts that locate the rear suspension independently of housing rotation.
It is standard equipment on dirt Late Models and Modifieds running four-link, swing-arm or Z-link rears. DIRTcar’s Late Model rules allow one axle-housing mount per side, aluminum or magnetic mild steel, assembled as a single piece, and require that it “attach directly to the axle tube with clearance only to permit rotation” with no fore, aft or vertical movement. Up to two radius rods per side attach to it, spaced 6 to 12 inches on the birdcage. Moving those link and shock holes — indexing the cage — is a real setup change, and the bearings and seals inside are consumables that dirt and heat eat. Some designs float the bearing so it can swivel and shed side load.
The birdcage is where rear geometry, shock travel and forward bite all get decided, and a dry or notchy bearing in one quietly changes the car's balance.