Why Did Blue Tits Open Milk Bottles? A British Mystery

How did the milk-bottle habit begin?

The first known report comes from 1921, from Swaythling, a district of Southampton in southern England, where tits were seen removing the caps of milk bottles left at the door. In the 1920s and 1930s more reports came in from other towns, and by the 1940s the habit was widespread in Britain.

At the time, milkmen delivered fresh milk early each morning, and bottles often stood on doorsteps for hours. The milk was not homogenized, so the cream separated and floated to the top as a thick layer. For a small bird that needs a lot of energy to survive a cold night, this was a rich, easy meal.

The birds used several methods. Some pecked a hole through the thin metal foil and drank through it. Others tore the foil or cardboard cap away in strips, or lifted it off completely. Blue tits sometimes followed the milkman along the street and attacked bottles soon after they were left.

What did scientists find?

In 1949, the ornithologists James Fisher and Robert Hinde published a famous study of the habit in the journal British Birds. They collected reports from across Britain and mapped where and when birds had been seen opening bottles.

Their survey showed that the behavior did not spread evenly from one starting point. Instead, it appeared in several separate places and then spread outward from each one, more quickly than birds could move on their own. Blue tits and great tits were by far the most common culprits, but a variety of other species, including coal tits, house sparrows, robins and blackbirds, were also recorded. Similar reports came from the Netherlands, Sweden and Denmark.

Fisher and Hinde concluded that a few birds had probably invented the trick independently, and that others then picked it up from them. Because tits are short-lived, the habit could only last for decades if new generations kept learning it.

Was it social learning?

The milk-bottle story became one of the best-known examples of animals learning a new behavior from each other, often called social learning. But how the birds learned has been debated.

There are several possibilities:

• Imitation: a bird watches another open a bottle and copies the exact movements.
• Local enhancement: a bird sees another feeding at a bottle, or finds an opened one, and is drawn to bottles. It then works out how to open them by trial and error.
• Independent invention: curious birds discover the trick on their own.

In 1984, the researchers David Sherry and Bennett Galef tested these ideas with black-capped chickadees, close American relatives of the blue tit, using small cream containers with foil lids. Birds that had watched a trained chickadee open a lid, or that had simply found lids already opened, learned the task more often than birds with no such experience. This suggested that much of the spread could happen without true imitation: seeing that bottles were worth investigating was often enough.

Later work showed that tits really can learn foraging methods from each other. In a 2015 study in Wytham Woods near Oxford, researchers trained a few wild great tits to open a puzzle box in one particular way. The trick spread through the local population, and the birds kept using the same method over the following years, a kind of local tradition.

Why were tits so good at it?

Blue tits and great tits are naturally inquisitive. They feed by pecking, prying and tearing at bark, buds, seed heads and leaves to reach hidden insects, so stripping a foil cap uses skills they already have. They are also light and agile enough to cling to a narrow bottle top. Our Eurasian blue tit profile describes this acrobatic feeding style.

In winter, tits move around in mixed flocks and spend a lot of time near houses and gardens. That gave them many chances to watch each other and to come across opened bottles. The cream itself was an excellent winter food, rich in fat and energy.

Why did the habit fade?

Blue tits have not forgotten how to peck, but their opportunity has mostly gone. Several changes made milk bottles much less rewarding:

• Fewer doorstep deliveries: from the 1970s onward, more people bought milk in supermarkets, in cartons and plastic bottles, and fewer bottles stood outside.
• Homogenized milk: most milk is now treated so the cream does not separate, so there is no rich layer at the top.
• Lower-fat milk: semi-skimmed and skimmed milk became popular, leaving even less fat to drink.
• Covered deliveries: some households used covers or boxes to protect their milk.

The behavior is still reported occasionally where doorstep milk deliveries continue, but it is no longer the everyday sight it was in the mid-20th century.

What the story tells us about birds

The milk-bottle episode showed scientists that small songbirds can adopt a completely new behavior and spread it through a population within a few generations. It helped launch research into animal innovation and culture, which today includes studies of crows using tools and whales sharing songs.

It also shows how closely blue tits live alongside people. They nest in garden boxes, raise very large families each spring, as explained in how many eggs does a blue tit lay?, and remain among the most familiar garden birds in the UK.