At the beginning of the 20th Century milk was delivered to British doorstops in bottles that had no tops so birds had easy access to the fat rich cream that settled at the top of the bottle. Birds lack the enzyme necessary to digest lactose, and milk will actually cause birds to suffer diarrhoea. The cream, however, has little lactose in it and is fully of energy – this is what the birds were after.

Two species of British garden birds learnt to siphon up the cream from the tops of the open bottles; blue tits and robins. And although they were not as intolerant to the cream as they are to milk, both blue tits’ and robins’ digestive systems did undergo some adaptation to cope with the new fat rich nutrients. This process certainly happened through Darwinian selection, a brilliant example of evolution in action.

Then after the First World War dairies began to seal the bottles with aluminium foil bottle tops to keep the milk fresher and potentially this new food source could have been cut off from the birds.

However, by the 1950s the entire UK blue tit population had learnt how to pierce the bottle tops to reach the cream, whereas the robins never did. Occasionally an individual robin learnt how to pierce the milk bottle seal but the skill never spread to the whole population as it did with blue tits.

Blue tits, as a species, had gone through an extraordinary learning process. Although individual robins could be as innovative as blue tits they had not passed on what they had learnt to other robins. The difference between the two birds could not be attributed to their ability to communicate. Blue tits and robins communicate in similar ways through colour, behaviour, movement and song. The difference could only be explained in the way blue tits pass on their skills from one individual bird to the species as a whole.

When bringing up their chicks blue tits will live together in couples. Once the young blue tits are flying and feeding by themselves blue tits will move about in flocks of up to ten birds. The flocks remain intact and will stay together for two to three months.

Robins, however, are territorial birds. A male robin will not allow another male to enter his territory and when threatened will warn another robin off. Their communication is much more aggressive with fixed boundaries that are not to be crossed.

Blue tits had learnt to peck the milk bottle tops by copying each other whereas young robins had had very little opportunity to do learn this skill. Birds that flock together appear to learn faster and increase their chances to evolve and survive.

With the introduction of semi-skimmed and skimmed milk and the decrease of doorstop deliveries this phenomenon has practically died out.