|Film Bulletin, 4 March 1963 (via)|
No, Hitch had really meant "The Birds is coming". Technically, a grammatically correct phrase, The Birds being the title of the film, thus a singular subject taking a singular verb. However, like the Universal executive, a lot of people didn't see it that way. With billboards everywhere advertising "The Birds is coming", language lovers were appalled by what they considered a grave grammatical error and even sent letters to newspapers to express their indignation. In the end, Hitch's slogan and the commotion it caused gave the director exactly what he wanted — more publicity for his film with the interest of the general public piqued. (As Evan Hunter said: "It was pure genius, a seemingly ungrammatical catchphrase that combined humour and suspense.")
Apparently the slogan not only caught the attention of adults but also of children, including a group of school children in New York. On 5 March 1963, a Manhattan school class wrote a letter to Hitchcock (seen below), asking him to correct his error so children wouldn't learn incorrect English.