For 60 years the blue tits of Britain broke into the milk on your doorstep, and all they ever took was the cream. It was first seen in Swaythling, near Southampton, in 1921. A blue tit landed on a bottle, pecked through the cap and drank. By the 1950s every tit in the country seemed to know the trick. Housewives put out saucers, stones and upturned cups. The birds learned to follow the milk float down the street and be waiting at the next step. You will remember the bottle. Silver foil on top, and beneath it 2 inches of thick yellow cream that had risen overnight, with the white milk underneath. The first one down in the morning got it on the porridge. That cream was the whole point of the burglary. A bird cannot digest milk. It has no means of handling the sugar in it. Cream is nearly all fat, with very little sugar, so the tits drank down to the line and stopped. A wild animal weighing 11 grams, with every bird table and berry bush in the street to choose from, worked out how to open a sealed container to get at butterfat. Then the habit died, in about 10 years. Nobody trapped them or changed the caps. The milk changed. It was homogenised so the cream no longer rose, then skimmed so it was hardly there, then moved to a plastic jug in a supermarket. The blue tits inspected the red top, found nothing in it worth stealing, and went back to the hedge. They were the last creatures in Britain to judge a pint of milk on what was actually in it.