Ever wondered why birds are born to peep, chirrup and sing? Surprisingly international avian experts have shown this to be true, literally, after finding fluctuations in bird species' heartbeat responses to their parents' calls -- from inside the egg. Using non-invasive techniques, a study found evidence of prenatal auditory learning in embryos of three vocal learning species (Superb fairy-wren, red-winged fairy-wren and Darwin's small ground finch) and two vocal non-learning species (little penguin and Japanese quail).