It occurred to me the other day how long it is since I saw an example of this. Of course, there are far fewer foil-topped milk bottles being left out these days - but even so there are some, and all appear intact from my observation, both here in Bristol and in Hampshire. (Reports from elsewhere are of course welcome.) There are 15 million blue tits in the UK - why aren't they helping themselves?
My theory is that once milk bottle numbers had declined below a certain critical mass the opportunities to learn it by watching other tits at work became so few and so isolated that the knowledge was lost, like the reading of Linear-A. In tit terms this represents the loss of an ancient craft, a sad cultural impoverishment.
As to how to put things right, I can only suggest that we capture some young and impressionable blue tits, let them loose in a cinema and play them video of the practice on a loop. If, indeed, any such video exists: all I could find on Youtube was this Second Life version, which omits the crucial information about how to break through the foil.
A bit more Googling yields this article, which mostly confirms my hunch, but makes me wonder additionally whether the homogenization of milk (when did that come in?) has something to do with blue tits dropping it from their diet.