Two Dutch researchers, Dr Jaap Graveland and Professor Rudi Drent, have found that 40 per cent of Great Tits (one of Europe's commonest birds) breeding in woods with calcium-poor soils near Arnhem, Holland fail to hatch any young. Another 10 per cent didn't even lay eggs, apparently because they couldn't form shells at all. Yet, when the nesting birds were supplied with fragments of snail and chicken eggshells - both rich calcium sources - the proportion of failed egg clutches fell to around one in 10. Without calcium supplements, each pair of Great Tits reared, on average, 1.8 young birds. The ones given supplements raised six per pair.
Dr Graveland and Professor Drent are convinced not only that this calcium deficiency is the result of acid rain - formed from air pollutants such as sulphur dioxide and nitrogen oxides dissolving in rainwater - but that other woodland birds may be suffering a similarly reduced breeding performance.
While some of the calcium needed for eggshell formation in female birds is stored in a special type of marrow in the birds' longer bones, this is not enough. Chickens, for example, prior to egg- laying, need to obtain extra calcium in their diet. Ornithologists used to believe that most birds could get enough calcium from their normal food, but this is now not believed to be so. Birds have to search for calcium supplements such as snail shells or other sources, such as calcium- rich grit and soil.
Naturally acid soils, however, contain only small amounts of calcium. If what little calcium that remains in the soil is then removed by acid rain, finding enough to form eggshells is almost impossible. According to Dr Graveland, egg-laying female Great Tits on calcium-poor soils spend 43 per cent of daylight hours searching for calcium-rich items to eat, almost twice as much as females with sufficient calcium. "As a result," he says, "the birds might lay smaller or fewer eggs."
Dr Albert Beintema and his colleagues at the Dutch Institute for Forestry and Nature Research have found calcium deficiency in Black Terns, small, fork-tailed seabirds which nest around inland lakes and pools. Their food, mainly water beetle larvae, water boatmen and damselflies, contains only a minute amount of calcium. They calculated that, to meet the growing needs of the Tern chicks, each one would need to be fed one prey item every minute by its parents. But each chick was actually fed once every four to six minutes. This explains, they believe, why most of the chicks they were studying around pools on acid, wet moorland lost weight from 10 days old onwards and usually died by their 25th day. Post-mortems showed that severe rickets was the cause of death with poorly developed skeletons and broken bones.
Roughly four times as many Black Tern eggs hatched from nests around lowland pools on calcium-rich soils compared to those on the acid moorlands, a finding which is similar to that for Great Tits in Dr Graveland and Professor Drent's studies.
Lakes, rivers and land polluted by acid rain are by no means confined to the Netherlands. In Britain, large tracts of uplands in Scotland, north Wales and the north of England - and parts of southern England on thin, sandy soils - are naturally acid. What little calcium they contained has been depleted even further by acid rain.
In the uplands of Wales, for instance, around 12,000km of streams and rivers are artificially acidified by acid rain, according to Dr Steve Ormerod of University of Wales, Cardiff and an expert on the impact of this type of pollution on stream wildlife. Some lakes are affected too, and many ecologists believe that changes in the plants and invertebrates of moorland and mountain vegetation may be at least partly attributable to acid rain.
The most acidified streams and lakes are akin to freshwater deserts; many invertebrates and fish cannot survive in them. In some parts of north- west Wales, soil pH has fallen by as much as 1.8 units (on a scale where a fall of one unit is a 10-fold increase in acidity) over the last 70 years.
Little wonder then that British conservationists are beginning to worry about the impact that acid rain is having on birds such as Great Tits, 1.6 million pairs of which are currently nesting in our woods and gardens. "Because of the Dutch findings," says Dr Humphrey Crick of the British Trust for Ornithology (BTO), "we are putting together a two- to three-year research study with scientists at Imperial College, London in order to look at any links between acid rain and breeding bird data."
If it gets sufficient funding, the BTO will analyse breeding records for a range of woodland, garden and moorland birds and widen the correlation with acid rain to other European countries including Finland, Germany, Spain and Poland.
But, while such links between calcium deficiency and birds breeding on land are only now being proven, the impact of calcium shortages for Dippers - plump, insect-eating birds of fast-flowing rivers - has been known since the mid-Eighties. This pioneering research on streams in Wales and Scotland, was led by Steve Ormerod.
What he found was that Dippers laid smaller clutches of eggs and reared fewer chicks on acidified streams compared with those on non-acidified streams. Nestling growth was also slower. What's more, Dippers on acid streams never laid second clutches of eggs, compared with one in five pairs on non-acid streams. "Wherever acidity was greatest," comments Dr Ormerod, "shells were thinnest."
Dr Ormerod thinks that ecologists shouldn't confine themselves to looking for calcium shortages in birds. Many species of bats, he points out, forage for insects flying over freshwater and wet habitats. Where such habitats have been acidified by pollution, the numbers of insects emerging from water-living larvae is likely to be low. Those that do will contain lowered levels of calcium. Unless the bats forage widely, they in turn may become calcium deficient which could limit the growth and survival of their milk- suckled youngsters.
The current concerns about eggshell thinning remind ornithologists of the enormous declines of many birds of prey - such as the Peregrine and Sparrowhawk - which began after the Second World War. Shell thickness declined by as much as 20 per cent. The cause was shown to be the then widely used insecticide, DDT, which accumulated at high levels in the body tissues of animals at the top of food chains and which, somehow, reduces incorporation of calcium into the shell. As DDT use has been phased out in most western countries, shell thicknesses have recovered and, with them, the populations of the birds of prey affected.
Both the Dutch researchers and Dr Ormerod are sure that the shortages of calcium this time round are due to acid rain and are not linked to pesticide and while one component of acid rain, sulphur dioxide from power stations and industry, is declining, the other - nitrogen oxides mainly from vehicle exhausts - is rising. Unless governments are prepared to act decisively, reduce vehicle numbers and improve public transport, acid rain, and its destructive effects, are here to stay. !