How to spot the signs of global warming in your back garden

Global warming doesn't only mean melting polar ice caps - it's changing the plants, birds and insects in your own back garden. Peter Marren reports

Spring is coming. Or is it? Many traditional signs of the new season have already appeared in the south of England. Trees are coming into leaf ever earlier. Animals and insects are emerging from hibernation in mid-winter. The grass is growing. And hands up everyone who used a lawnmower before mid-March last year (admittedly this year has been slower to get started).

What event defines the spring for you? Maybe it's the first primrose. Primroses traditionally appear around Mothering Sunday. But last year the countrywide average date for the first primrose was 28 February. And even in these climatically challenged times, February is not many people's idea of spring.

How about frogspawn? Well, if you live in Cornwall, frogs start to feel the urge around Christmas time. By the time spring comes to the south-west, garden ponds are full of tadpoles.

Rooks cawing from the treetops? Well, rooks have become early birds. Many are finishing off their colonies by the end of January. By the time of the traditional spring the eggs are in the nest.

First cuckoo anyone? Well, the cuckoo at least is fairly reliable. They still arrive, from Africa, at the traditional time in mid-April. It is not so much that the cuckoo has abandoned spring, but that spring has abandoned the cuckoo. In recent years mid-April marks not the coming of spring but the arrival of the first heatwave. Besides, fewer and fewer of us are lucky enough to hear a cuckoo. Since the 1960s, the number of these birds visiting our shores has dropped by more than half.

We can be precise about all these things thanks to the UK Phenology Network, run by the Woodland Trust and the Centre for Ecology and Hydrology research laboratory at Monks Wood in Cambridgeshire. Phenology is about recording the dates of natural events, whether they be the first primrose, the appearance of frogspawn or the first flowering of purple lilac.

The network is even monitoring the first time we mow the lawn. The results enable us to see how wildlife responds to changing seasons. And in these days of concern over global warming, it also tells us exactly what is happening as wild animals and plants struggle to adapt themselves to new conditions.

The work of the UK Phenology Network has become well-known thanks to BBC Television's Springwatch programme. Last year, in the company of Bill Oddie, Kate Humble and Simon King, we watched the various signs of spring sweeping across the country. Some 70,000 volunteers took part in the survey via the network's website, many of them contributing records from their own back gardens. In a sense the lawns and garden ponds of suburbia have become Britain's makeshift outdoor laboratories. The first primrose is no longer just a date. It is data.

According to the network, Britain is 1.3C warmer on average than in the 1960s. The average March temperature is 5.6C now, compared with 4.2C then. This may not seem a big increase, but its effect has been to move spring forward by 19 days. Early butterflies such as the holly blue and orange tip appear three weeks earlier than 30 years ago. And the growing season is extending. The network says we used to mow our lawns for the first time around 1 April. Now many gardeners are getting out the mower in March or even February. In winter 2003-2004 a third of Phenology UK's recorders cut their lawns before 1 April.

But isn't warmer weather good for wildlife? Not necessarily. The problem lies in "synchronicity". Like us, wildlife relies on the calendar. Birds time their nesting to coincide with plentiful food. Woodland flowers appear within a narrow window of opportunity when the soil is warm but before the ground is heavily shaded.

Imagine you are a blue tit. You can take advantage of the garden nut-feeder to fatten up for the breeding season and use that handy nest-box for the nursery. But nuts are no good for your nestlings. Growing birds need plenty of protein, and the only baby food out there in sufficient quantity is caterpillars. But caterpillars grow even faster than baby blue tits, so the birds have to time it just right. And that's the snag. A temperature rise of only a degree or two means caterpillars emerge earlier and feed up faster. Hence the lives of blue tits have become that bit harder. Mistake the timing and your babies starve.

Blue tits can always nest again. Things are potentially more serious for species that only breed once a year, such as frogs. Spawning in December may seem like a good strategy but it is risky. The pond may freeze, killing the spawn, while tadpoles face the coldest time of year when they need to fatten up fast before they get eaten. So climate warming could be bad news for frogs.

Life might seem easier for a plant. But plants live in a highly competitive environment. The big winners in the climate-change game are plants that get a head start by growing in mild spells through the winter. That is why wild flowers such as cow parsley or hedge garlic may be a growing problem in your garden. Like barley, they have switched from being spring-growing plants to winter ones.

And for every winner there are losers. Cow parsley is tall with frothy blossom and big leaves. The earlier it appears, the more likely it is to crowd out other plants - such as primroses and bluebells. Where cow parsley abounds the primrose's window of opportunity is closing.

The warming climate also affects woodland trees. "Ash before oak, in for a soak," is a country saying. There is no danger of that happening today, for oak bursts its buds at least a week ahead of ash. And every rise in average temperature widens the gap between the two trees, so that oak is becoming more competitive at the expense of ash. Not that we will be any drier as a result: they also say: "Oak before ash, we're in for a splash."

But it's in our own backyard that we experience climate change up close and personal. With the grass growing all year, we will spend more time with the mower. That means we will be sneezing more from cut-grass allergens and grass pollen, especially as grasses are flowering earlier and for longer. Finding enough daffodils left for Mothering Sunday will become a problem. At least in the south, and gradually further north, winter and spring are merging into a long, mild year's beginning. We might call it Springter. Soon the first signs of spring may follow the latest apples, or the last crop of berries.

What to look out for

Harlequin ladybird

This orange, spotty ladybird first appeared in a pub garden in Essex two years ago. It has already spread over East Anglia and the Home Counties and is set to become as familiar as our red ones. Sadly, it eats the red ones. Spot it on bushes in spring and in autumnal stored fruit.

Early hawthorn

Hawthorn is also known as may blossom, but the UK average for first flowering is currently 28 April. We may have to rename it.

Queen bees

Fat queen bumblebees (left) used to be among the first signs of spring. Today they are often active in mid-winter.

Blackcaps

This sweet-singing warbler used to be a summer visitor, but 40 per cent of us now see it in the winter. Milder winters are one reason; certain late-fruiting berries are another.

Butterflies waking up

What is arguably our prettiest butterfly, the red admiral, used to be a migrant. Now it is here all year round in some places and regularly wakes up from winter hibernation to sun itself and look for a snack of nectar or rotting fruit.

Early birds

Some birds seem to be nesting up to a fortnight earlier than previously. In particular, keep a look out for blackbirds, blue tits, great tits, starlings and chaffinches carrying nest material in their beaks.

Not so dead-nettle

The white dead-nettle is an important source of nectar for early insects like bees. In the past it flowered around April. Now it seems flowers can appear all year round.

To take part in the UK Phenology Network, visit www.phenology.org.uk