How do baby birds learn the song unique to their species? Is it drilled into their tiny brains by their parents, or do they hatch pre-programmed?

On Monday, researchers offered intriguing evidence that the distinct melodies of at least some songbirds are encoded in their genes.

Flycatchers taken from the nest as eggs, then hatched and raised by a foster family from a different species, consistently harboured a lifelong preference for the tell-tale tune of their genetic kindred, a study has found.

This can only mean one thing: the ability to distinguish between songs is innate, not taught, said study co-author David Wheatcroft of Uppsala University in Sweden.

"Song discrimination depends on genetic inherited factors," he told AFP.

For scientists, bird song is a model for studying the behavioural, neural and genetic mechanisms that drive vocal learning -- the ability to absorb and reproduce language, whether human or animal.

Songbirds are able from an early age to distinguish between the chants of their own species and those of others.

Researchers have long been trying to unravel whether this ability is inborn or learnt -- the age-old Nature Vs Nurture debate.

For the latest study, Wheatcroft and colleague Anna Qvarnstrom swapped eggs in the nests of two flycatcher species -- pied and collared.

The infants were isolated from bird sound until 12 days after hatching. They were then played recordings of their biological species' song, and the tune of the parents that raised them.

The baby birds reacted more to their own species' call, the team found.

This was measured by how much they chirped to beg for food, how often they looking up at the nest opening, and how much they moved around.

"Nestlings discriminate songs... even when they have been raised completely by parents from the other species," concluded Wheatcroft.

"This shows that song discrimination does not depend on early experience or learning."

When the birds reached adulthood, the team monitored which song they decided to perform themselves.

"They almost universally sing the songs of their own species," said Wheatcroft -- rejecting their surrogate parents' chant.

"Our study is the first to demonstrate that song discrimination depends on genetic differences across species," the researcher concluded.

From an evolutionary perspective, the ability likely helps birds identify members of their own species, and to ensure they do not mate with another. Hybrid flycatchers are sterile.