As shown by this illustration in the SR-71 flight manual, its inlet cones (spikes) retract as its speed increases:
Meanwhile, the Wikipedia page for the MiG-21 describes the opposite behavior for its intake cone:
On early model MiG-21s, the cone has three positions. For speeds up to Mach 1.5 the cone is fully retracted to the maximum aft position. For speeds between Mach 1.5 and Mach 1.9 the cone moves to the middle position. For speeds higher than Mach 1.9 the cone moves to the maximum forward position.
No source is provided for this part of the Wikipedia article. If the info about the MiG-21 is accurate, why does its intake cone behave differently than the SR-71's? Do the two aircraft's intake cones serve different aerodynamic purposes?