The phrase 'τὴν Βασίλειαν' is an elliptical construction where the substantive noun is omitted. In this context, 'Βασίλειαν' functions as a substantive adjective referring to 'τὴν Βασίλειαν πόλιν' (the royal city) or, more likely given the play's plot, 'τὴν Βασίλειαν ἀρχήν' (the royal power/kingdom). This usage relies on the audience's immediate recognition of the specific political entity being established by the birds, rather than the standard grammatical expectation of an explicit noun following a definite article and adjective. The omission serves a rhetorical purpose, emphasizing the abstract concept of sovereignty as a tangible object to be possessed alongside the protagonist.