Why do penguins give stones to each other?

THERE'S NOT A GREAT CHOICE OF NEST-building materials in the Antarctic. Pretty much the only things available to gentoo, Adélie and chinstrap penguins are pebbles, which at least serve to raise eggs off the frozen ground.

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Pebbles don't collect themselves and can be hard to find. The males tend to do the bulk of the work and will readily steal from their neighbours to fulfil their parental duties. There are plenty of videos out there of penguins passing the stones from beak