This is the first example of speciation that scientists have been able to observe directly in the field.

Researchers followed the entire population of finches on a tiny Galapagos island called Daphne Major, for many years, and so they were able to watch the speciation in progress.

The research was published in the journal Science.
The group of finch species to which the Big Bird population belongs are collectively known as Darwin's finches and helped Charles Darwin to uncover the process of evolution by natural selection.

In 1981, the researchers noticed the arrival of a male of a non-native species, the large cactus finch.

Professors Rosemary and Peter Grant noticed that this male proceeded to mate with a female of one of the local species, a medium ground finch, producing fertile young.

Almost 40 years later, the progeny of that original mating are still being observed, and number around 30 individuals.

"It's an extreme case of something we're coming to realise more generally over the years. Evolution in general can happen very quickly," said Prof Roger Butlin, a speciation expert who wasn't involved in the study.

This new finch population is sufficiently different in form and habits to the native birds, as to be marked out as a new species, and individuals from the different populations don't interbreed.

Prof Butlin told the BBC that people working on speciation credit the Grant professors with altering our understanding of rapid evolutionary change in the field.

In the past, it was thought that two different species must be unable to produce fertile offspring in order to be defined as such. But in more recent years, it has been established that many birds and other animals that we consider to be unique species are in fact able to interbreed with others to produce fertile young.

"We tend not to argue about what defines a species anymore, because that doesn't get you anywhere," said Prof Butlin. What he says is more interesting is understanding the role that hybridisation can have in the process of creating new species, which is why this observation of Galapagos finches is so important.

The researchers think that the original male must have flown 65 miles from the large cactus finches' home island of Espaola. That's a very long way for a small finch to fly, and so it would be very unlikely for the bird to make a successful return flight.