India, May 3 -- In the 18th century, the seasonal disappearance and reappearance of birds was a phenomenon that stumped naturalists. What could be happening to them?

The Greek philosopher Aristotle (384-322 BCE) believed that robins and redstarts in Greece were perhaps the same species, miraculously transforming into each other as seasons changed. Robins in winter would become redstarts in summer, he theorised (although robins did migrate to Europe at the beginning of summer, coinciding with when redstarts started flying in from Africa, this was of course an inaccurate guess on his part).

He also assumed that certain birds, such as swallows, possibly hibernated, perhaps at the bottom of ponds, in colder months.

What changed this prevai...