Climate change is hitting robins. With warmer winters and earlier springs, these much-loved garden birds are now laying their eggs in their mossy nests almost a week earlier than they were 40 years ago, according to the latest report from the British Trust for Ornithology.

The average date for the robins' first egg is now April 22 - no longer April 28, as it used to be. They are not the only ones. Chaffinches are starting to lay on May 2, rather than May 11.

Why are they doing this? And is it good or bad news? Some recent studies of great tits by Oxford ornithologists have given an interesting answer.

Great tits feed their young in the nest mainly on the caterpillars of the winter moth, which swarm over oak trees munching on the leaves in spring. Now the caterpillars, responding to the early warmth, are emerging earlier. So it was feared that the beautiful synchronisation of great tit hatching dates with the appearance of the caterpillars, which has gone on for countless generations, would be wrecked and the young great tits would starve.

Not at all. The great tits have adapted themselves to the change, and in turn started nesting earlier. Almost certainly something similar has happened to the chaffinches and robins. So the earlier nesting might have been bad news, but in fact it is good. Birds can adapt much more quickly than it was thought.

However, not all of them. Researchers in the Netherlands have found that their great tits have not been adjusting anything like as well. One can hardly suppose that birds are affected by their native country's politics, but one cannot help feeling that this is something of a triumph for British pragmatism.

Another caution should be added here. One cannot expect to go out and observe these changes easily. These are average figures - and you cannot see an average. From one year to the next, nature's dates may swing about violently. This year, with a cold, wet April, most birds were in fact nesting later than usual. The pattern of the weather and the underlying climate change may go in quite opposite directions.

There is, however, one effect of climate change on birds that is more easily seen. As temperatures rise, birds that flourish in warmer climates are beginning to come into Britain from further south, and some birds, such as woodlarks, hitherto found only in southern England, are spreading further north.

There has just been a spectacular example of this. In 2004 the British Trust for Ornithology predicted that birds such as the great reed warbler, with its foghorn of a song, the black kite and the cattle egret, a little white heron from Spain and North Africa, might soon becoming breeding birds here. Now, this summer, it has been discovered that two pairs of cattle egrets have nested successfully in the Somerset levels. This is the first time this has happened. A good prediction!

Taking a longer view, it has been suggested that all European bird species will move 550km to the north-east by the end of the century. This means that we may have such things as gorgeous hoopoes breeding here regularly, and colourful ortolans singing in our hedges (though not, we must hope, appearing on our plates, as they still do secretly in France).

Drier summers may have effects not only on birds. That great guru of the woodlands, Oliver Rackham, recently observed that after hot summers there were fewer primroses around Cambridge - but not in the wetter woods of the west side of Britain. Sustained dry summers might have a slow, enduring effect on our flora.

To a large extent, though, the effect of climate change on nature seems to suggest that Britain may look different in the future, but not distinctly worse. Extreme Greens cry out at any prospect of change - but one cannot say that what seems likely to happen in this respect will all be bad.

The most serious effect so far of climate change on birds has been seen on the coasts of northern England and Scotland. There have been dramatic breeding failures among puffins, kittiwakes, guillemots, razorbills and terns, with whole populations in some of the northern isles failing to rear young in recent summers.

This has apparently been due to a shortage of sandeels in the waters in which they fish. These little fish are the main food of the the nestlings. But as the northern seas have got warmer, the sandeels have moved further north to the colder waters that suit them and the plankton that they, in turn, feed on.

To compound the problem, another fish - the pipefish - has moved into the waters from which the sandeels have been departing, and the parents birds have been bringing these to their young. But they are stiff, bony fish and the young birds cannot eat them easily. Visitors have seen the depressing spectacle of dead chicks lying with uneaten pipefish all round them. Here, at least, it seems clear that climate change can have devastating consequences on wildlife that we love.

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