By Sarah Mukherjee
BBC News environment correspondent
Changes in bird numbers can be difficult to explain
Ornithologists say they are increasingly concerned about the "alarming" decline of some of Britain's woodland birds species.
What is most perplexing them is that while there are many theories, there appears to be no obvious explanation, so it is very difficult to try to halt the trend.
Something is happening in the dappled glades and airy canopies of Britain's woodlands.
Fragments of the dense forest that once covered this country, they are now oases for species like the Spotted Flycatcher and the Tree Pipit.
But the numbers of these birds appear to be declining, catastrophically in some cases. While there are lots of theories, there appears to be no one obvious explanation for the trend.
"Ironically, these species are declining just as the amount of forest is increasing," says Derek, one of the RSPB's researchers.
He is looking at the possibility of more in-depth research on the decline of one species in particular, the lesser spotted woodpecker, which is down by nearly 80% in the last 30 years.
Now two major bird conservation charities, the RSPB and the British Trust for Ornithology, are joining forces, appealing for birdwatchers to get in touch if they see these now elusive birds.
Derek is monitoring a site in Cambridgeshire where a pair of woodpeckers are brooding a clutch of chicks.
Watching them fly in and out of their nest 40ft up in an old beech tree, the intricate black, white and red colouring of the male flashing in the sunlight, is an awesome sight - and one that, these days, birdwatchers are lucky to see even once in their lifetime.
Many theories have been put forward as to why the decline of the woodpecker and many other species has been so rapid.
Some experts have suggested that the sharp increase in the number of deer is to blame. Deer are voracious feeders, and can strip out bushes and young trees from woodland very rapidly.
Other scientists say the change in woodland management is causing the decline. Modern practice is to remove dead wood - the wood in which the bark beetles live, that the birds in turn feed on.
But on the other hand, more woodland is being left to its own devices than ever before.
Whatever the reason, the decline is now of serious concern. The RSPB is currently funding research to find out exactly what is happening, before, ornithologists say, it becomes too late.