Peacocks hibernate through the winter and can be seen in the coldest winter months, although most individuals become active in late March and April. As well as being a harbinger of the coming spring, Peacock butterflies are interesting in many other ways. They are probably the longest-lived butterflies in Britain, with adults surviving from late July, well into the following spring, perhaps into June. Thus, contrary to popular belief that butterflies only live for a few days, some Peacocks may live to see their 11th month (albeit having spent five or six months of their adult lives asleep in hibernation).

The Peacock butterfly has fared well in recent years. Not only has it increased in distribution, pushing northwards in northern England and central Scotland, but populations have also increased significantly in size at sites monitored by butterfly transects. It is even emerging from hibernation earlier than it was 20 years ago. There is little shortage of food for the black, spiny Peacock caterpillars, which feed primarily on Common (stinging) Nettle, and it seems highly probable that the good fortunes of this butterfly are due to climate change.

Although a common visitor to garden buddleias, particularly in the autumn whilst building up fat reserves for hibernation, Peacocks are nomadic butterflies that range widely though the countryside. They often find their preferred breeding habitats (large nettle beds in sheltered but sunny situations) in the shelter of woodland clearings, rides and edges.

The Peacock's name comes directly from that of its avian namesake, thanks to the similarity between the eye patterns on the bird's tails and those on the butterfly's wings. As long ago as the late 1600s, the butterfly was called the Peacock's Eye. Even the scientific name of the butterfly is derived from a Greek myth involving a Peacock bird.

Life cycle and behaviour

Adults emerge from hibernation from March onwards with numbers peaking in late April. They quickly mate and females lay eggs. The resulting offspring emerge as butterflies from late July onwards (later in the north). A small second brood is possible in the south in very favourable years. This was seen in a few places in the hot summer of 2003, when Peacock caterpillars found during September hatched out in early October.

This is supported by experiments that have demonstrated the role of daylength in determining the development of caterpillars into adults that either will breed immediately or will hibernate. In the laboratory, caterpillars from southern England have been induced to develop into reproductive adults when exposed to long periods of artificial daylight (equivalent to the longest days of summer). Therefore, a partial second generation may occur when favourable weather allows the rapid development of the first brood during the long days of mid-summer.

Males feed and disperse in the mornings, then establish territories on the ground in the early afternoon. These territories are invariably in sunny spots in the corners of woodland or hedges and are vigorously defended against other males. When a female is encountered, the male will abandon his territory and give chase.

Eggs

Eggs are laid in large, irregular clusters on the undersides of young nettle leaves. In contrast to the laying sites of the Small Tortoiseshell, large vigorous plants in the middle of nettle beds are often selected, although these must be in full sunlight at midday, when egg laying usually occurs.

Caterpillars

Caterpillars build communal webs over the growing tips of the nettles on which they feed, migrating to fresh plants as each is stripped bare.

The feeding damage, swathes of web, and the black, spiny caterpillars are very conspicuous in June. Eventually the caterpillars disperse to pupate suspended from vegetation.

Emerging Adults

The emerging adults congregate in large numbers at good nectar sources, feeding voraciously to accumulate fat reserves for hibernation. The butterflies also spend time prospecting for suitable hibernation sites, often in hollow trees, crevices in bark or stone, and in unheated buildings. Once a site has been found, the individual will remain in the vicinity, roosting in the site and feeding nearby during the day, until it enters hibernation. Most appear to begin hibernation in early September, although individuals may be seen in October.

The Peacock does not live in discrete colonies and individuals range widely across the countryside in search of suitable habitats. In marking experiments in Germany, Peacocks have been recovered over 90km away from the place where they were first caught and marked.

Observations have revealed a tendency to fly to the north-west in the early part of the season and to the south-east later in the year, and some migration to and from continental Europe is likely. Nevertheless, fewer individuals are probably involved in these movements than is the case for the Small Tortoiseshell or the true migrants, the Red Admiral and Painted Lady.

Distribution and trends

The Peacock is widespread and common in the south of Britain and Ireland, but less frequent to the north and at higher altitudes. In Scotland, it is resident in the west and on islands of the west coast, as far north as Argyll. It is probably also resident in the south-east of Scotland. Scattered modern and historical records show that the butterfly has reached many northern parts of Scotland, including Orkney and Shetland, but these individuals are probably vagrants. In January 2004, a Peacock was found in hibernation in a stack of cut peat on the island of Lewis in the Outer Hebrides!

Little change in distribution occurred in the southern part of its range during the twentieth century. However, in the north, the Peacock expanded its range from an apparent low point in the latter part of the nineteenth century. This expansion began in the 1930s and has continued to the present day in some places, notably Northern Ireland. Here the butterfly was recorded in only one-quarter of 10 km squares in 1970-82, but in over three-quarters of squares in the period 1995-9. The butterfly is more widely recorded now across the Pennines and has become much more common in the central belt and highlands of Scotland in recent years.

As with many butterflies that are currently expanding their range, data from the Butterfly Monitoring Scheme show a significant overall increase in the abundance of the Peacock since 1976. This trend is strong despite dramatic population crashes following wet and dull summers in 1985 and 1986, which caused extinction at several northern BMS sites and scarcity in many areas, for example around Dublin. Further falls in the collated index, such as in 1993 and 1994, did not prevent a record value being achieved in 1996. As with the Small Tortoiseshell, the largest increases in abundance have been in eastern England.

European trend

The Peacock's distribution is stable in most countries but there is evidence of expansion at the northern edge of its range, particularly in Finland (>100% increase in 25 years) and also in Latvia and Lithuania (25-100% increase). There has been a small decrease in Sweden (15-25% in 25 years).

Interpretation and outlook

The changes in the distribution of the Peacock appear to be driven by climate. The timing of its contraction and expansion relate well to periods of minor climatic deterioration and amelioration. Furthermore, although less dramatic, the Peacock's expansion in the northern part of Britain and Ireland has occurred in broad synchrony with that of other species, such as the Speckled Wood and the Comma. The Peacock, like the Small Tortoiseshell, has gregarious caterpillars that are able to raise their body temperatures above that of their surroundings. This lesser reliance on ambient temperature may explain the relative stability of the Peacock's distribution in comparison with that of the Comma.

The butterfly's distribution is not constrained by the range of its caterpillar foodplant, which is common and widespread across Britain and Ireland. In eastern England, agricultural intensification may have improved conditions for nettles and this may explain the significant increases in abundance at BMS sites in this region. However, these trends may not reflect higher abundance in the countryside as a whole. Monitored sites tend to be areas of semi-natural habitat and Peacocks rely on such areas for critical periods of their life cycles, notably courtship, mating, and pre-hibernation feeding. Suitable areas are rarer in eastern England, so the densities of Peacocks congregating at them might be expected to be greater, even if the actual number in the wider countryside is the same as elsewhere.

During the twentieth century, the Peacock expanded to occupy all of its former range by the early 1980s. The BNM survey indicates that this expansion has continued in parts of Northern Ireland and Scotland and the species may now occupy more territory than at any time in its recorded past.

Text adapted from the Millennium Atlas of Butterflies in Britain and Ireland (Oxford University Press 2001).