At one time milk was delivered to British doorsteps in bottles with a card disc as a stopper. In the 1920s, bluetits learned how to remove the card stoppers to get at the cream. It seems that the habit began independently in as many as three different bird populations in London. But by the 1930s, birds across much of southeast England were carrying out these doorstep raids, too.
When card tops were replaced by foil caps, the tits learned to pierce the foil. Later, many other bird species, including magpies, learned to do the same. Within a matter of decades, the ability to remove milk-bottle tops spread through the nation's tit population.
It seems likely that, rather than learning the technique from watching others, individual birds solved the problem of getting at the milk for themselves. Why they should all think of it at once is not clear. Maybe a bird removing a bottle top attracts the attention of others to the fact the food is to be had by solving a problem. Each bird then works out a solution for itself by trial and error. Just seeing the result, and not the behaviour, seems to be enough for many birds.