In a penguin colony, a good pebble is worth stealing. And nobody’s pebbles are ever quite safe.
By Chinstrap Collective
Penguins steal pebbles because a stone nest keeps their eggs up out of the meltwater, and nests built higher with more rocks raise more chicks. Good stones near a crowded colony run out fast, so chinstraps and Adélies take them from the nest next door, and each pair guards its own pile in shifts.
Picture a rocky hillside by the sea, crowded with penguins. Every one of them is building the same thing: a small, round pile of stones. Then one of them leans over to the nest next door, takes a pebble, and carries it home as if it had always been there.
Welcome to the great pebble heist. It happens every nesting season, and the scientists who study it took it seriously enough to put it in the title of their research.
Why do penguins build nests out of stones?
A chinstrap doesn’t build with twigs. Its nest is a ring of stones about 40 centimetres (16 inches) wide and up to 15 centimetres (6 inches) high (Animal Diversity Web). That ring is where a pair lays its two eggs in late November, and where the two parents take turns keeping them warm for the next five weeks (Australian Antarctic Division).
Why stones? Their Adélie cousins show the reason. In spring, Adélies build their nests from pebbles and pick sloping sites, so that meltwater runs away from the nest instead of into it (Australian Antarctic Division).
And a better pile pays off. In a camera-based study of Adélies, high nests built with lots of rocks were more likely to raise one or two chicks than low nests with only a few (Australian Antarctic Division, 2022). A bigger pile of stones is not vanity. It is a better chance of raising a family.
Which penguins steal rocks?
So what do you do when the good stones nearby are already taken? If you are a penguin, you borrow them, permanently, from whoever is closest.
Chinstraps steal nest stones from each other. Two papers published in the journal Polar Biology in 1995 carry the title “Nest maintenance and stone theft in the chinstrap penguin” (Moreno and colleagues; Carrascal and colleagues). Adélies do it too: the Australian Antarctic Division notes plainly that Adélie penguins take rocks from their neighbours’ nests (AAD).
Notice the two halves of that title. A nest is never really finished: it needs upkeep. And the quickest supply of stones is the nest next door, which belongs to a neighbour who is also doing upkeep, and who is also looking at yours.
Who guards the pile?
If everyone is borrowing, someone has to stay home. Here the penguins have a system any tired parent will recognise: they take turns.
Chinstrap males and females share 33 to 36 days of incubation. Their shifts start at 5 to 10 days each and get shorter as hatching approaches (Australian Antarctic Division). While one parent is off duty, the other sits tight on the eggs, and on the stones.
Gentoo penguins are no pushovers either. When pebbles are available, gentoos build their nests with them and with other objects, lay two eggs, and defend their nests fiercely (British Antarctic Survey, 2018). A gentoo on guard is not a neighbour to borrow from.
Do penguins propose with pebbles?
You have probably seen the story: a penguin finds the perfect pebble and offers it to the one it loves. It is a lovely idea. But BBC Wildlife’s Discover Wildlife looked into it, and its verdict on the proposal story is that “any evidence that it’s actually true is hard to come by” (Stuart Blackman, Discover Wildlife).
What the science does show is less romantic and more practical. In a colony where every pair needs a pile of stones, a pebble is building material, and building material gets borrowed. A gift, a theft, a renovation: to a penguin, it may all be the same pebble.
What the heist teaches us
It would be easy to call it bad behaviour. But the pebble heist is really a story about how much a home is worth when you build it from almost nothing. Every stone in the ring had to be found, fetched and then guarded, by two parents who each spend days at a time sitting on it.
It is also a long game. Adélie penguins return to the same nest and the same mate if they can (Australian Antarctic Division), and gentoos usually pair up with the same partner as the year before (British Antarctic Survey, 2018). Chinstraps first breed at about three years old and can live to be 16 to 20 (Australian Antarctic Division). That is a lot of nesting seasons, and a lot of stones to keep track of.
So the next time you find yourself guarding your favourite mug at the office, or quietly reclaiming your phone charger from a family member, remember: the penguins got there first.
Penguin pebble FAQ
Why do penguins steal rocks from each other?
Because good nest stones near a crowded colony are scarce, and a higher nest with more rocks is more likely to raise chicks (AAD, 2022). Taking a stone from the nest next door is the quickest supply.
Which penguins use pebbles for nests?
Chinstraps and Adélies build their nests from stones, and gentoos use them when they are available (ADW; AAD; BAS).
Do penguins give pebbles as gifts?
That is the popular story, but the evidence for a pebble “proposal” is hard to come by (Discover Wildlife). Stones are nest material first.
How big is a penguin’s pebble nest?
A chinstrap’s is about 40 centimetres (16 inches) across and up to 15 centimetres (6 inches) high (Animal Diversity Web).
More Field Notes: How do penguins sleep? 10,000 naps a day · Chinstrap penguin facts: a day in the life
Sources
- Coulson (2001), Pygoscelis antarcticus, Animal Diversity Web: nest size.
- Australian Antarctic Division, Chinstrap penguin: two eggs in late November, shared incubation and shifts, first breeding at three, lifespan.
- Australian Antarctic Division, Adélie penguin: pebble nests on sloping sites; taking rocks from neighbours’ nests; the same nest and mate.
- Australian Antarctic Division (2022), Penguin heaven: multi-decade science reveals secret lives of Adélies: high, rocky nests raise more chicks.
- Moreno and colleagues (1995), Nest maintenance and stone theft in the chinstrap penguin, Polar Biology.
- Carrascal and colleagues (1995), Nest maintenance and stone theft in the chinstrap penguin, Polar Biology.
- British Antarctic Survey (2018), Study of penguin colonies at Antarctic island shows decline: gentoo nests and pair bonds.
- Stuart Blackman, Do penguins really “propose” with pebbles?, BBC Wildlife’s Discover Wildlife: the evidence for pebble proposals.