Small blue macaw

World's rarest bird 56cm long Dark cobalt blue plumage, black bill, dark grey legs Lives in Brazil's arid tropical mountain savannah

The bird went missing more than five months ago in the semi-arid Curaca region in northeastern Brazil.

Brazilian Environment Ministry officials say that if they are unable to find the bird, the macaw will join the list of species thought to have vanished from the wild.

There are 66 of the birds still in captivity - six of them in Brazil. But with no wild population, reintroducing them will be extremely difficult, experts say.

Endangered species

The species - Cyanopsitta spixii - has always been rare since the birds were first recorded by an Austrian naturalist in the last century. Their numbers have since dwindled because of destruction of their habitat and hunting by bird collectors.

In December a team from the Brazilian Environmental Institute (IBAMA) tried in vain to find the bird in the Curaca region, some 600 km (370 miles) north of Salvador de Bahia.

Severino had never before left his territory for more than 15 days. Scientists were hoping early next year to reintroduce five other birds into the area which have been raised in captivity. But without Severino to show them how to survive in the wild, that will be extremely difficult.

Severino had built up a mass of knowledge on where to find water, which nuts to eat and where to find them and how to avoid predators, the BBC's Brazil correspondent Tom Gibb reports.

Biologist Francisco de Assis Neo, quoted by the French news agency AFP, said that at this time of year "the birds make their nests and become more timid and, therefore, more difficult to find".

Severino had previously mated with a female parrot, but the pair never produced any offspring.

One problem facing the scientists is that small blue macaws are difficult to sex. But Dr Richard Griffiths and Bela Tiwari of Oxford University have devised a technique to sex the endangered bird from minute samples of DNA taken from moulted feathers.

According to Mr Griffiths, it is "not a very adventurous bird and it's a bit of an evolutionary loser".

Ninety species of parrot are threatened with extinction and zoos worldwide are working hard to try to save them.