I was riding by the other day on my spotted mare when I saw a butterfly. I think it was a small tortoiseshell, though I can't be certain because it was going like the clappers at the time. The butterfly, not the horse, though we made up for that later.

A butterfly! Mid-February! Is that the earliest butterfly I have ever seen? The answer is that I don't know. That's because I don't keep records. That's the sort of thing that real naturalists do: the rest of us just go oh and wow and how absolutely bloody amazing and walk or gallop on.

There is a listing, dating, note-taking tendency in natural history, and you might, if you were severe, write it off as so much nerdishness. But you'd be wrong. It may have been nerdishness for 300 years or so, but these days it is right up there on the cutting edge: vital information about our planet, what is happening to it and how it is changing.

Robert Marsham started it all. He was born 300 years ago in Norfolk, and had a small estate at Stratton Strawless. He was one of life's write-downers: his habit of meticulousness added up, in the end, to a kind of genius. He kept regular records of the natural events that indicate the passing of the seasons, and did so for more than 60 years. Gilbert White called him “a painful and accurate naturalist”.

As I marvel at the year's first goldcrest song - impossibly high and thin, far up in a line of conifers - so others go to the trouble of writing it down beneath the correct date. If you keep information, you create patterns: and as the year turns, so we see quite clearly that it turns earlier than before.

Norfolk Wildlife Trust has asked its members to report first sightings. One of these is the orange-tip butterfly: the male unmistakably lives up to his name. Norfolk records show that the butterfly appears two weeks earlier than it did 30 years ago. Oak trees are appearing in full leaf three weeks earlier than they did 50 years back. Swallows turn up ten days earlier than they did 30 years ago.

If you don't live in Norfolk, you can report a different suite of first appearances for Springwatch at the BBC and the Woodland Trust. One of these is the red-tailed bumblebee; I saw one of the white-bummed species in London the other week, nosing round a flowerpot outside an antique ship. So it doesn't count for the survey, and besides, I failed to make a note of the date.

Ah well. This practice of compulsive note-taking of times and dates and first flowerings has evolved from individual obsession to full-grown science: Marsham was the world's first phenologist. Conditions change: and suddenly the overlooked and unimportant becomes central and vital. The same thing happened 65 million years ago, when a meteor struck the Earth. The dinosaurs were wiped out and the scruffy, seedy, obscure little mammals took over. Humans are just one of them: the one that is changing the planet.

The white-crowned sparrow that turned up at Cley and brought a million twitchers in its wake is to become immortal. Voluntary donations at the site where it turned up have passed £5,000, which goes to the Cley Church Restoration Fund. It is enough to restore the West Window; there are plans to include an image of the sparrow. Twitching has always been a form of prayer and thanksgiving and glorification.

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