Jane drain

The migration of women out of an area, leaving a majority of men (cf "brain drain").

Man 1: There seem to be fewer girls around than there used to be...
Man 2: That's Jane drain for you—the women are all moving to the cities.

Jane Drain

A lack of energy or enthusiasm in an office when Jane is not at work.

A: Hey, what's going on today? It's so quiet in here.
B: Yeah, I felt that Jane Drain too. I hope she is back soon.