The vocative is often like the nominative in the singular; in the plural it is always the same. Nominative, vocative, and accusative have the same form in neuter words, and always have α in the
plural. In the dual there are two forms, one for nominative, accusative, and vocative, the other for genitive and dative.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].