DECIDUOUS WOODLAND

Nature, 2005

The birds are here, though. Many are in quiet mode, moulting behind the safety of the tired leaves and avoiding excessive external effort while their bodies work hard on plumage change. They have no need for song or display and are not yet stressed by food shortage, so, overall, they can be difficult to locate.

But the wood is not in slumber, and there are birds hereabouts that are feverishly active. Some of them are even on 'holiday' - or at least as near as a bird can get to one. These are the young birds of the year, and they are going through a rite of passage known as post-juvenile dispersal. Having left their parents' territory, they have begun a period of wandering - not necessarily travelling very far, but at least avoiding the immediate neighbourhood of where they were born. As they move, they gather into parties with other wandering birds - of the same age but not necessarily the same species - and follow a beat along paths or woodland edges each day, familiarising themselves with places that might one day be breeding territory. Many do not travel more than a few kilometres away from where they were hatched, but this, even for the most sedentary of birds, such as marsh tits and nuthatches (species that, once settled, never leave their territory), is nevertheless a formal time of travel and movement.

Many of these wandering species are attracted at the beginning of each August day by the calls of the flocking 'carrier species' -birds, such as blue tits or long-tailed tits, that form the nucleus of the gatherings and guide the movement along, acting like gurus to hippies. Their calls are readily answered, as flocking can be highly beneficial. For example, as they move, members of the flock watch their peers feeding and learn novel foraging techniques or rich places to search. More eyes are also better for predator detection. The members of the flock each have their eyes opened a little wider to their world; one might equate the month of August to a young bird's gap year.

For some woodland birds, much longer and less informal travels have begun in earnest. Several migrant species go through a rapid moult after breeding and are already on their way south towards their wintering grounds by August; the majority leave the country before the month is out. One of these is the garden warbler, a bird that seems to be a perpetual migrant. It is on the road for up to six months a year, travelling slowly between Europe and Africa and never settling down. Some of spring's most famous players are also notable for their early departures, including Beethoven's Pastoral stars, the nightingale and cuckoo. Nightingales stop singing in June and, no longer holding our rapt attention, melt away from our consciousness like faded pop stars. In common with many migrants, they can fly long distances, so it is possible that we may wake up one August morning to find they've gone. Cuckoos, also silent now, depart with equal surety. We are more likely to see them leave than the skulking nightingales because, prior to departure, they often visit woodland glades and edges to collect their late-blooming tipple - hairy caterpillars, fresh on the nettle. It's an odd diet - but then the cuckoo is an odd bird. These caterpillars are no doubt as nutritious as others, but are coated with noxious hairs that are a deterrent to other birds. The cuckoo, however, swallows them in droves, mashing the caterpillars in its gizzard. Once inside, the hairs adhere to the cuckoo's stomach lining. Eventually that peels off, and both lining and hairs are ejected together as a pellet.

The bloom of hairy caterpillars is only one of several feeding booms that begin in August. The main berry season is also launched, with several trees and shrubs beginning to burst into fruit. On the wood edges elders and rowans ripen, attracting a variety of fruit-eaters from the shadows, such as blackbirds and song thrushes. Young starlings, kept off prime grassland by the adults, take to such trees too, and the less noticeable species, such as robins, blackcaps or even spotted flycatchers, come one by one.

These smaller species, which can be kept off the bounty by their heftier competitors, may also need to feed on berries undercover. Several plants of the deep wood, such as honeysuckle and even lords-and-ladies, produce specialist fruit for the discerning palate, their wares discreetly hidden from the crowds as if their place in the shade were equivalent to some classy backstreet cafe.

August is also the seed season and, for a few species, the strengthening supply of seeds may actually lengthen the breeding programme. Bullfinches, for one, sometimes have young in the nest even at the end of August. Their progeny are raised mainly on the seeds of the herb layer, such as dog's mercury, and tree seeds.

There is one more late summer bloom of food around -- the birds themselves. At this time of year, with breeding adults and juveniles intermixing, there are more bodies around than usual, and the glut provides rich opportunities for the woodland's predators. Young sparrowhawks and tawny owls, for example, have largely completed their early summer hunting apprenticeships, and have a great opportunity now to practise what they have learned from their parents. Theirs is not an easy diet to satisfy, and they must become accomplished very quickly Nevertheless, August favours them for all the reasons explained here. Many songbirds, for example, are moulting, leaving gaps in their feathers with potentially big consequences for impaired flight. Secondly, there are juveniles moving about in dispersing groups, each with limited skills in predator avoidance. And thirdly, those very gluts of plant produce that benefit the fruit-eaters also concentrate their customers, making them unusually vulnerable.

No wonder, then, that an August wood may seem to be a quiet place. Perhaps, with such risk about, the birds are being unusually cautious.