Deer Behind Britain's Great Bird Decline?

James Owen in the United Kingdom
for National Geographic News
March 3, 2003

This spring leading ornithologists will begin surveying bird life in 350 woods across Britain. The study aims to reveal the reasons for a startling decline in many woodland species.

Since the 1970s the woods have been losing their spring chorus of birdsong. Willow tit numbers have fallen 78 percent, woodcock by 74 percent, and song thrushes have more than halved. This is despite a significant increase in tree cover—up more than 20 percent in England alone over the last two decades.

Possible causes for dwindling bird populations include changes in forestry practices, egg predation by grey squirrels, and the effects of climate change. Yet the chief suspect in the minds of many ornithologists is that picture of doe-eyed innocence—the deer.

Deer numbers are soaring in Britain. Two hundred years ago the roe deer was extinct in England and Wales, the victim of over-hunting and forest clearance. Today the population stands at around 700,000.

It's a similar story for the red deer, Britain's other native species, which now festoons many Scottish hillsides.

The growth in tree cover, increased cereal farming (an important source of food), reduced hunting levels, and better winter survival rates due to a warming climate are all thought to be factors behind the deer's revival. The same goes for two introduced deer—the fallow and muntjac—which are now widespread in Britain.

The two conservation nonprofits behind the survey, the Royal Society for the Protection of Birds (RSPB) and the British Trust for Ornithology (BTO), say these four species are chewing away huge chunks of woodland bird habitat.

"Birds like nightingales and warblers are adversely affected," said Rob Fuller, BTO's director of habitat research. "They need low, dense vegetation in which to nest—deer remove this.

"We also think deer have a negative impact on food resources. By biting off buds and flowers they reduce the amount of seed and fruit available in autumn and winter."

Plant Variety Is Crucial

Deer can alter the entire structure of a wood. Certain flowers, shrubs, and trees are pruned back, leaving only less palatable species. Fuller says plant variety is crucial to maintaining the diversity of other woodland wildlife.

"The woods where I work in England contain roe and muntjac deer," Fuller said. "The plants they really clobber include ash, hazel, and sallow. And it's characteristic of all woods with high deer densities that bramble gets heavily browsed. That's bad news for birds as many of them love bramble."

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