Harbingers of spring are aflutter

This is a marine, annelid worm called a sea mouse (Aphrodite aculeata). The scales on its back are covered with fine grey hairs, and the spines on its flanks have iridescent green, blue and gold hairs. It burrows on the sea-bed into mud or muddy sand, and can get washed ashore in large numbers after storms.

`The Millennium Atlas is the most thorough picture of butterfly fortunes ever compiled for these islands'

Now that we've forfeited the right to complain about the weather (this having to be improvised from day to day while Gaia retires, muttering, to the engine room), words like fickle and reluctant are doubly redundant as adjectives for spring. Oh, for the innocent Nature Notes of bygone days! I find myself longing for a first butterfly. In 1997, my nature diary has reminded me, a pair of tortoiseshells were twirling round the pond on April 1st, and a red admiral lazed on the aubrietia - no out-of-season joker, but a startling, overwintering pioneer. Surely, by now, I could hope for the first peacock, dashing darkly about the garden and pausing to flash those hallucinatory eyes from wings of Victorian velvet?

Given any proper sunshine, it seems, peacocks have been coming out earlier from hibernation in recent years - one of many morsels of good news in the magnificent Millennium Atlas of Butterflies in Britain and Ireland, just published (at £30 sterling) by Oxford University Press. Indeed, while there is much doleful stuff in this book about the man-made threats to our butterflies, its impact is essentially joyful and enriching. A joint project by OUP and Butterfly Conservation, the atlas started out as a five year survey, using records by thousands of butterfly-watchers in UK territory only. But for the final two years, the Republic was admitted to the ecological communion of Britain and Ireland, and the Dublin Naturalists Field Club took on the mobilisation of volunteer recording in the south. The result is the most thorough picture of butterfly fortunes ever put together for these islands. It is modelled on the intensive, big-format atlases on breeding and wintering birds, and its brilliant photographs of butterflies and their habitats, and generous species information give the book a very wide appeal.

For the Irish butterfly-fancier, its pleasures are mixed. On the one hand, the picture of decline so marked in Britain has yet to be paralleled here (and we even have a few positive surprises). On the other hand, there is the familiar regret that so many of Britain's butterflies have failed to make it to Ireland, no matter how many east winds blow. The resident species across the Irish Sea total 59, compared with 28 on this side. In global terms, both figures are almost trivial (Europe alone has about 560 different species), but they are still the most closely-studied sets of butterflies in the world.

The bigger British total is justified by the extra sun and warmth of southern England, and differences in geology and plant life. We have no chalk downs with short turf and a special flora, and very little dry, sun-baked heath. Such things help to explain England's extra blue butterflies, for example, and gorgeous creatures like the white admiral and purple emperor. They don't quite show why Britain has half-a-dozen lovely skippers with grasses as food plants while we, with the same grasses, have only (and literally) the dingy one.

Butterflies broadly divide between abundant, mobile species with widespread food plants - such as the stinging nettle - for their caterpillars, and specialised species tied to scarcer plants and habitats. In both islands, the loss of semi-natural grasslands through "improvement", with perennial rye-grass mown for silage, has been a huge impoverishment of butterfly territory.

None the less, most Irish species are holding their own, and some are even expanding. The orange-tip, which takes wing later this month to start laying eggs on cuckoo flowers, and the speckled wood of woodland shadows are both doing well. The peacock, too, has been expanding in Northern Ireland and may now hold more territory on the island than at any time on record.

Two of our habitat specialists are also thriving: the holly blue and the purple hairstreak (which uses oak trees). The delicate, fluttering wood white, doing so badly in the heavily shaded woods of England and Wales, is actually more widespread here, and is expanding northwards, using the semi-shaded edges and rides of young conifer plantations.

However, two Irish specialists which live in colonies on peatland rate special concern. One is the dusky large heath butterfly, declining sharply in England and Wales, and threatened here by loss of its cotton-grass food plant as bogs are drained, overgrazed or forested. The other is the beautifully-patterned marsh fritillary, whose food plant is the devils-bit scabious of cut-away bogs and damp grassland. Declining fast in Europe and Britain, its scattered colonies along Ireland's bog roads are among the last landscape networks for the species.

Climate change is already showing its effects, as 15 butterfly species have expanded their range over the past two decades. Most of them are the wider countryside species, able to breed among the grasses and common plants and the shrubs of field margins, hedges and roadside verges. The northward spread of the speckled wood in Britain and Europe follows the moist warmth which keeps its numbers up in this island.

Ireland is rather too small and stable in its temperatures to demonstrate the northward shifts in range that can be drawn on the map of mainland Europe. But climate change, which already persuades a few migrant red admirals to try over wintering here, may coax the painted lady and clouded yellow to follow suit.

It may also bring us new colonists from Britain. The comma butterfly (like an orange tortoiseshell with scalloped wings) has been rapidly recolonising the north of England and has wandered across to the coast of Co Down. Its spread has been helped by summers that let it breed an extra generation.

Finally, what of the increasing early-autumn records in Ireland of the striking monarch butterfly with its big "stained-glass" wings, especially along the south coast? This is the amazing North American migrant that overwinters in its millions in Mexico. Influxes to Ireland and south-west England in 1995 and 1999 coincided with easterly Atlantic winds that also brought us exceptional falls of North American birds. They represent, says the Millennium Atlas, "little more than a delightful curiosity", and there isn't a chance of breeding or colonisation.