You cannot hide on a gravel area, unless you become invisible. This is how the common terns do it. A nest of branches would be too obvious. Therefore, the eggs are placed in a shallow hollow. This prevents the eggs from rolling away. The camouflage colour then makes the eggs look like pebbles and the clutch is difficult to detect. Some common terns still give their nest a personal touch and decorate small stones around the nesting site. Nest sites nearby to plants or larger stones are also popular. Probably the birds can thus better recognize their nest.
Even the little chicks are well camouflaged. At first, they duck into their nesting site and rely on their down plumage, which makes them look like stones from a distance.