Adélie penguins breed on bare rock, not ice, and line a shallow scrape with stones. The stones lift the eggs clear of meltwater, which matters because nests can sit in pooled water when snow thaws. A nest may hold hundreds of pebbles, each carried in the bill from wherever the bird can find one.
Suitable pebbles are a limited resource near dense colonies, so theft from neighbours is routine and has been recorded repeatedly by observers. Birds returning to a nest will pick up stones from unattended nests along the way. The result is a persistent low-level churn of material through a colony rather than a steady accumulation.
The story is often told as if the penguins were being deliberately deceitful, or as if stones functioned as courtship gifts in a transactional sense. The observed behaviour is simpler: a scarce building material that is easy to move and hard to guard. What remains less settled is how much nest quality, as opposed to timing and parental condition, actually determines breeding success.