The feat itself does not specify. The Flavor text suggests it modifies the Change Shape ability, which is supernatural, but that is not mentioned in the rules section.
As an adjunct to that question. Can a Kitsune apply 'fast shifter' Alternate Racial Trait to Fox Shape?
PRD wrote:
Fast Shifter (Su): You were born with an innate talent for switching between your natural forms. Using your racial change shape ability is a move action instead of a standard action. This racial trait replaces kitsune magic.
When not stated, all feats and abilities are assumed to be (Ex). This one likely should be (Sp) or (Su), but lacking the designation defaults it to (Ex).
Fast Shifter does not apply to Fox Shape because Fox Shape does not modify the kitsune Change Shape ability. It probably should, but it does not state that it does, so it is an additional ability over and above Change Shape.
Now, this is RAW. Talk to your GM. At my table, Fox Shape would be a modification of Change Shape, and thus be part of a (Su) ability and also benefit from Fast Shifter.
