Only in the US. In the EU, in particular, APIs cannot be copyrighted[0].
Namely: "For the avoidance of doubt, it has to be made clear that only the expression of a computer program is protected and that ideas and principles which underlie any element of a program, including those which underlie its interfaces, are not protected by copyright under this Directive."
Namely: "For the avoidance of doubt, it has to be made clear that only the expression of a computer program is protected and that ideas and principles which underlie any element of a program, including those which underlie its interfaces, are not protected by copyright under this Directive."
[0] http://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:320...