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Two species of bird have been observed brooding in the same nest and raising offspring together.

Such cooperative breeding between different species has never been documented before, says at the Stazione Zoologica Anton Dohrn of Naples, Italy. “It is a very strange and rare situation, in which the brood is mixed between the two species,” he says.

The nest-sharing parents were a pair of common redstarts (Phoenicurus phoenicurus) and a pair of black redstarts (Phoenicurus ochruros). Giacomo Bruni, who also worked on the study, spotted them sharing a nest box he had put on the wall of his house in Tuscany, Italy.

The researchers think the two species built the nest together. “Probably one started the work and the other joined later, adding the material,” says Balestrieri.

When they were away from the nest, the two species tried to chase each other away, but when they were on the nest, their interactions appeared to be collaborative.

Three black redstart chicks and two common redstart chicks hatched in the nest. Using video cameras, the researchers observed that each parent fed both its own chicks and those of the other species.

All five chicks successfully fledged. When they collected the nest later, the researchers found that one common redstart egg had failed to hatch.

About 3 per cent of all bird species are known to practise cooperative breeding, in which one or more individuals of the same species help to rear young that aren’t their own. Balestrieri and his colleagues aren’t aware of any other examples of cooperation between two different species, however.

Unlike with parasitic species such as the cuckoo, which tricks other birds into raising its chicks, it seems that both redstart species benefitted from the relationship.

“The young of both pairs successfully fledged. That makes us think of a real collaboration in this extraordinary, rare extended family,” says Balestrieri. “This is something very interesting, which deserves to be studied further.”