Biologists at Lund University, together with their colleagues from Portugal and the UK, have now proven that the endangered São Tomé grosbeak is the world's largest canary -- 50 per cent larger than the runner-up.

The São Tomé grosbeak is one of the rarest birds in the world and can only be found on the island of São Tomé in the West African Gulf of Guinea. After the bird was discovered in 1888, another 101 years went by before it was spotted again by birdwatchers.

Until now, it has been categorised as Nesospiza -- "the new finch" -- but new DNA analyses, performed by the researchers, show that it is a canary or seedeater of the genus Crithagra.

The São Tomé grosbeak is distinguished by its size (20 cm long), flat head and very large beak.

The island nation of São Tomé and Príncipe has never been attached to the mainland. Its 1,000 square kilometres contain a total of 28 endemic bird species. This can be compared to the 22 endemic species found on the Galápagos, which is 100 times larger.

Because the small islands have been isolated for so long, several species have evolved rapidly and distinguished themselves from their relatives on the mainland -- a phenomenon known as the "island effect." The seclusion of an island involves an evolution by which some species develop so-called gigantism -- they become giants. The opposite evolutionary process -- that animals become smaller -- is also common.

São Tomé and Príncipe have been inhabited for more than 500 years, but have remained fairly intact. In fact, there is still no documented extinction of a species on these islands, although presently some species are critically endangered.