TODAY (December 21) is the shortest day of the year and throughout the UK, hundreds of birdwatchers were up at dawn to record the first birds to arrive in their gardens.
The reason: ornithologists know that, in mid-winter, birds must feed and drink as soon as daylight arrives if they are to survive. During the long, cold nights - and there were heavy frosts in the Yorkshire Dales last night - many birds burn up large portions of their fat reserves merely to keep warm.
Unless this resource is quickly replaced, birds can die, say scientists from the British Trust for Ornithology (BTO). But to get more data on which species need to feed first, the trust is working with listeners to the BBC Radio Four Today programme to hold a nationwide snap survey.
Anyone interested in taking part can also help by getting up before dawn tomorrow (Wednesday) for the difference in day length is very small. To take part, log onto: http://www.bto.org/gbw/shortest-day