Why has spring arrived nearly a month earlier this year than it did in the 1940s?
ITV News Reporter Geraint Vincent has been finding out what the birds' egg laying habits tell us about the seasons
Some birds in one of the most highly studied woodlands in the world are laying eggs a month before they did seven decades ago, suggesting spring is coming earlier and earlier each year.
Ever since 800 acres of woodland was bequeathed to Oxford University in the 1940s, scientists at Wytham Woods have tracked the eating habits of Great Tits living there.
They say that when the birds were first monitored in 1947, the first eggs appeared on April 27, but this year, they were spotted 30 days earlier, on March 28.
Scientists say that the caterpillars the Great Tits are eating are appearing sooner because the oak tree leaves they eat are also appearing earlier on in the season.
The rest of the woodland appears to be in sync, with the caterpillars that the fledglings eat also appearing sooner.
Experts say this is evidence of our changing climate and could signal the start of spring coming more quickly than we are used to.