“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.”

www.britishbirdlovers.co.uk/articles/…