Fortunately this turns out not to be the case. I've seen occasional blue tits feeding since then, and the level of food has been dropping slowly. And today they have been at it more or less non-stop, at one point there were three or four jockeying for position. I'm going to need to fill it in a day or two.
The only problem is that pigeons are landing on the window sill, attracted by occasional seeds the tits drop. Not sure what I can do about that, other than fitting some plastic spikes or something.