When do frogs emerge from hibernation (and how do we know)?

In the June 1999 issue of British Wildlife, Ken Tyson and Roger Smith (1999) described the seasonal emergence pattern of Common Frog Rana temporaria spawn. They related the first spawning dates for a site near Okehampton in Devon between 1990 and 1999 to the accumulated day degrees of temperature above freezing from the shortest day (21st December) to