A rock pool on a British shore at low tide with seaweed, limpets and a small hermit crab in a periwinkle shell on the sandy bottom
AI-generated image, not a photograph of a real person or place.
liveborrow

BRITAIN'S HOME-BORROWERS

Somebody else's
home.

BORROWED shells, nests, colonies

  • Crabs borrow empty shells
  • Cuckoos borrow nests
  • Cuckoo bees borrow colonies
  • Three ways to live borrowed

Some animals never build a home of their own.

On British shores and in British meadows, a few creatures live in shells, nests and colonies made by someone else. Some borrow what is empty. Others move in while the owner is still there.

Updated

Meet a borrower

THE SHORT ANSWER

Why build
when you can borrow?

Borrowing a home saves building one.
Some borrowers pay the owner back. Many do not.

ON THE SHORE

The hermit crab's shell

Hermit crabs live in empty sea snail shells, especially whelks and periwinkles, and move into ever larger shells as they grow.

Meet a borrower

IN THE NEST

The cuckoo's egg

Female cuckoos build no nest. Each removes a host's egg and lays her own in its place, often in a Meadow Pipit's nest.

See the numbers

IN THE COLONY

The cuckoo bumblebee

A female cuckoo bee enters a social bumblebee nest, kills the queen and takes it over. The workers then raise her young.

Spot them yourself

MEET A BORROWER

How much does it
take from the owner?

Pick a borrower to see how it gets its home. The bar shows how much the borrowing costs the original owner, from nothing at all to a lost nest or colony.

Borrower check

Choose a borrower
Hermit crab, empty shellCosts nobody

The crab moves into a sea snail shell that is already empty, and swaps it for a bigger one as it grows.

Hermit crab, shell fightEviction

One crab may knock on another's shell and fight to evict the owner. The contest goes on until one of them wins.

Cuckoo chickTakes all the food

The cuckoo egg hatches after just 12 days and the chick takes all the food the host brings. Its foster parent keeps feeding it for two more weeks after it leaves the nest.

Cuckoo bumblebeeTakes the colony

The female kills the resident queen and takes over the nest. She makes no workers of her own and collects no pollen.

Sources: RSPB Cuckoo page; Bumblebee Conservation Trust cuckoo bumblebee guide; North Wales Wildlife Trust and Scottish Seabird Centre hermit crab pages.

Based on RSPB, Bumblebee Conservation Trust, North Wales Wildlife Trust and Scottish Seabird Centre species pages.

TWO NUMBERS WORTH KNOWING

Many nests,
many borrowers.

How many nests one cuckoo borrows in a season,
and how many UK bumblebees live borrowed.

12–22

Eggs a female lays in a season

The RSPB says a female cuckoo lays between 12 and 22 eggs over a season, each in a different nest, and tends to choose the same host species that reared her.

CUCKOO · RSPB

6 of 24

UK bumblebees that are cuckoo bees

The Bumblebee Conservation Trust says six of the UK's 24 bumblebees are cuckoo bees. They are pollinators too, and not a major threat to social bumblebee populations.

BUMBLEBEES · BBCT

The cuckoo is Red-listed in the UK. BTO, which has satellite-tracked cuckoos since 2011, says their numbers have fallen by over 30% since 1995.

FOUR WAYS TO SPOT THEM

Look twice
at the shell.

A dark-winged bumblebee with a white tail resting on a purple knapweed flower in a summer meadow
AI-generated image, not a photograph of a real person or place.

You do not need special kit, just a slow look at the right place in the right season.

  1. Watch for a walking shell

    In a rock pool, a periwinkle or whelk shell that crawls is probably a hermit crab. They are found around all UK coasts, all year.

  2. Look for the big right claw

    The common hermit crab is reddish brown and its right pincer is larger than its left.

  3. Listen in late spring

    Cuckoos lay from late April through June. Only the males call 'cuck-oo'; females make a bubbling sound.

  4. Check the bee's legs

    Cuckoo bumblebees collect no pollen, so they have no pollen baskets on their back legs.

BE A GOOD VISITOR

Leave the home
as you found it.

A small ground nest in grass holding several dark speckled pipit eggs and one paler egg
AI-generated image, not a photograph of a real person or place.

Hermit crabs sometimes end up in rock pools after rough weather scatters them from their usual homes, which reach down to about 150 metres below the surface.

The Scottish Seabird Centre asks rockpoolers to leave everything as they found it, and that includes putting a hermit crab back with its borrowed shell where it came from.

Questions

Questions people ask

Where do hermit crabs get their shells?

From sea snails. Wildlife trust pages describe them living in empty or discarded sea snail shells, especially whelks and periwinkles, and moving to larger ones as they grow.

Why does a hermit crab need a shell?

It has tough pincers but a soft body, which it coils up inside its borrowed shell, gripping on with its hooked tail.

Which birds raise young cuckoos?

The RSPB says Meadow Pipit nests are particularly favoured, and Dunnocks are among other hosts. Females tend to choose the species that reared them.

Why do adult cuckoos leave Britain so early?

With other birds raising their young, adults are free to leave early. Most head to Africa during June; young cuckoos leave later, once fully fledged.

Are cuckoo bumblebees bad for bees?

The Bumblebee Conservation Trust says they are important pollinators in their own right and not a major threat to social bumblebee populations.

Sources

Where the facts come from

  1. RSPB: Cuckoo bird facts
  2. BTO: Cuckoo Tracking Project
  3. Bumblebee Conservation Trust: Cuckoo bumblebees identification guide (PDF)
  4. North Wales Wildlife Trust: Hermit crab
  5. Scottish Seabird Centre: Common hermit crab

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