Each year thousands of people in the UK contribute towards the science of phenology, reporting the first frogspawn, swallow, butterfly, and so on, so that comprehensive maps can be compiled of the timing of such events.
Predictably most spring events happen first in the West Country and spread north, while in the autumn the reverse is true. The variations can be large from year to year. The 2009 spring was well in advance of 2010, mainly because January and February 2010 were the coldest for 30 years. Last year most species "sprung" later than the long-term average by a few days, but many were 10 to 19 days later than the very mild 2009. These wide annual variations account for why it takes so long to be sure that the climate is changing.
The scientific approach to mapping the seasons began on 10 April 1736, when a country squire noted the arrival of the first swallow in his village of Stratton Strawless, north of Norwich. Robert Marsham was 28. Each year from then on, for another 62 years, he noted the swallows, and 26 other "indications of spring". This popular science really took off when he reported his records to the Royal Society in 1789 and many other country gentlemen took up the pastime.
Remarkably his family kept a continuous record until 1958 of these "indications", giving an unrivalled database from which to track the trends that climate change will bring.
The progress of 2011's spring is reported at www.naturescalendar.org.uk.