Great skuas obtain much of their food by kleptoparasitism. A skua singles out a gannet, guillemot or gull returning with a full crop, pursues it closely, and seizes a wingtip or tail until the pursued bird disgorges. The gannet is substantially larger, so the attack works by making flight unmanageable rather than by overpowering.
Piracy is only part of the repertoire. Great skuas also kill seabirds outright, including puffins and small petrels, take eggs and chicks at colonies, and scavenge behind fishing vessels. Diet at Scottish colonies shifted noticeably as fishery discards declined, with more predation on other seabirds, which turned a scavenger into a significant colony predator in some places.
On the breeding grounds the same aggression is directed at intruders: skuas strike walkers who approach nests on open moorland. What remains unsettled is how much of the increased predation on smaller seabirds is a discard-driven substitution and how much reflects changes in fish availability more broadly. The species was also hit hard by highly pathogenic avian influenza from 2021 onward.