Two birds, which have never been recorded in the UK at the same time and place, have been spotted in the Isles of Scilly, off Cornwall.
A red-footed booby, native to the Galápagos Islands, was spotted on top of Bishop Rock Lighthouse on Monday, with a brown booby at the bottom steps.
The rare red-footed species of tropical seabird had been recorded for the second time in the UK near the Isles of Scilly on 16 August.
Lucy McRobert, from the Isles of Scilly Wildlife Trust, said having a "pair of boobies" at one location was an "unbelievable sighting".
She said: "This is a bird of tropical waters ... they shouldn't be seen around the UK at all, so the fact that there's one healthy and it's fishing was an unbelievable sighting."
Ms McRobert said the red-footed booby had been sitting atop Bishop Rock "reasonably reliably" for a fortnight, and had then been joined by the brown booby.
She said about 600 twitchers - people who goes to great lengths to view new bird species - had come to see the red-footed seabird.
"It is just unprecedented in birding terms; you couldn't script this, it is unbelievable."
Ms McRobert said the trust believed "warming sea temperatures have pushed the birds out of their natural territories ... it's the only thing we can assume right now".