Here's a slightly scary looking Cuckoo on the RSPB reserve at Otmoor. It is a wonderful thing to live in a medium that transmits sound to us like air does. The speed of sound in air depends on temperature. An important reason for this is that the speed of sound in a gas is inversely proportional to the square root of the density of the gas. As the air temperature increases, the air will expand slightly and thus be less dense.
