Numbers Game
One of the places that is responsible for my continued love of birds is Skomer Island off the coast of Pembrokeshire – and it is here that I first learned about the Breeding Bird Survey (BBS), or as it used to be known, the Common Bird Census.
The first time I went to the island, I had only been into bird-watching for a couple of years; I was just getting started on a new hobby and there was so much to learn about.
One of the things I learned was how to ‘guestimate’ the number of birds you’d seen if there was a big group or flock of the same species. You take a portion of the group that you can count and then guess how many times over the same size portion would fit into the whole group to make up the total figure.
I remember asking a conservation worker, who shall remain nameless, if it was significant to see ‘about 20’ cormorants on a particular rock to the south of the island. His surly response was that it wasn’t 20, but 17, and there ended the conversation.
Later, a much friendlier conservationist explained what was going on; not why cormorant man was in a bad mood, but why he was so bothered about the exact numbers.
He was a volunteer counting birds for the Breeding Bird Survey. His grumpiness might have been down to the fact that he had to get up early to do his survey, but walking two lines through a 1km square, counting all birds encountered, noting their habitats and any mammals he might have seen on his way didn’t seem to be worst job in the world!
The status of wild bird populations is an important indicator of the health of the countryside. The BBS, a joint initiative by the BTO, the JNCC and the RSPB, was launched in 1994 to provide better habitat coverage than the main survey running at the time, the Common Birds Census. BBS trends are produced each year for over 100 species, and the results are increasingly used by Government and non-governmental organisations to set conservation priorities.
Last year’s results presented a mixed picture for the South West’s birds – with some species declining by as much as 50% on the previous year. Cuckoos, starlings and willow warbler numbers are all down, but the precise reasons behind this are hard to define without a further look at the birds in question.
The RSPB’s Peter Exley said: ‘It’s not always immediately obvious why bird numbers have gone up or down, but surveys like this one are vital for helping conservation organisations keep an eye on what birds need most attention.’
Other species seemed to be doing extremely well though. Great spotted woodpecker numbers appear to have risen by nearly 150% and there are 60% more swallows than there were in 2006. Ravens, Canada geese, pheasant and mallard numbers also appear to be on the rise.
As well as the regional drop in numbers, cuckoos are also in trouble nationally. The species seems to be being hit regardless of what habitat it is using. In Dorset it is widespread, and can be found at diverse locations from the likes of Christchurch Harbour, to places including Bovington Heath and Wareham Forest.
The British Trust for Ornithology’s John Tully, one of the regional BBS organisers, said: ‘In 2007, over 75,000 birds were counted but only 17 were cuckoos. Over 30 hours of bird-watching was needed for each cuckoo!’
It has been suggested that a decline in its host species (cuckoos lay their eggs in other species’ nests, ejecting the host’s own eggs leaving young to be brought up by surrogate parents) may be to blame; but some such as the dunnock appear to be increasing – illustrating just how hard it can be to define the reasons for bird declines.
Peter Exley added: ‘These annual surveys are like a barometer revealing the fortunes of the birds we treasure and enjoy around us. As the pressures mount on our birdlife, from climate change to development, thousands of volunteers are out there giving their time to keep an eye on which birds are coping well, and which are not. The priority now is taking action for those birds in the greatest trouble.’
The British Trust for Ornithology is keen to recruit more volunteers to help with the BBS. For details of how to get involved visit their website www.bto.org