91Èȱ¬

By looking at what happened in previous big freezes, ornithologists have identified five factors which will influence how bad it is for Britain's birds. Starting with the most important, they are:

1. Prolonged low temperatures and severe frosts, without breaks
2. Ice on trees and the ground
3. Freezing of rivers, lakes and shorelines
4. How much of Britain is under ice and snow
5. The timing of the freeze: the later it comes, the more birds die

So far, we can say that this winter ranks pretty highly on factors 1 and 4 - the cold spell has gone on for several weeks, and covered most of Britain. The other factors are less critical at the moment - though of course if the cold weather continues things may change.

And what about the coming spring? Are we likely to see numbers down for many of our common and familiar birds? In spring 1963 some species bred in much lower numbers than before: especially waterbirds such as the and , and smaller birds such as the wren, goldcrest and long-tailed tit.

Things may be different this year - garden bird feeding will certainly help long-tailed tits, for example. It will certainly be interesting to see how the birds got on when we return for Springwatch.

Most importantly, how will bird populations be affected in the longer term?
Surprisingly, perhaps, the evidence here is that many species will recover very quickly - especially those that have several broods. Species such as the wren, robin and blue tit are very short-lived, and die off in huge numbers each winter anyway - so in three or four years time numbers should have returned to normal.

In fact what has been abnormal is the unprecedented run of very mild winters we have experienced over the past 20 years or so. From 1986 to 2008 Britain (and especially the south) experienced an unprecedented run of very mild winters, which has led to changes in our wildlife patterns:

• Early emergence of flowers and insects (see Springwatch surveys.)
• Early breeding of many birds (sometimes before Christmas).
• 'Summer visitors' overwintering (eg chiffchaff)
• A major fall in numbers of winter visitors (eg Bewick's swan and white-fronted goose), as birds stay further east of the UK.

Cold winters are also valuable for what some people see as restoring the balance of nature. For example:

• Hibernating creatures (bats, butterflies, bumblebees etc) are less likely to emerge and then get killed off by a cold snap, as has happened in the past few mild winters
• Birds are unlikely to start nesting too early (again, as happens in mild winters)
• Flowers are less likely to emerge and then get killed off by late frosts
• Viruses, parasites etc are killed off, which will benefit their hosts. (Again, mild winters tend to allow disease vectors to multiply)

Please tell us about your experiences of our wildlife in the Big Freeze

Stephen Moss is a series producer at the 91Èȱ¬ Natural History Unit, with a special interest in British wildlife. He is author of the book Birds and Weather (Hamlyn, 1995).