Saturday, 11 February 2006

HIbernating butterflies

Although winter can be a difficult time, there are species that manage to avoid the worst of its ravages by going into hibernation. Many are familiar to us: newts that retreat into the spaces under log piles, hedgehogs that bury themselves in last autumn’s leaves or ladybirds that squeeze into the cracks of a window frame. Most of these creatures will spend the winter unnoticed by homeowners and gardeners, but others may be chanced upon as logs are brought in from the woodpile or something is fetched from the garden shed.

This winter, a small tortoiseshell butterfly has chosen my shed and it is interesting that this butterfly should choose to overwinter as an adult. Very few of our resident butterflies overwinter as adults; the majority spend the winter as caterpillars, although nine species overwinter as eggs and eleven as pupae. It is also interesting to note that, with the exception of the speckled wood, the hibernating phase is always the same in a given species. The speckled wood can hibernate as either a caterpillar or a pupa. Along with the small tortoiseshell, the other species to overwinter as adults are comma, peacock and brimstone. To these can be added large tortoiseshell (though now thought to be extinct as a resident species), red admiral (though only in the extreme south of the country does overwintering appear to be successful) and Camberwell beauty (which overwinters occasionally). Many of these species will spend the winter in hollow trees or outbuildings but the beautifully camouflaged comma prefers to overwinter on exposed branches or among dead leaves and the brimstone hibernates deep within an evergreen shrub. During hibernation these butterflies must be vulnerable to predation by birds and this may be why many have evolved their camouflaged underwings. The peacock goes a step further and has evolved a warning display. If disturbed, the peacock will open its wings to their fullest extent to reveal the large ‘owl-like eye’ pattern on its wings. At the same time it produces a hissing sound by rubbing its wings together, something that is usually enough to scare off a would-be predator.

Once we receive a run of several days of warm, spring-like, weather many of the butterflies will emerge from hibernation to seek out nectar sources to replenish reserves lost over winter. None of the species is able to mate immediately on waking but needs to feed and bask in the sun to develop the eggs or sperm needed for the reproductive act. The butterfly can always re-enter hibernation if spring has not really arrived, although it will have lost vital reserves and be less likely to survive the remaining winter period.

Friday, 10 February 2006

Winter songsters

There have been a few warmer days of late, premature hints of a spring that is still a number of weeks away. Such teasing glimpses make February burdensome – winter can’t still be upon us. I want to get out and watch nature springing into life. I am not the only one eager to get going, for outside in the early morning darkness a song thrush is singing; its strident notes striking out above the plaintive winter song of a robin. The robin is one of the few birds to defend a territory year round and this is why its wistful tunes can be heard throughout the winter months. But the song thrush is a less regular songster at this time of the year and its performance is an optimistic sign that winter will soon be over.

The impression ones gets when listening to a song thrush is of a singer that enjoys the act of performance, something that is reflected in the bold clarity of the delivered phrases and the deceptive simplicity of structure. Each song thrush will have a repertoire of some 100 or so different phrases and appears to select from these almost at random, putting several together and often repeating a sequence a number of times over. This fondness for a rhythmic repetition of repeated phrases may be one of the reasons why the song thrush features so prominently in poetry. One of the best examples of this comes in Tennyson’s ‘The Throstle’ but others, including Thomas Hardy, Robert Browning and Edward Thomas, all draw on this English songster. Edward Thomas, undoubtedly the least well-known of these poets, featured our various thrushes in 15 or so of his 142 poems and his use of colloquial speech rhythms is well-suited to the repetitive nature of the song.

The presence of repeated phrases is characteristic of the song thrush and very useful for separating this species from another late winter songster, the mistle thrush. The latter species has a song that is more reminiscent of a blackbird, though harsher in tone with a faster tempo. This is delivered from the upper branches of a tree and is loud and far-carrying. Many authors have commented upon the feeling of sadness that derives from the mistle thrush song. Perhaps this is added to by its habit of singing on bleak, overcast days or during periods of wind and rain, a behaviour that has earned it the local name of ‘stormcock’. Although I admire the mistle thrush for singing, indifferent to the harsh backdrop of a late winter storm, I prefer the optimism offered by the song thrush: “Spring is coming, spring is coming, spring is coming”.

Thursday, 9 February 2006

On the trail of the Otter

There have been more signs of otter activity locally in recent weeks, encouraging news for a species that is recovering from a decline that began fifty years ago. The introduction of persistent organochlorine pesticides was part of a post-war drive towards agricultural self-sufficiency. The otter population suffered the consequences of our lack of understanding of the impacts of these toxic chemicals on the wider environment. Now that the use of such chemicals has ceased, we are seeing a recovery in the otter population, albeit with something of a helping hand in the form of a reintroduction programme. Even though the signs are encouraging, many of the Norfolk rivers that may once have supported otters have yet to be reoccupied. Periodic surveys, together with the conservation efforts of the Otter Trust and the Anglian Otters and Rivers Project, show that the population is recovering and expanding slowly.

Seeing an otter is a rare occurrence. The species is secretive and mainly nocturnal and this is one of the main reasons why encounters are so rare. Here is a creature perfectly adapted to its riverine existence and well able to slip away unnoticed if disturbed. Roger Deakin, in his wonderfully evocative book Waterlog, writes of a momentary encounter with an otter while he was swimming in the River Waveney. Deakin describes the otter’s “puckish, Dionysian habit” of veiling itself from view, something to which I can relate from my own encounters with these spirits of the river. While Deakin’s otter slipped from the floating log on which it had been sunning itself to disappear from sight, my most engaging encounter was with an otter that seemed positively curious about what I was doing. The otter was hunting in a deep pool that I often scan for pike and I saw its dark silhouette below the surface. I waited patiently and then the otter surfaced. It was clear that it had seen me against the sky but, nonetheless, it seemed inquisitive, almost pleasantly surprised by the creature that stood staring down from the bridge above. Having given me a rather prolonged once over, the otter slipped away into the deep of the pool and was lost in the shadow of the bridge.

I walk along this stretch of river everyday, to and from work, and yet I have only seen the otters themselves on a handful of occasions over the last five years. Their droppings, placed at prominent locations to advertise their residency to other otters, are more commonly encountered so I know they are still there. Knowing this, generates a sense of anticipation each time I walk along the river and I wonder if I am being watched from the shadows under the bank.

Wednesday, 8 February 2006

Norfolk home to grey geese

Norfolk has a reputation for its wintering geese. This winter, the reputation seems particularly well-deserved, with huge numbers of geese to be seen at sites across the county. Holkham, in particular, has exceptional numbers this year, with tens of thousands of pink-footed geese present. The sight of huge skeins of geese moving between their roosting and feeding grounds is one of the most sensational experiences for any birdwatcher. The numbers of pink-feet wintering here are internationally important. The entire breeding population of Iceland and eastern Greenland winters here, representing more than 85% of the World population. Favoured roosting and feeding areas have become established over the years, with the birds choosing to roost on secure sandbars and shingle ridges off the North Norfolk coast. Their principal diet during the winter months is sugar beet tops, a food they first began to utilise in 1966. At Holkham, the remaining sugar beet tops tend to be ploughed in during February and the geese switch to feed on permanent pasture and winter cereals.

Pink-feet are wary birds and favour sites where disturbance is minimal. They prefer large fields and avoid those with tall surrounding hedges. If undisturbed, feeding will continue throughout the day, with the birds returning to the roost at dusk. However, on moonlit nights they may remain in the fields, making the most of feeding opportunities and good visibility. Although the birds tend to use a small number of favoured roosts, the roosts themselves are not self-contained and individuals may shift between them. A very small number of individuals from the Svalbard breeding population, which winters in the Netherlands and Belgium, is known to join our wintering flocks.  These are not the only geese to mix with the pink-feet, since small numbers of white-fronted geese (of two distinct races) and genuine wild barnacle geese may also be present.

Norfolk is also fortunate in having wintering bean geese. Two different races, known as tundra bean goose and taiga bean goose, winter within the county. In recent winters, some 100-150 taiga bean geese have used the marshes on the northern side of the River Yare, typically at Cantley and Buckenham, and are the only regular wintering flock of these geese in England. Individuals of the tundra race were first recognised as wintering here in 1987 when up to seven individuals visited Buckenham Marshes, remaining separate from the local taiga flock. Small numbers are reported annually, often mixed in with the flocks of pink-feet at sites on the North Norfolk coast. The different ‘grey’ geese can represent something of an identification challenge but, even if you just view them as ‘grey’ geese, they make a spectacular sight.

Tuesday, 7 February 2006

Keep an eye out for our smallest mouse

The winter months provide an ideal opportunity for collecting information on the distribution of Britain’s smallest species of rodent. Weighing just six grams when fully grown, the diminutive harvest mouse is rarely seen alive and many of the records that I receive, as county mammal recorder, relate to individuals taken by pet cats. The harvest mouse is the only British mouse to build a nest of woven grass leaves well above ground level and it is these nests that can be found by careful searching during the winter months. Two types of nest are built: a breeding nest (up to ten centimetres in diameter) and a smaller non-breeding one. Both are spherical in shape and composed of grass leaves, carefully woven together from living material by the female. There is no obvious entrance hole in a fresh nest and the female may enter and leave by forcing her way through the weave. After the female has abandoned the young, usually at around 15 days of age, they will continue to use the nest for several more days. During this time, obvious entrance holes may develop as the nest receives something of a battering from the young.

As the vegetation dies back in winter, harvest mice abandon the stalk zone in which they have spent the summer searching for insects and seeds, and move down to ground level where they use runs made by other small mammal species. Here they may build temporary non-breeding nests in the base of tussocks. A few years ago, I spent time studying small mammals in an area of wet meadow and it was only during the winter months that I tended to catch harvest mice in my live-traps, positioned at ground level. These russet-coloured mice are ideally suited to a life in the stalk zone and are equipped with a semi-prehensile tail, which they wrap around stalks to steady themselves as they move through the vegetation.

Because harvest mice need long vegetation, it is worth searching for their nests in areas of rough grassland, alongside the base of hedgerows or ditches and in reed beds. The well-constructed nests often last well into the winter and careful searching of clumps of cocksfoot grass can prove rewarding. Anne Brewster, who has been recording the presence of harvest mice around her village since 1976, has now found the species to be widespread locally. The species is probably more widespread within Norfolk than the records gathered to date would suggest. So, why not check out a few likely sites in your area and let me know if you find any abandoned nests. As county mammal recorder I am keen to improve our understanding of this often overlooked species.

Monday, 6 February 2006

A scatter of winter thrushes

It was wonderfully bright and crisp the other morning, as I explored the fields around Harling. Mobile tit flocks were much in evidence within the small patches of woodland, while great spotted woodpeckers could be heard drumming in the distance.  Near to the Dower House at West Harling I came across a mixed flock of birds that was feeding on an area of overwinter stubble. The flock was not overly large but was spread out over a reasonable area and, as I scanned across it with my binoculars, I was able to pick out a number of different species. Brightly coloured yellowhammers stood out against the stubble, foraging alongside less obvious greenfinches and the occasional chaffinch. Starlings shuffled about with a busy action, searching for food, among similarly sized redwings and larger fieldfares.

The fieldfare is one of my favourite birds, with a gait and pose similar to a mistle thrush but marked more distinctively. The bold, grey head and rump contrast with a rich chestnut brown back, dark wings and a black tail. These wary birds often stand with the neck and head held high, the wings drooped at their side and the tail held horizontally. They have a rapid, almost bounding gait and, when disturbed, they will take to the top of the nearest tree. The redwing is much smaller and more subtly marked than its larger relative. Slightly smaller than a song thrush, the redwing is darker in colour and has a distinctive white stripe above the eye (known as the supercilium). The chest has dark streaks, rather than spots, and the underwing is smudged rufous-orange.

Both species are winter visitors, arriving in autumn from breeding grounds that stretch across Fennoscandia and east into Russia. Arable fields containing permanent pasture and stubble are favoured and the birds feed on earthworms, snails and other invertebrates taken from the surface of the soil. In early winter, fruits and berries feature prominently in their diet but stocks are usually exhausted before the year’s end. Redwings, in particular, struggle during cold weather and will make nomadic movements to escape the worst of the cold. In some years, individuals will venture into gardens in search of food. Although more robust, Fieldfares may also visit gardens to feed on windfall apples.

This flock was one of few that I have seen this winter and there seem to be fewer around this year. It will only be a number of weeks until these wonderful birds begin to depart, though some may linger through into May and occasional birds have summered here in previous years. While their departure heralds the arrival of spring, it will be a shame to see them go.