The last swifts I saw this summer were so high that they were almost invisible, zooming south on their migration, past soaring herring gulls and against a glorious blue sky. That day, much nearer the ground, hundreds of common blue butterflies were on the wing, busy in the warm sunshine.
Scattered among them were several chalkhill blues and some of the much rarer and even more radiant Adonis blues. Many fed alongside painted ladies on the few black knapweed flowers that hadn't yet gone to seed, while they ignored the clustered bellflowers that were in full bloom nearby.
This scene wouldn't seem out of place in The Country Diary of an Edwardian Lady, Edith Holden's influential naturalist study of 1906. And there is a lot more going on amid these details of the natural world than a just a nice walk in the countryside. If you know how and where to look, all the clues are there to indicate the soil type, the geographical location and the time of year. These special clues are known as "biological indicators", or "bioindicators" for short.
If we popped that esteemed Edwardian lady into a time capsule and dropped her into my time and place - Beachy Head, August 23, 2003 - I would lay bets that within two minutes, and with no technical aids, she would be able to tell the date to within maybe four days either way, as well as the place to within 50 miles or so. There would be no need for her to check the setting position of the sun or its angle overhead at midday - she would be able to make accurate guesses thanks to her detailed knowledge of natural signs around her.
But wildlife can indicate much more than approximate dates and places. The concept of bioindicators - of living organisms able to reveal things about our world that might otherwise be more complicated or expensive to measure - has made huge gains in credence and practical application over the past couple of decades. The hypersensitivity of certain species to various physical conditions and stimuli makes them the "litmus paper" to gauge all sorts of environmental situations and changes. They tell us not only about the state of our planet, but also about our impact on it - so much so that bioindicators are now used widely in industry, and even in politics.
However, we should remember that humankind has used these signs for millennia, and long before scientists invented technical terms for them.
Before modern technology, people living close to nature relied heavily on wildlife clues to find food, to avoid danger and to predict the weather.
Bioindicators would often have been - and for some people still are - the difference between life and death.
Many indicators that people once used have survived through folklore. One of the first I learned about as a child was the use of seaweed to predict rain. Bladder wrack is one of the best species to use as it has little brown capsules which, if tautly inflated, presage rain. And if the leathery fronds are slack and wrinkly, then dry weather is said to follow. This simple rule has a fair degree of accuracy because seaweed is hygroscopic, which means it tends to absorb moisture from the atmosphere. This is only possible in periods of humidity, which usually occur before and during periods of rain.
In fact, bioindicators for forecasting rain seem to be quite common. You may have heard it said that if the cows in a field are lying down, then it is going to rain. This is not especially accurate - imagine the leg-ache cows would get during prolonged dry spells. Even so, there is some truth in this. In fact, cows can sense falling barometric pressure far better than humans can, and they do like to keep an area of ground dry to rest on.
The red-throated diver, a waterbird which breeds on lochs in northern Scotland, has long been known to local people as the "rain goose" on account of its habit of calling in advance of wet weather. But it seems to be one of those truisms nullified by the tendency for it to rain so frequently in Scotland.
Another bird with weather-forecasting abilities is the swallow. The old saying, "When the swallows fly high, the weather will be dry" has some validity - and a simple explanation. In fine weather, insects are swept up on thermal currents to great altitude, way beyond human sight. At such times, swallows follow in order to feed on them. So it follows that, "When the swallows fly low, we're set for a blow."
In today's world, where wet weather is something we experience briefly en route from porch to car, it is easy to forget how only a couple of generations ago, so many people were employed on the land or cultivated their back gardens that understanding the weather was crucial for achieving a successful harvest. In agricultural terms, bioindicators have been useful for predicting the suitability of land for cultivation. Pliny the Elder, the Roman scholar, noted in AD50 that land that supported wild plum, elder and oak was likely to be suitable for wheat production.
This spotting of agricultural potential without having to resort to trial and error was of great importance to peoples moving into new areas, such as the North American settlers. They realised that forests of beech, birch and maple sit on richer soils, so these were chosen in preference to areas in which the native vegetation was pine, while tall-grass prairies were selected for cereal production, and areas of tussock grass avoided altogether.
Various plant species have proven to be excellent indicators of climate and soil nutrient levels, as well as pH levels and the presence or absence of certain low-level elements. Plants are also one of the tests used to determine whether a woodland is "ancient" (meaning a woodland that may have been managed but has never been cultivated since at least AD1600). Certain species, such as the bluebell and wood anemone, are so typical of woodland environments and so intolerant of disturbance that their presence is a good indicator that the woodland has stood for a very long time. The greater the number of ancient woodland indicator species, the greater the likelihood of the woodland's "ancient" provenance, and so the greater the conservation priority given to it.
If we can use bioindicators to tell us about characteristics of the world they are living in, we can also find indicators that show that all is not well with that world. Probably the most extensive use of bioindicators is as pollution monitors. Again, the idea is not new one. The most famous example from the past is of the canary in the coal mine. Carried down in small cages into the mineshafts, their function was as early-warning systems for carbon monoxide (CO). Having such a small volume of blood compared with humans, small birds become poisoned by CO much quicker than we do. In an atmosphere containing only 0.1 per cent CO, canaries lose the co-ordination needed to land on their perches. In higher concentrations, canaries lose the power in their legs and fall flat on the cage floor. On seeing these signs, miners knew they had to get out quickly.
Many species groups have since proven to be good indicators of a wide range of potentially harmful pollutants. Lichens, seaweeds, fish and diatoms (a group of single-cell aquatic algae) are all extensively used. In the US, raccoons are used as bioindicators of radionucleide and metal contamination. Indeed, bioindicators may be some of the most sensitive measuring instruments available to us. They are like barometers, thermometers and every other "ometer" you can think of - all rolled into one and with only one read-out.
Sometimes bioindicators can be useful even if it is not immediately clear what they are indicating. If the initial sign you are looking for is that "all is well" or "not well", bioindicators can help. This principle is at the heart of the Government's report, A Better Quality Of Life (see www.sustainable-development.gov.ukuk_strategycontent.htm), which offers a kind of barometer to gauge whether or not we are living our lives sustainably. Among almost 150 indicators, including health, jobs, educational achievement and economic prosperity, there are 15 "headline" indicators, one of which is wild bird populations.
This indicator looks at the populations of common birds found in farmland and woodland, as well as all native species as a whole. These populations are regarded as a good indicator of the broad state of wildlife and countryside because they occupy a wide range of habitats, they tend to be at or near the top of the food chain, and because a lot of information on bird populations is available from previous research. With numbers of farmland birds showing severe declines since the 1970s, this indicator clearly suggests that all is not well, and these findings have influenced politicians, conservationists and farmers to work together to reverse the birds' fortunes. Like the canaries in the coal mines, birds are now helping to identify problems in the world above ground.
Alongside the fortunes of birds, the budburst times of trees, the flowering of common native plants and the distribution and emergence times of butterflies, moths and other creatures, are now being used as bioindicators for what is potentially one of the most profound changes the world has ever seen: global warming. Our Edwardian country lady, deposited on Beachy Head in the 21st century, could be forgiven for misjudging the date slightly. It is almost a century since she was observing nature, and the average dates for butterfly emergence, blooming flowers and migrating birds have all shifted - in some cases dramatically.
This phenology - the study of the timings of recurring wildlife activities - may prove to be of great help in learning to read nature's book. Watch this space - and those bioindicators.
Adrian Thomas is a wildlife journalist and press officer for the Royal Society for the Protection of Birds
RIVPACS: ENVIRONMENT AGENCY'S SYSTEM OF ANALYSIS
The Environment Agency (EA) conducts its biological surveys using a computer program known as RIVPACS (river invertebrate prediction and classification system). It is at the sophisticated end of a number of similar methodologies which require samples of river life to be collected and identified. Identification right down to species level is specialist stuff, and unnecessary; instead, it is quite straightforward to ascribe individuals to major types.
Some organisms are known to be wiped out quickly by even small levels of organic pollution. Others can tolerate up to moderate levels, and some species survive or even thrive in high concentrations. The RIVPACS method ascribes pollution tolerance values to 80 groups of invertebrates. From samples collected in autumn and spring, the computer program analyses these data together with some physical characteristics of the sample site and estimates how varied the aquatic life is compared with what we would expect the river to support if there was no pollution.
An Environment Agency survey in 2000 found that 94 per cent of our rivers were of good or fair quality, compared with 87 per cent in 1990. Some 5 per cent were poor and 1 per cent bad.
What this kind of monitoring reveals is that in order to use bioindicators you have to understand exactly what the bioindicators are responding to.
That often relies on extensive research in the first place. For example, if the presence or absence of caddisfly larvae are to be used to gauge the levels of organic pollutants, you need to know at what level of pollution the larvae begin to suffer. You also need to ensure that some other factor such as water pH, temperature, pathogens, turbidity or some other factor is not at work too.