Saturday, 14 January 2006

Looking for wintering bugs

The other morning seemed suitably inviting to get out and about in search of overwintering bugs. Insects can seem a bit thin on the ground during the winter months but, if you know where to look, there are plenty to be found. I made my way to one of my favourite patches, armed with a stout stick, a pooter and a large white net. A pooter is a rather ingenious bit of kit that can best be described as a mini-hoover, with which you can suck up small insects into an observation pot. I started by working my way along the edge of a reedbed, stopping every few feet to position the net low down and to then tap the vegetation above the net with the stick. A fairly firm tap is usually sufficient to dislodge any insects and spiders that have chosen to overwinter in the vegetation. The white of the net enables you to spot those insects that are dislodged as they crawl around amid the bits of plant material that have also been dislodged. Within ten minutes or so I had several dozen individuals crawling around in the net: at least three different species of spider, two species of beetle, a caterpillar and various heteropteran bugs. Many of these were very small – one of the beetles was about the size of a pinhead – and most would need to be examined back at home under the binocular microscope before I would know what they were.

One of the spiders was familiar to me. Its elongated body and the way in which it held its legs – two pairs of legs fully extended forwards of the body and the other two pairs fully extended backwards – made it look like a fragment of grass stem. This was one of the two species of Tibellus spider known from the county. The pale straw colouration of these spiders helps them to perch inconspicuously on dried vegetation and ambush passing insects. Identification of this spider would definitely need a microscope, so subtle are the differences between the two species.

One species that would not need to be taken home for identification was a brownish-green bug, which had dropped into the net as I tapped a gorse bush. This was the aptly named gorse shieldbug, readily identifiable by its appearance and by the forward-pointing spine on its underside. As you might expect, it derives its name from an association with gorse and from the modified wings covering its back that give the appearance of a shield. Overwintering adults are brown in colour but come summer they turn a beautiful yellow-green. It had been a worthwhile visit, with a few more species to add to the list of those recorded from the site.

Friday, 13 January 2006

A late winter spectacle

As I have noted before, the long winter nights can cause problems for small birds and a number of species are known to roost communally to reduce heat loss. Such roosts can provide a spectacle for the winter birdwatcher, as tens, hundreds or even thousands of individuals congregate at favoured sites. Many readers will know of the huge starling roosts that may form at this time of the year but how many are aware of the roosts of pied wagtails or reed buntings that form each night throughout the winter months? Pied wagtails may gather to roost on large buildings, especially on some of the county’s larger supermarkets, and their loud ‘chis-ick’ calls fill the late afternoon air as individuals arrive from scattered feeding sites just before dusk. These congregations provide an ideal opportunity for teams of licensed bird ringers to catch and ring large numbers of birds, helping researchers to understand more about bird movements and survival.

Last Sunday afternoon, I was part of a team catching and ringing pied wagtails and reed buntings at a small roost in southwest Norfolk. The roost was in a reedbed, an ideal place to spend the night perched above open water and safe from predators, and the site has been used over a number of winters, although not always by the same species. A couple of winters ago, there were large numbers of starlings using the site. This winter there are very few starlings but fairly good numbers of wagtails and buntings. We arrived to set up our nets at two-o’clock and, once erected, all we had to do was wait for the arrival of the birds. Periodic checks of the nets during the intervening time revealed a number of wrens, all of which were local birds that had been caught at the site during previous summer sessions. Wrens often move into reedbeds during winter because this is one of the best places to find the invertebrates on which to feed.

As dusk approached, wagtails and buntings began to arrive. Parties of wagtails gathered together and wheeled about the sky calling, a spectacle that lasted for half an hour or so. The numbers of birds increased and then groups of individuals dropped down into the reeds, with some ending up in our nets. Quickly and efficiently our team removed birds from the nets and processed them, noting down details of wing length, weight and age (not an easy thing to discern in a pied wagtail) before letting them go so they could return to the roost. Other members of the team had, by this stage, packed away the nets and we were left listening to the occasional shuffles of birds moving to favoured positions in the reeds.

Thursday, 12 January 2006

Pigeons feast on ivy berries

The end wall of my garden is hidden beneath a mass of ivy. Arising from a single trunk, whose girth far exceeds that of my thigh, the ivy has spread nearly thirty feet, bushing up and out along the wall. Over recent weeks the ivy has become the favoured dining location for the local woodpigeons, which have come to feast on the unripe berries. The antics of these birds are a delight to watch and I have been surprised by their agility. Individuals clamber about among the foliage, sometimes bracing themselves with half-open wings and stretching head down to reach more distant fruits. They rarely seem to over-balance but do, on occasion, appear less than dignified in their posture.

A hungry woodpigeon will fill its crop with a very large meal, eating up to 450 berries in a sitting, and will follow this with a long period of rest and digestion. Individuals that have had their fill loaf about in the tall trees beyond the end of the garden, sitting high and alert to occasional forays by the local sparrowhawk. It is during late winter that woodpigeons typically turn to ivy. The berries are nutritious and have an especially high fat content. Interestingly, when ripe, they are known to be moderately poisonous to mammals and at least some birds, if eaten in any quantity. It is not clear if the woodpigeon is immune to the toxins or whether it is able to deal with them by being particularly careful in how it treats the fruit once eaten. Perhaps the berries contain fewer toxins when unripe. The berries are an important stopgap, a food that is available at a time when other favoured foods have become scarce. Certainly, the pigeons seem to move onto ivy berries earlier in years when the autumn beechmast crop has been poor. 

Ivy is not the only winter food that woodpigeons have learnt to utilise. They have also taken to oil seed rape in a big way and the introduction of this crop has made a big difference to our woodpigeon population. Historically, under farming regimes with over-winter stubbles followed by the spring sowing of cereals, the lack of suitable food between January and March was known to be the controlling factor behind woodpigeon population change. Many woodpigeons would starve during this period and the population would be kept in check. With the introduction of this new crop, the brake halting population growth has been removed allowing our woodpigeon population to expand in the manner that it has over the last ten years. This may be one of the reasons why so many woodpigeons seem to descend on my garden at this time of the year.

Wednesday, 11 January 2006

Are Norfolk's deer becoming a problem?

There has been quite a bit of discussion over recent months about the numbers of deer within the county. This has been part of a wider debate about deer numbers nationally and concerns have been expressed about the impact that they might be having on forestry and farming interests, other wildlife (by damaging woodland vegetation) and through collisions with motor vehicles. Just last week, there was an article on the subject in one of our local newspapers. The debate is an interesting one, particularly so for me in my role as County Mammal Recorder, and it raises a number of important issues centred around the interaction between humans and the other wildlife with which we share the countryside. 

Introduced species, such as muntjac and Chinese water deer, seem to be doing very well within the county and both species have increased in numbers and range over the last few years. Drive along any of the roads through Thetford Forest on a summer evening and you will see several dozen muntjac grazing on the verge during the course of your journey. These small, dog-sized, deer are surprisingly solid and any collision with a motorcar is likely to be damaging to both parties.

Research has demonstrated that at high densities, deer populations damage deciduous woodland by preventing natural regeneration and by removing much of the ground flora. This, in turn, reduces opportunities for other wildlife and is thought to be one of the reasons for the declines seen in a number of woodland bird species. Experiments using deer fences to exclude deer from small blocks within woodland show just how pronounced the impact of deer grazing can be. 

There may be good ecological and commercial reasons for the numbers of deer to be controlled and control through shooting has been an important component of forest management within the county for many decades. As with any control programme, it is important to know how many individuals there are in an area in order that a suitable level of control can be established. In the case of deer this can be something of a problem. Deer populations are not easy to monitor, in part because of their secretive nature, but also because populations may range over large areas. Traditional methods of monitoring mammals, such as recording field signs like tracks and droppings, are not always readily applicable to deer and require a high degree of skill on the part of the observer. Having said this, efforts are being made to build up a more complete picture of the number of deer in our region and various organisations are working together, through the Deer Initiative, to improve our understanding of these wonderful animals.

Tuesday, 10 January 2006

Worrying decline of much-loved bird

The largest and most majestic of our breeding finches, the hawfinch, is a now an increasingly rare delight for the Norfolk birdwatcher. With its cinnamon and chestnut plumage, black bib and extremely stout beak, this wonderful bird is one of the real gems of the bird world. Unfortunately, its elusive and secretive nature, coupled with an alarming decline in breeding numbers, make this a difficult bird to see, especially during the breeding season. It is during the winter then, that it is well worth visiting one of the handful of sites which support small wintering flocks. Within Norfolk, Barnham Cross Common and Lynford Arboretum, both in the west of the county, are the most reliable sites. However, even here the birds can be difficult to see. 

During the winter months, the hawfinch feeds on the fruits and seeds of various trees, favouring those of hornbeam, field maple and cherry. The bird uses its stout bill and very well developed facial muscles to crush open cherry stones. The stone is held within the bill, balanced on four horny pads attached to the inside of the upper and lower mandibles, and the bird then applies a crushing pressure reported to be more than 1,000 times greater than its own body weight! This ability is reflected in the bird’s Latin name of ‘Coccothraustes’, which derives from two Greek words meaning ‘a kernal’ and ‘I break into pieces’. The power of the bill, coupled with an aggressive nature, is probably the reason behind the rather peculiar courtship display of this species. The display is highly ritualised and involves a large number of submissive gestures and a very cautious approach on the part of the male.

The Hawfinch has a very patchy distribution in Britain and the most important populations are now located outside of Norfolk. These populations have declined, possibly because of changes in the quality of favoured woodland habitats, increased competition for food and nest predation by introduced grey squirrels. During the 1970s and early 1980s large flocks would be fairly commonplace in winter but flocks in excess of 10 birds are now most unusual. This pattern can be seen in Norfolk. In the winter of 1974/75 a flock of some 183 birds was present at East Wretham but by 1986 very few birds were using the site. Numbers at the Lynford and Barnham Cross Common sites are also far smaller than they once were – both in single figures. For how much longer will we hang onto these wonderful birds? However, there is one piece of encouraging news. Following a tree planting campaign, numbers have been increasing on the Dutch polders. Perhaps the hawfinch may benefit from similar schemes here in East Anglia.

Monday, 9 January 2006

Good numbers of wintering duck

On Thetford’s southern edge, squeezed between Barnham Cross Common and Shadwell Stud, lies the Nunnery Lakes Reserve; a series of flooded gravel pits. The lakes are very handy, just a ten minute walk from home, and I visit them throughout the year. An early New Year visit is usually productive, with a good chance of seeing the goosanders that use the reserve as a regular wintering site. Just now there are seventeen of these fish-eating ducks on one of the larger lakes.

The goosander is the largest of the sawbills, so-called because of the serrated inner edge to their bills that helps them grasp the small fish on which they feed. With a relatively long neck and a flat-backed body, the goosander is well-adapted to catching fish. When searching for prey, a goosander will swim along with the head and neck held underwater, scanning for shoals of small fish, before diving and pursuing its prey. It is this dependence on small fish that has brought this elegant piscivore into conflict with game-fishing interests. In some areas, goosanders are shot under licence to reduce their perceived impact on certain fisheries. However, the degree to which the control measures benefit the fisheries is unclear, as is the impact on the wider goosander population.

Although goosanders breed across much of Northern Britain, they do not breed in Norfolk. Wintering numbers in the county tend to peak between mid-December and late February, making the goosander one of the latest winter visitors to arrive. The birds almost invariably winter on inland waterbodies and a number of Norfolk’s inland lakes, rivers and broads will hold one or two individuals in most winters. Only UEA Broad, the Nunnery Lakes and the Great Ouse relief channel regularly support more than a dozen individuals. While most of the British wintering population is drawn from the British breeding population, those birds wintering in Norfolk are thought to originate from the Fennoscandian and Russian breeding populations. 

One of the most interesting aspects of their annual cycle is the pattern of movements made by the males. Once the females begin incubating their eggs, males from across much of the European breeding range migrate to northern Norway, where they gather to moult in huge numbers. This moult takes several weeks and the birds do not return south until November. The females, faced with the business of rearing their young, remain on the breeding grounds to moult once the young become independent. The two sexes then meet up again at the chosen wintering sites. The good numbers present this year may be boosted later into the winter, especially if birds wintering on the Continent are pushed further west by another spell of very cold weather.