Saturday, 23 September 2006

From blackberries to airmiles

Despite the fact that we enjoyed a couple of summer-like days earlier this week, there is no escaping the fact that autumn is upon us. The telltale signs are there; browning of the bracken, the scent of mushrooms in the woods and a change in bird behaviour. For many birds, the time has arrived to start out on an autumn migration that will take them south from our shores, into Europe and beyond into the dark heart of Africa. In most species, this is not a journey upon which they can embark without first making some important preparations. Flight requires fuel and small birds, like migrant warblers, must take on board the fuel necessary for at least part of the journey, perhaps stopping en route to top up their reserves. Only those that can feed on the wing, such as swallows and swifts, can avoid having to lay down large reserves of fat in advance.

Many of our small migratory birds will add to their fat reserves very quickly, perhaps adding between 10 and 13pc of their body weight each day during this period of fattening. Some of the increase comes from eating more food, with birds making the most of autumn’s bounty. At the same time, many species make better use of the food they ingest, either through changes in their metabolism or by selecting foods that are easy to digest. Many fruits are ideal; low in fibre, abundant at this time of year and easy to digest, they also tend to be rich in sugars (which help in the deposition of fat). Hedgerow shrubs heavy with berries will be well-attended by warblers and thrushes over the coming weeks. Other species, such as the sedge warbler, feed selectively on aphids, a behaviour that may be related to the relatively high sugar content of these invertebrates.

Bird ringers catching birds at this time of the year often comment on just how fat some of these small birds can become. While a species like a garden warbler would normally weigh about 17g, just prior to migration it may reach 34g, raising the question of how it manages to get off the ground. This highlights the trade-off that a migrating bird has to make. The more fuel it takes on board, the heavier it becomes and the slower it flies (making it a target for predators). There must be an ideal fuel load for the journey, something that will depend on how far the bird has to go, whether it can refuel en route and how likely it is to meet a predator. It is a good job that things are less complicated when we go on holiday!

Friday, 22 September 2006

One for sorrow, two for joy

The old saying begins “One for sorrow, two for joy” but the other morning I find myself wondering what would seven magpies bring. The seven in question were gathered in the middle of the road around the corpse of a brown hare, presumably killed by a passing car. They took to the air as I drove towards them; young birds, their tails not yet fully developed, with at least one adult bird alongside them. Perhaps they were learning the ropes, discovering the easy meals to be had at this time of the year on Norfolk’s roads.

Despite striking plumage and resourceful nature, the magpie is often viewed with suspicion or even outright loathing. It has long been said that the magpie has a drop of the Devil’s blood in it. This is not the only religious association. The magpie was the one bird that refused to enter Noah’s ark, choosing instead to perch on its roof and chatter and swear at the drowning of the land. To add to this, Christian tradition has it that the magpie did not go into mourning at the time of the crucifixion. The habit of caching food for use in more difficult times, a strategy adopted by a number of bird and mammal species, has given the magpie the reputation of being a thief. This is perhaps most famously celebrated by Rossini’s opera ‘La Gazza Ladra’ – ‘The Thieving Magpie’.

You have to have respect for the magpie. An incredibly adaptable bird, it is able to feed on a very wide range of foodstuffs and has managed to exploit all sorts of opportunities. The magpie was one of the first species to peck open foil-covered milk bottles to reach the cream inside, a behaviour that extended to raiding egg boxes left alongside. However, the habit of raiding the nests of other birds brings an angry response from some quarters. While such behaviour can ruin breeding attempts, there is no evidence that this act of predation has had any effect on long-term changes in songbird populations, despite several scientific studies looking hard at this very issue. Our long history of rearing game no doubt contributes to a collective anger directed at the magpie, with persecution of this species fortunately less intense that it once was.

So what of the rhyme? What would seven magpies bring? There are many variants of the rhyme but the oldest version that I can find (and the one I use here) dates from 1750. It runs “One for sorrow, two for joy, three for a girl and four for a boy. Five for silver, six for gold, seven for a secret never to be told.” Don’t tell anyone but I rather like magpies.

Thursday, 21 September 2006

Celebrating Norfolk's moths

There is something deeply rewarding about running a moth trap. The sense of anticipation that grips me each morning, as I come to empty the trap of its nocturnal visitors, reminds me of being a child on Christmas Day, eager to open presents waiting under the tree. Perhaps, in some way, catching moths rekindles the ancient instincts of Man the hunter. More likely though, it reflects a need to discover, identify and “collect” (in this case photograph).

Running a moth trap requires little effort on my part. The trap itself is a wooden box, partly covered with two perspex sheets, above which a bright mercury vapour bulb is placed. Set at dusk, the trap does its work through the hours of dark, leaving me to rise at dawn and check the contents. Moths are attracted by the light and, flying into the bulb, drop into the box below. Here they seek shelter amid the pile of old egg boxes, remaining dormant until my arrival. Unless you have ever seen a moth trap in operation, or rather the results of its work, you will have no idea of just how many beautiful and diverse moths there are in Britain, both in terms of species (about 30 for every species of butterfly) and number of individuals. Many of our moths are more beautiful in appearance than their diurnal cousins but because they are largely nocturnal, they are under-appreciated.

All that may change if, as seems likely, increasing numbers of people continue to take up moth trapping. This coming weekend sees an event that is sure to encourage more people to take up this fascinating hobby. Saturday 23rd is National Moth Night – a celebration of Britain’s moths and of moth recording in general. The organisers, Butterfly Conservation and the publishers of Atropos (a journal for moth enthusiasts), hope that people across the country will either visit planned moth events or set up their own simple moth trap (a bright light shone on a white sheet). The timing of National Moth Night differs from year to year, to vary the range of species encountered – moths have defined periods when they are on the wing. While September may not be the best month for catching large numbers of moths, it offers some rather colourful species, particularly migrants from across the channel. The results collected by participants are pooled into Britain’s largest survey of which species are flying and where, highlighting the conservation value of trapping.  Judging by the sorts of species being reported across Norfolk at the moment, it should prove to be a very interesting event. To get involved yourself, why not grab an old sheet, a bright torch and visit to find out more.

Wednesday, 20 September 2006

Disease outbreak hits finches

I have received an increasing number of calls over recent weeks from garden birdwatchers concerned at the appearance of dead and dying finches. In most instances these reports have involved greenfinches and chaffinches but there have also been occasional reports involving other species. This is not the time of the year when we would normally see birds dying from common diseases like salmonella and e-coli (these are normally seen in late winter), which suggests a different source. Thanks to the efforts of researchers working on the Garden Bird Health initiative, we have discovered that the disease involved is trichomonosis. This disease is caused by a parasite and it has been documented in a range of bird species, most commonly in doves and pigeons (where it is more commonly known as ‘canker’). The disease was first recognised in finches in Britain last summer, when a small outbreak was noted but this summer the outbreak seems to be that much larger and this has caused some concern.

Individual birds showing signs of the disease appear lethargic, fluffed up and reluctant to move away from feeding stations. They may have difficulty swallowing and may drool saliva or regurgitate food, something which can cause the feathers around the bill to become wet. Although in captivity the disease can be treated, it is virtually impossible to treat wild birds with an effective dose of medication. This means that prevention is the only practical option and garden birdwatchers should aim to reduce the risk of disease transmission between birds by adopting sensible hygiene practices. These include keeping bird feeders and bird tables clean (with a disinfectant or detergent) and by insuring that clean, fresh water is available each day, provided in vessels that have been thoroughly cleaned. It has been suggested that you should stop feeding altogether if you find any sick or dying birds but I feel that it is better to feed birds at a clean feeding station than force them to move on and potentially feed elsewhere where no hygiene measures are in place.

Most of the reports that I have received so far have come from western and central England but we should be alert here, in Norfolk, to the possibility that the disease might arrive. The parasite itself cannot survive for long outside of the body and it appears to be transmitted through salival contamination of food and water. This may be why it has been more widespread this summer – with the dry conditions reducing the availability of drinking pools and forcing more birds to congregate at those which remain.  More information on the disease and on sensible hygiene measures can be obtained from the British Trust for Ornithology and UFAW.

Tuesday, 19 September 2006

Dragonfly gives me the once over

It is quite a strange feeling to be given the once over by a dragonfly. On one very warm afternoon last week this is exactly what happened. A dragonfly, about seven centimetres in length, approached me with a very direct flight and hovered just in front of my face. After a brief moment it adjusted its position, viewing me from a slightly different angle before turning and moving away to continue its patrol. As it turned I could see the paired spots, one large pair on each segment of the elongated abdomen. Most of the spots were a pale leaf green in colour. Those on the last three segments were pale blue and the last two pairs of these were each fused into a single larger spot, all characteristic features of a male southern hawker dragonfly. This is an inquisitive species that patrols low down around its favoured small, shaded pools and ponds. As such, it is often common in wooded areas and in urban gardens, where cats frequently catch it.

This was not the only species of hawker that I have seen in recent days. The similar, though somewhat smaller, migrant hawker has been much in evidence, patrolling over local ponds and getting caught in our mist nets, used to catch swallows on the local fen. The male migrant hawker also sports paired spots, though his are pale blue along the full length of the abdomen and all remain unfused. A yellow wedge, shaped like a golf tee, appears in the middle of the second abdominal segment, a feature absent in our other hawkers and useful when dragonfly watching. Migrant hawkers appear to be less inquisitive than their larger relative, patrolling low only around breeding ponds and preferring to search for food up along the edge of trees and hedgerows. As its name suggests, the migrant hawker is, to some extent, a summer visitor, with large numbers reinforcing our resident breeding population.

These two ‘blue-green’ hawkers are joined in Britain by another ‘blue-green’ species: the common hawker, which, despite its name, is anything but common within Norfolk, being restricted to a few localised sites in the east of the county. There are also two brown-coloured hawkers to be found in Norfolk, one of which, the Norfolk Hawker, is not found anywhere else in Britain. With its brilliant green eyes, this striking beast is well worth looking for in spring around the Broads. Any brown-coloured hawkers seen on the wing at this time of the year will be a brown hawker, a more widespread species. A number of related species may turn up in the county as vagrants and, with global warming, some of these may yet become established.

Monday, 18 September 2006

Icons of the northern summer

Over the past few weeks I have spent a number of very pleasant evenings out on the fen. Working with a team of bird ringers, I have been involved in an important international project tracking the autumn swallow migration. The British Trust for Ornithology’s ‘Swallow Roost Project’ sets out to assess the pre-migratory strategies of different populations of swallows by collaborating with teams of bird ringers in other parts of Europe. Our own efforts on the fen over the last few years have varied in their success. On some evenings the swallows play ball and fill our nets. On other evenings, the nets remain empty.

The swallows come into roost just before dusk, favouring the edge of a reedbed where, with water beneath them, they feel safe from predators. Small parties of swallows, moving through the river valley, are drawn down to the reedbed by the swallow calls emanating from our tape-lures but it requires knowledge of where they prefer to roost to make sure that we place our nets correctly. If we get it right then the birds are caught in the nets as they pass low over the reeds. Once all the birds have been safely removed from the nets, we begin the process of ringing the birds and taking the all-important measurements that will help us establish how these long-distance travels prepare for their epic journey.  After we have processed each swallow it is placed in a roosting box where it will spend the night, before being released early the next morning. Preliminary examination of the information that has been collected suggests that British swallows, in contrast to many other small migratory birds, do not put on much weight before they depart our shores. Instead, they appear to fatten up on the Continent before making the journey across the Mediterranean, into Africa and across the great expanse of the Sahara.

Sometimes our efforts do not succeed because the swallows choose to roost in a different part of the reedbed. On other occasions, the presence of a hobby may frighten the swallows off, such that they choose to roost somewhere else entirely. The hobby is a superbly adapted bird of prey, with an explosive burst of speed and well-able to catch a swallow. As the season progresses, hobbies seem to appear more frequently and the number of swallows using the roost becomes more erratic. Just last week we had at least two hobbies over the fen and the 60 or so swallows remained high, refusing to venture down into the reedbed roost. With the hobbies present the swallows eventually moved off elsewhere. Even on those evenings when we fail in our task, there is always plenty to see, making the effort worthwhile.