Via Google News, I found this wonderful piece of “The Birds” memorabilia for sale on the Movie Poster Studio web site…
An article on the CinemaRetro web site goes into more details…
The concept behind this particular bird-brained scheme was to allow members of the public free entry to screenings, as long as they were wearing the mask, and as long as they could match a number printed on the back of the mask with a list at the cinema box office.