Even though almost half a century has passed, I can still recall in vivid detail the events of a hot, sunny afternoon in August 1970. My mother and I were visiting Brownsea Island, off the Dorset coast. We entered a dark hide, opened the window and looked out across the lagoon. And there – shining like a beacon – was a Persil-white apparition: my first little egret.

Back then, this ghostly member of the heron family was a very rare visitor to Britain. Nowadays, little egrets are so numerous that we hardly give them a second glance. On my local patch, the Avalon Marshes in the heart of Somerset, I have seen up to 60 in a single feeding flock. And, according to the magazine British Birds, there are now more than 1,000 breeding pairs, as far north as the Scottish border.

As it turned out, that little egret was a trailblazer for a host of other herons to cross the Channel and establish themselves as British breeding birds. Today, no fewer than seven species breed within a short cycle ride of my home. They include great white and cattle egrets (the ones we usually see in nature films in Africa, perched on the backs of buffaloes), little bittern, and the latest arrival, the nocturnal night heron, which nested in Britain for the first time in recorded history last year. I also regularly see glossy ibises, while the odd white stork has recently turned up. Both are, like the egrets, visitors from continental Europe. None of these species was here when we moved to Somerset a decade ago.

To put this in perspective, when I saw my first little egret, there were only two species of heron breeding in Britain: the familiar grey heron and the impossibly elusive bittern. Now bitterns are so common that my son George has managed to video them on his smartphone. Part of their success is down to the creation of swaths of wetland habitat on the Avalon Marshes, which this year deservedly won the national lottery award for the best environment project.

European Bee-eater. Photograph: Alamy

But the factor that brought most of these species here is undoubtedly climate change, which is altering our birdlife at a faster rate than at any time in recorded history. For along with those herons and egrets, we are also seeing black-winged stilts, hoopoes and even bee-eaters, a species usually associated with the Mediterranean, yet which has nested in Britain several times during the past decade.

These new arrivals are one of the headline messages in the latest report from the RSPB, The State of the UK’s Birds 2017. And there’s no doubt that the colonisation of so many new species is good news – especially for the growing army of birders who flock to the Somerset Levels. Indeed they have proved such a big attraction that I am now running guided tours.

It’s not just these newcomers that are benefiting from a warmer climate. Earlier springs have encouraged a host of resident species to begin nesting several weeks ahead of schedule. For birds such as the blackbird and robin, which can raise two or more broods of chicks in a good year, this early start can be a real advantage.

But for migrant birds, many of which travel here each spring from sub-Saharan Africa to raise a family, the picture is a lot more complicated. Again, some appear to be responding to change: observations made by keen amateur birders for the British Trust for Ornithology show that swallows now arrive in spring roughly two weeks earlier than they used to in the 1960s – clearly in reaction to a warming world.

Yet as the RSPB report notes, there are major downsides to climate change. Numbers of several species clinging on as British breeding birds are falling rapidly. These include the dotterel, a rare example of a bird where the female takes the upper hand in courtship; the whimbrel, a smaller cousin of the curlew; and the Slavonian grebe, now confined as a breeding bird to a handful of Scottish lochs.

Even the chiffchaff and blackcap, whose numbers have more than doubled since 1970, may struggle to cope if predictions of more extreme warming come to pass.

The eminent German scientist Peter Berthold, who has made a lifelong study of migrating birds, once suggested that the first stages of global warming would lead to a “honeymoon period”, during which migrants will benefit from earlier springs and the greater availability of insect food.

But if temperatures continue to rise inexorably, Berthold has predicted that the resulting droughts, habitat changes and food shortages are likely to bring major population crashes. That is especially true if, as seems highly likely, the Sahara desert – the major obstacle all long-distance migrants need to overcome on their twice-annual journeys – continues to expand.

Snow bunting. Photograph: Rex

One of the biggest issues facing both migrants and breeding birds is what scientists call “asynchronous response”. This occurs when different species react to the same change in conditions in different ways. For example, oak moth caterpillars, on which both resident great tits and migrant pied flycatchers feed their chicks, are appearing earlier and earlier each spring.

Great tits are already responding to this change by shifting their own breeding cycle earlier too. But for pied flycatchers, which spend the winter in west Africa, deciding when to head north to breed is usually governed by subtle shifts in day length around the spring equinox.

The likelihood that they will be able to bring forward their departure from Africa and arrival in Britain, so that the hatching of their chicks coincides with the earlier peak of caterpillar availability, is a long shot, to say the least.

Meanwhile, while we may be excited by exotic new species in the south, the picture is a lot less rosy farther north. The RSPB report paints a bleak picture for Scotland, home to the southernmost breeding populations of species normally found in the Arctic.

The snow buntings and ptarmigans nesting on the high tops of the Cairngorm plateau rely on regular snow to create the specialised ecosystem on which they depend. In a warming world, if that snow disappears, then so will they.

I find this frustrating, because more than 20 years ago, in my first book Birds and Weather, I forecasted many of these changes. It seemed obvious to me that if temperatures rose as rapidly as was then being predicted (and has indeed come to pass), then the consequences for birdlife were going to be serious.

Even so, I never imagined that the changes to both our climate and bird distribution would be so rapid, or so extreme. In only a couple of decades, we have seen new arrivals in the south and population declines in the north, together with the continued loss of many farmland birds, for which global warming may be the final factor that tips them over the edge. So given the speed of change in the past quarter century, what differences in our birdlife can we expect by 2040? Again, I think the story of new colonists in the south and departures in the north will continue. I would not be surprised if there were soon a dozen species of long-legged waterbird, including spoonbills, squacco and purple herons, strutting around the Avalon Marshes.

But even in the south, we are likely to lose some familiar birds, as they move north. The latest BTO Atlas survey, from 2007-11, showed that birds such as the willow warbler – by far our commonest summer visitor – have shifted their ranges north and west, and may soon cease to breed in south-east England.

And for my Scottish friends, the losses are likely to outweigh any gains. While nuthatches have now arrived in southern Scotland, and reed warblers are hot on their heels, this hardly compensates for the imminent loss of those specialist birds of the high tops. Nor does it make up for the potential disappearance of other classic Scottish species, such as the common scoter and black-throated diver.

Spotted Flycatcher. Photograph: Alamy

Most worryingly, if the sand eels on which many seabirds feed their chicks continue to shift northwards to cooler seas, then the spectacular seabird colonies of the Scottish islands – aptly described by veteran conservationist Roy Dennis as “our Serengeti” – may disappear too. That would be a tragedy.

In the end, climate change does far more damage than simply shifting the timing of the seasons and making some locations suitable for new species or unsuitable for existing ones. Its most momentous consequence is the way it changes the very nature of delicately balanced ecosystems.

As temperatures continue to rise, familiar habitats will be transformed out of recognition, so that specialist species, which have evolved to be perfectly suited to their particular home, will not be able to adapt rapidly enough to cope.

Conversely, the species that will do well are generalists, able to adjust their feeding and breeding behaviour in response to higher temperatures. I admire gulls, crows and wood pigeons for their ability to live alongside us; but I don’t want to live in a world where they are the only birds I see.

Back in 1995, I ended Birds and Weather with a sentiment that seems more apt than ever today.

Ultimately, we may find that we are powerless against the combination of pressures we have set in motion. Like passengers on a rollercoaster, we can only hope that the ride will eventually come to an end, and that when it does, we and the birds will have survived.

Stephen Moss leads tours for Somerset Birdwatching Holidays. His latest book is The Robin: A Biography (Square Peg)

HERE TODAY… GONE TOMORROW?

PTARMIGAN

The toughest British bird, perfectly suited to living all year round on the high tops of the Scottish Highlands, could struggle if the frozen ecosystem on which it depends disappears.

▶ PUFFIN

Warming seas around the British Isles are forcing sand eels, one of the puffin’s main sources of food, farther north and out of the reach of breeding colonies. Breeding productivity has plummeted and a population crash is predicted to follow.

▼ SPOTTED FLYCATCHER

One of our latest migrants to arrive, in mid-May, this aerial acrobat may not be able to cope with earlier springs. By the time it has chicks in late June, the glut of insects that it must catch to feed them will have already gone.

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SNOW BUNTING

Another specialist of the high tops, it breeds in snow-filled corries, where no other small bird can survive. Warmer temperatures could allow lowland species to move up the mountain and compete for food.

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SNOWY OWL

In the late 1960s, a short spell of climatic cooling allowed a pair of snowy owls to colonise the Shetland island of Fetlar. Soon afterwards, the trend towards warmer summers forced this magnificent bird to retreat back north, almost certainly never to return.

CUCKOO

Already suffering a huge decline because of drought in its African winter quarters, and the loss of large moth caterpillars due to intensive farming, it is vulnerable to even greater shifts brought about by climate change.