Why have all the birds gone quiet?

A rustling in the depths of the blackcurrant thickets signals a last mopping-up of berries by the acre's tribe of blackbirds. Their surreptitiousness and silence, so at odds with the jubilant festival of early summer, are part of a general skulking that overtakes the garden at this time. For people newly tuned to the life of birds, the sudden dip in mood can be quite worrying - where has everything gone?

Birds have rhythms beyond our entertainment. The springtime flood of song served its purpose: space was shared out and defended, nests built and family reared. Even the song-thrush, after raising two or three broods, falls quiet in August and September.

But the compelling reason for the sudden reticence of robins and wrens, the chilling-out of chaffinches, is the annual moult: a complete change of feathers after the final wear and tear of the breeding season. Even this summer's juveniles are changing into adult plumage. Moulting takes energy, as the bird's metabolism speeds up to grow new feathers and push out the old ones. The birds become lethargic, reluctant to fly very far, and spend much of the day resting in deep cover.

Small birds can't afford to have their flightpower diminished, so they change their feathers a few at a time. Starlings, for example, which spend so much time on the wing, started shedding their main flight feathers (the primaries) in late June and will take 100 days to change them all. Just about now, they are beginning on their tail feathers, one pair at a time.

The starling's new feathers have pale tips, which give the bird its speckled, winter look. By next spring, these tips will have worn away, leaving the plumage sleek and iridescent. In other birds, such as tits and finches, this abrasion of dull, discreet winter plumage will reveal the bright colours of the next mating season.

Some of the bigger birds, such as waders, have such great lift in their wings that they can afford to drop half-a-dozen primaries at a time and fly around with gaps in a normally sleek silhouette. The curlew is one of these, and has to speed up its wing-beats by perhaps 40 strokes a minute until its primaries grow back.

Geese, ducks and swans all need maximum lift for their big bodies and have to take the gamble of dropping all their primary feathers at once, trusting they can swim away from harm while they are flightless. Barnacle geese, for example, moult beside tundra lakes in Greenland, and head out into deep water with their goslings when an Arctic fox comes by.

Ducks have other strategies. Mallard lie low in the reedbeds, the drake going temporarily into "eclipse" - losing the lovely bottle-green head-feathers, which might flash too brightly, and then growing them back as his wings return to use. Most of our estuary shelduck migrate to moult, joining rafts of their fellows on the sand flats of Heligoland, in the southern North Sea.

Irish birders, too, wait out these latesummer doldrums, longing for the autumn gales to bring them rare vagrants from America. Some, meanwhile, have turned their telescopes to dragonflies, to great effect. In this first summer of the Dragonfly Ireland Survey (see www.dragonflyireland. fsnet.co.uk), global warming and more such observers are producing some startling records.

At the coastal lakes and fen-fringed pools of Co Wexford, for example, bird artists Killian Mullarney and David Daly, and other birders, have met numbers of the big and beautiful emperor dragonfly, Anax imperator, from southern Europe. This dramatic creature has expanded its northerly range in England and these are its first Irish records.

The male emperor, with yellow face, green thorax and bright blue abdomen, is the largest of the hawker dragonflies, reaching an impressive eight centimetres and an even bigger wingspan. It flies for hours without resting, out over open water beyond the reedbeds of lake or canal. The slightly smaller, less brilliant female was seen laying eggs "almost at our feet" at a pool near Curracloe.

The season's other invertebrate excitements, fanned by warm winds from the south, began with an influx of hummingbird hawk-moths and continue with intriguing reports of some rare and lovely butterflies. In Co Down in June, there were two sightings of "probable" Camberwell beauties, the richly-purple butterfly with creamy margins to its wings, even bigger than peacock or red admiral.

In Co Fermanagh, there was also a "possible" monarch, the big amber-and-black butterfly whose migrations the length of North America are so spectacular. There are monarchs resident in the Canary Islands, and breeding populations have become established in Portugal and southern Spain, but vagrants to Ireland usually arrive in the same autumn tail-ends of hurricanes that carry American songbirds off course. Here, too, it is birders on their vigils along the Co Cork coast who are likely first to spot the strangers in the sky.

The rest of us, meanwhile, can look forward to a hatch this month of another generation of the migrants already here. Red admiral caterpillars have been chewing on nettles as far north as Malin More in Co Donegal. Painted ladies, too, which lay on thistles, should produce an Irish generation.

This has also been an excellent year for clouded yellows, whose caterpillars feed on clover, and a successful hatch this month could produce substantial numbers. Any reports would be welcomed by David Nash, who is co-ordinating butterfly records with the Heritage Council's support. He is at the Dublin Naturalists' Field Club, 35 Nutley Park, Donnybrook, Dublin 4, and its Butterfly Guide is available at £3.50, inc p. & p.