Seaton Devon
Feb 1 2010
The Big Freeze - Impact On Birds
by Dr Mark Avery
Conservation Director, RSPB

Big Freeze - Impact On Birds

The weather is hardly tropical yet but it's thawing in most parts of the country. Whether we've seen the end of snow and ice for a while is yet to be seen. What are the early indications of impacts on birds?

It's too early to tell, for sure, but it's fun trying to work it out from the evidence that we have at the moment. One of the problems in coming to any view at all is that there are at least three things happening to our birds: they are moving to new places because of the cold weather, they are behaving differently and they are dying. Added to which our behaviour is different too - we may spend more time at home and less at work because of travel problems and visit different places to record our birds for the same reasons. These factors could all affect bird reports and that's why I'd be cautious to draw any conclusions at all until the standardised, long-running, regular bird surveys come through over the next weeks and months - and into next year!

The monthly BTO/WWT/RSPB/JNCC Wetland Bird Survey counts are coming up. This survey counts waterfowl at thousands of sites across the UK every month. It will eventually show us whether birds have shifted their geographic ranges in reponse to the freeze - for example are the lapwing which are missing from my local patch in east Northants sitting at Chew Valley Lake in Somerset where I went birdwatching as a boy? Or at least have populations shifted from the north and east of the country to the south and west and maybe even into Ireland? But these data take a long time to come in and be analysed, it's a very big job, so don't hold your breath for definitive news!

At the end of the month the RSPB's Big Garden Birdwatch swings into operation with sometimes as many as 500,000 people recording the birds in their gardens for an hour on the same weekend. This survey has highlighted changes in common bird numbers in the past (eg the decline of the song thrush) and may give us a very interesting insight into how many common birds have fared. We aim to have a quick look at the data to come up with some provisional findings within days of the BGBW weekend.

In the summer the BTO/JNCC/RSPB Breeding Bird Survey will monitor the population levels of a wide range of our breeding birds and will tell us more definitively whether populations have been affected by the freezing temperatures and snow cover. But these analyses won't be available until the summer of 2011!

But what can we guess at the moment?

Here's a fun statisitic. Lots of people are talking about seeing redwings and fieldfares in their gardens and the bird identifier on the RSPB webpage experienced a 5-fold increase in traffic for these two species. Not a count of birds - but a strong indication that there are more of these birds coming into our gardens to escape the freeze. Does that mean that there are also more of them in the country - they arrive from Scandinavia in the autumn - or does it mean that they are dying off and struggling to find food? Or is there a bit of both happening?

Another place to go for real-time insights into bird numbers is Birdtrack - a BTO/RSPB/Birdwatch Ireland project mentioned several times before in this blog. Using the 'Maps and Reports' tab on the home page there is the ability to look at reporting rates of many individual species ona weekly basis. Reporting rates are simply the proportion of all bird lists that included the target species, so it gives a very rough handle on actual numbers but that's fine for these purposes - after all I'm only looking at the figures through curiosity and for fun!

One more caveat - the reporting rates can change retrospectively if people add more data - for example if you have a lot of bird counts from last weekend sitting in your notebook and enter them next weekend (and many others do the same) then that may alter last week's reporting rates.

However, there are early indications for some of our common species such as wren, long-tailed tit and goldcrest that their numbers (no - reporting rates!) have fallen a bit in the cold weather and that might mean that they are dying off in bigger than usual numbers. Look at the graphs and you might persuade yourself of the same for some other species too. I'd have some faith in the wren and long-tailed tit graphs because these are common and widespread species which are known to be affected by cold winters, don't make big movements in cold weather and are easily identifiable. I wonder what the BGBW will show us?

Bitterns and woodcock look as though they are being recorded more frequently than usual - I'd put this down to changes in behaviour caused by the icy weather.

And what about fieldfares and redwings? Their reporting rates have shot up in the cold weather as they flock into gardens and towns. But maybe you knew that already by looking out of the window!