They're hungry, probably.
Thousands of dolphins have appeared off the Welsh coast in recent days, to the consternation of locals - but not experts. "This seems like feeding behaviour," says Dr Mauvis Gore, a zoologist at the University Marine Biological Station, Millport. Such displays are not unprecedented in waters with plenty of fish. Areas with a lot of mackerel tend to attract large groups of dolphins such as this one. A similar display was seen in the Firth of Clyde, near Glasgow, last year.
It is very hard to track dolphins, Gore explains, and little is therefore known about how widely they travel. While dolphins often return to the same areas to calve, their migration between feeding areas is far less predictable - so these animals may well have come from far away. Global warming may have a part to play too, she says. Growth of phytoplankton, the microscopic plants at the bottom of the food chain, is promoted by warmer seas, and this may have boosted fish numbers in the Irish Sea.
Tourists hoping to see the cetaceans should be careful: "Dolphins have high cognitive awareness," Gore says. Boats sailing too near are likely to frighten them, and they might never come back.