Common terns nest on the ground in the open, on shingle beaches, sandbars and low islands, with only a shallow scrape for a nest. That leaves eggs and chicks visible to gulls, crows, raptors and mammalian predators. The colony's principal defence is collective attack rather than concealment.
When an intruder crosses the colony boundary, birds lift off in a wave, hover above the threat and dive at its head, calling continuously and often defecating on it. Terns will strike a human scalp with the bill firmly enough to break the skin, which is the practical reason that seabird islands are fenced or closed during nesting. The response scales with proximity to nests rather than with the intruder's actual danger.
The common misconception is that this is individual bravery. It works because it is joint: a lone pair mobbing a gull achieves little, and nest survival within a colony tends to be higher toward the dense centre than at the thinly defended edge. What varies, and is not fully predictable, is when a colony abandons a site altogether rather than defending it.