Wednesday, 31 May 2006

Scheme charts nesting success

On a Saturday, by accident, I found my first yellowhammer nest of the year, situated just above the ground in a hawthorn bush and surrounded by rank grass. The presence of the nest was revealed when a female yellowhammer shot out of the bush I had just tapped with my butterfly net. Tapping bushes is a useful way of discovering hidden butterflies and other insects.

The nest was made of grass stems, woven together and lined with fine grass and hair. The front edge sported a ‘doorstep’, often characteristic of the species, and inside were three off-white eggs. Each egg was beautifully marked with a mixture of thin and thick squiggles, almost as if some child had taken a fine pen to a blank egg and added a random design. Having counted the number of eggs and checked that they were warm, I moved away to let the female back on. From a distance, I made a mental note of where the nest was located and jotted down a few notes in my field notebook. This would enable me to make further visits to the nest over the coming weeks and to collect information of value to the British Trust for Ornithology’s (BTO) Nest Record Scheme. Through the scheme, the BTO is able to gather essential information on breeding success and productivity for a large number of our breeding species. This information is then used alongside that from other BTO surveys to help find out why particular species are in decline or on the increase. The network of Nest Recorders operates under a strict code of conduct. This governs the way in which nests are visited and standardises the way in which information is collected. Recorders typically make a number of visits to each nest throughout the nesting period, in order to collect information on the number of eggs laid, the number which hatch and the number of chicks which survive through to fledging. Perhaps the most important information concerns the outcome of the nesting attempt: was it successful or did the nest fail because of predation, weather or desertion?

The timing of my visit, coupled with the presence of an incubating female, suggested that the clutch was complete at three eggs (yellowhammers may lay up to five eggs but don’t start incubating until the final egg has been laid). This will enable me to plan the timing of my next visit so that I can collect information on hatching success and gain a better indication of when the first egg had been laid. Two more visits will follow this and then I will send in my completed Nest Record Card to the BTO to add this important dataset.

Tuesday, 30 May 2006

Lament of the summer crooner

Over the last two weeks, my early morning walks through the forest have been accompanied by the gentle purring call of a turtle dove.  This, our only migrant member of the pigeon family, is a summer visitor to Norfolk, arriving in late April or early May. By now, many pairs will have laid their first clutches of eggs and may just, if conditions are favourable, produce another before departure to their African wintering quarters. For me, the soft hypnotic purr is a reassuring part of summer, heard in my youth from southern counties on warm summer evenings and now a feature of my morning walks. Yet, there is a sorrowful undertone to the call itself, something that has resonance with many who hear it. Shakespeare famously used the image of a calling turtle dove in the final scene of The Winter’s Tale, when Paulina proclaims “I, an old turtle, will wing me to some whither’d bough, and there my mate, that’s never to be found again, lament till I am lost.”

The turtle dove has a breeding range that is largely restricted, within Britain, to the southeast of England. This reflects an association with arable land, the birds favouring areas with suitable thick hedgerows for nesting and abundant weed and (latterly) cereal seeds for food. Historically, it was believed that the arable weed fumitory was the most important food item but recent work by the Game Conservancy, carried out here in East Anglia, has shown that oilseed rape and cereal now dominate. Perhaps the increasing use of herbicides to control arable weeds has forced the birds to exploit new feeding opportunities. Other evidence also points towards the birds finding it more difficult to survive within a changed arable landscape. Individuals will travel up to 10km from their breeding territories in order to find good feeding opportunities and this, coupled with a decline in nest site availability, may be behind an 80% decline in numbers between 1967 and 1999.

It is not just within Britain that the turtle dove population has been under pressure. The wintering areas in Sahelian Africa have increasingly suffered from drought and the birds themselves are heavily persecuted during migration by hunters. Within France, where such hunting is illegal under European legislation, nearly 200,000 were shot in Autumn 1998, while a further 25,000 were shot on their return northwards the following spring.  For a species that is now having difficulty in rearing more than one brood in a season, such levels of persecution cannot be sustained. Against such a background, Shakespeare’s verse takes on a different meaning. The day may soon come when we lament the passing of the turtle dove’s song from our summer overture.

Monday, 29 May 2006

Russian doll parasites

I do not usually make an effort to identify the various flies that I encounter when looking for other insects. However, the other day I came across a large and rather distinctive fly, settled on a hawthorn. A quick flick with my butterfly net and the creature was soon potted up for closer examination at home, where I would be able to use my reference books to attempt an identification. My guess that the fly might prove readily identifiable turned out to be correct and, thanks to an old and rather battered copy of Colyer & Hammond’s “Flies of the British Isles”, I discovered that it went by the Latin name of Tachina fera and belonged to a group of flies with some rather unsavoury habits. Colyer & Hammond describe this group as containing some of the most handsome flies but with their hairy, almost bloated, abdomens I feel that the use of the word “handsome” gives rather more credit than is deserved.

Tachina fera is a rather common fly and may be abundant around water mint during late summer. The species, like other members its family, is parasitic upon the caterpillars of various moths and butterflies and has been recorded using autumnal rustic, pine beauty, black arches and large white, amongst others. Adult flies deposit their eggs in places frequented by suitable hosts and the larvae that hatch from these eggs search out and bore into a host. The parasite remains attached to the body wall of the caterpillar by its rear spiracles. These are the tubes through which it breathes. So, even though it is living within its host (and feeding on its body fluids) it still draws oxygen in from the outside. Other members of this group of flies may enter their host as eggs, being ingested as the host feeds on plant material. They then hatch within the intestine and ‘migrate’ to their final position. The presence of the parasite does not usually cause fatal damage straightaway. In fact, the host will typically continue to grow and develop as normal, with the parasite “creaming off” some of the resources being laid down. Ultimately, however, the parasite will need to pupate and emerge from its host and it is at this stage that death occurs.

While the discovery of such unpleasant habits was a little unsettling, Colyer & Hammond provided an unexpected twist to this tale of parasite and host. It turns out that adult flies in this family may be subject to parasitism themselves, typically by small wasps, and sometimes the fly larva, living inside its caterpillar host, may itself have a parasite living within it – a parasite within a parasite, a bit like a Russian doll.

Saturday, 6 May 2006

A sense of anticipation

There is a sense of anticipation in our household at the moment, for any day now will see the return of our breeding swifts. This is the first house that I have shared with swifts and their presence during the early part of the summer is one of the perks of urban living. To sit in the back garden on a still summer’s evening and watch the parties of swifts screech past is magical. No wonder Ted Hughes felt driven to write a poem about them – these birds are the true masters of our skies.

The first swifts have been in the country for a couple of weeks now but the main arrival comes at the start of May, with birds returning to nest sites used the previous year. Their stay is a brief one, just sixteen weeks, making them one of the last migrants to arrive in spring and first to depart in late summer. The largest colonies within the county can be found in Norwich, where the birds nest in cavities under the eaves of older properties. Many of these nesting sites have been lost over the years as renovations restrict access to nesting chambers. New properties tend not to be swift-friendly, although some local authorities have taken the lead in adding swift boxes to their new developments, something that is not only an encouraging sign but also an indication of the strength of the relationship between ourselves and these enigmatic visitors. The name ‘swift’ comes from the Old English word ‘swifan’ which means ‘fast moving’, an apt name for this slender-bodied bird with its long, swept-back wings. Edward Thomas, in his poem ‘Haymaking’, described their form ‘as if the bow had flown off with the arrow’ and to me, this seems a most fitting description.

Swifts feed at higher altitudes than either swallows or house martins and undertake long flights in search of food. Depending upon the weather systems in place at the time, swifts breeding in Britain may forage as far afield as Germany, tracking clockwise around low-pressure systems to seek out areas with the greatest abundance of prey. If weather conditions are poor, then adult swifts may be unable to find sufficient food for their chicks and the chicks will enter a state of torpor. This behaviour enables them to go without food for up to 48 hours. Immatures may not breed until their fourth year and it is thought that during this period they remain entirely on the wing, again highlighting their mastery of the air. Given this behaviour, and the short duration of their stay in Britain, we cannot really call them ‘our’ swifts. They are brief visitors and we should enjoy them while we can.

Friday, 5 May 2006

Overlooked Dipteran on the wing

Patches of early-flowering plants in sheltered locations provide an ideal opportunity to watch many of our often-overlooked insects. Such spots are typically a few degrees warmer than surrounding areas, something that suits those insects that may be limited by air temperature. In early spring, a couple of degrees difference in air temperature can mean that an insect is on the wing, rather than sitting immobile in a state of semi-torpor. With a bank of young nettles and a range of spring-flowering plants, one warm corner of the Nunnery Garden in Thetford holds a great deal of interest for the entomologist. Just the other afternoon, there were several species of hoverfly on the wing, along with queen wasps, five species of butterfly and our commonest species of bee-fly, Bombylius major.

Bee-fly - Bombylius major

Bee-flies are the dipteran equivalent of hummingbirds, able to manoeuvre with ease and adjust their positioning while hovering to insert their long proboscis into tubular flowers. Bombylius major is about 11mm in length, furry in appearance and ginger in colour. The long proboscis is a prominent feature and one that may give cause of alarm. However, this fly is harmless and does not sting or bite. The high pitched whine made by its flight does remind me of a mosquito though and has, on occasion, given me cause to flinch as one has drifted past my ear. Although this species reaches as far north as Scotland, like other bee-flies it is on the northern limit of its range and so is most commonly encountered in southern Britain. Research has revealed that it cannot take to the wing if the temperature is below 17ÂșC and will vibrate its wings rapidly to increase its body temperature if necessary.

Bee-flies can be difficult to approach and I find that the best thing to do is to sit still, just in front of flowers that are likely to be visited. The territorial nature of Bombylius major also helps (as it does when watching many hoverfly species), since an insect that has been flushed will often return to the same patch after a few minutes. Male bee-flies will make darting forays to intercept other insects entering their airspace, presumably in the hope that one of the intruders will turn out to be a female bee-fly. Mated females produce a large number of eggs which, for most species, are distributed in a rather speculative manner within areas likely to be used by nesting solitary bees and wasps. It is the nests of these species upon which young bee-flies parasitize. The strategy must work, since bee-flies seem to do well around here and I can usually count on seeing several within sheltered spots each spring.

Thursday, 4 May 2006

Reeling beauty

Foulden Common is a regular haunt for me during May and June. Home to a wide range of birds and insects, this mixture of short-turf grassland and scrub provides the perfect excuse for an afternoon spent wandering in search of wildlife. At the moment, there are a small number of sheep and their lambs on the common, brought in to help maintain the short turf and diversity of plants needed by resident insects, notably grizzled skipper butterfly. A visit on Sunday afternoon began in sunshine but, as the cloud thickened, the temperature dropped and few insects were on the wing. Despite the lack of invertebrates, there were plenty of birds to be seen and heard. In addition to the chiffchaffs, blackcaps, chaffinches and tits that were much in evidence, I was lucky enough to catch the reeling song of a grasshopper warbler. This grey-brown and rather anonymous looking bird has a habit of skulking low down in the vegetation; so it is fortunate that its reeling song, likened to the sound made by an angler’s reel, is so characteristic. The song, usually delivered in the evening or at night, has a rather unusual quality in that it seems to rise and fall in volume – almost as if the bird is close by and then suddenly much farther away. This effect occurs as the bird rotates its head, the volume appearing to increase when the bird is facing you.

The grasshopper warbler is a summer visitor to our region, with individuals arriving from their West African wintering grounds from the middle of April. Most birds have arrived by the beginning of May, and the male song can be heard from a range of habitats, including scrub, wetland margins, thick hedgerows and old industrial sites. The key feature of these habitats is the availability of low, dense ground cover, in which the birds can nest, coupled with some taller song posts and insect-rich feeding areas. The number of grasshopper warblers seems to fluctuate from year to year, perhaps reflecting a tendency to move between areas but, even allowing for this, this little warbler can best be described as an uncommon breeding species. The breeding season itself extends from late April through into August, potentially allowing a pair to have three nesting attempts.

The wetter margins of Foulden Common are, in many ways, ideal for the grasshopper warbler, making an evening visit to the site well worthwhile. If you need an additional reason to tempt you to the common, then the presence of nightingales should be sufficient. An evening serenade by two of our greatest songsters should be enough to tempt even the laziest of armchair birdwatchers – you don’t even need your binoculars.

Wednesday, 3 May 2006

A bloom to look forward to

For me, one of the best things about this time of year is the sense of expectation; species you hope to see and nature reserves you plan to visit. One of my regular trips in late May takes me just over the border into Suffolk to view one of Britain’s three colonies of military orchid. This plant has all of the qualities that make orchids so seductive for the botanist. Not only is the military orchid a very rare plant but it is also one of the most beautiful native blooms that you could hope to see.  In common with a number of related species, the flower resembles a tiny human figure, complete with a head, arms and legs. In the military orchid, the ‘head’ resembles an old-fashioned helmet, pale on the outside but flushed with purple on the inside. The ‘arms’ and ‘legs’ are pale mauve and a series of purple spots, arranged like buttons on a tunic, speckle the lip of the flower, giving the appearance of a tiny man in uniform.

Although this species was once fairly common across parts of southern England, over-collecting and changes in habitat management led to a sudden decline. By 1929 the species was considered extinct but in 1947 it was dramatically rediscovered in Buckinghamshire. J E Lousley, the discoverer, kept the location of the site secret, although it was subsequently ‘discovered’ by another pair of botanists a number of years later. The location of the colony was made pubic in 1975, by which time the local Wildlife Trust had ownership of the site and had implemented measures to safeguard the plants. Even more amazing was the discovery in 1955 of a colony growing in an old chalk pit near Mildenhall. This site, owned by the Forestry Commission, is now managed in conjunction with Suffolk Wildlife Trust and is known as the Rex Graham Reserve. Over the course of the bank holiday at the end of May, the Trust holds an open day, during which visitors can view the orchids. Although they flower from mid-May to mid-June the flowers are usually at their best in late May so it is a wonderful opportunity to see these striking plants. Military orchids are long-lived and typically first put up an aerial stem in their fourth year, followed by a flowering spike several years later. The plant may then flower annually for a number of years, although many have a dormant period, missing one or more years of flowering. Even if you only have a passing interest in orchids, I would urge you to visit the reserve on the late May open day. You can get further information from Suffolk Wildlife Trust 01473-890089.

Tuesday, 2 May 2006

Continued rise of the Collared Dove

Perhaps unfairly, I have always considered the collared dove to be something of a stupid bird. This view was reinforced the other day when I spotted one incubating eggs in a nest, precariously balanced on narrow branches above the middle of the River Little Ouse. Should the wind get up, I thought, then the nest would slip between the branches and into the river. Sure enough, two days later the nest was gone. Despite the fact that collared doves build nests in some pretty silly places, and that such nests are usually just a pathetically small platform of twigs, the species has done amazingly well over the last fifty years.

The collared dove was first recorded nesting in Britain in 1954, when two young were reared from a nest in a small garden at Cromer. Since then, the species has spread across the rest of Britain and it is now one of our most familiar birds. The colonisation of Britain was part of a wider expansion in breeding range that took place across Europe. Up until the 1930s, the species was largely restricted, within Europe, to Turkey and the Balkans. Over the following 20 years, there was a dramatic range expansion to the northwest, extending the known range by more than 1,600 km. Most of the initial spread within Britain was linked to villages and suburbs, suggesting that the provision of supplementary food at garden feeding stations and the availability of spilt grain around farmyards was helping the species. Breeding behaviour also seems to have played a part, since collared doves are known to have a large number of breeding attempts each year. One well-watched pair had seven attempts in one year, though most of these failed. Although nesting has been recorded in all months of the year, the main breeding season extends from mid-February to mid-October. The birds seem able to start a new breeding attempt before the previous one has finished, with females using breaks from incubation to attend to young from the previous attempt. Such persistence has clearly paid off, helped by a lack of competition from other species and easy access to food throughout the year.

The question of what triggered the initial expansion in range has been much debated. Some authors suggest that it was due to a genetic mutation; others put it down to climate change or link it to agricultural intensification. To be honest, we don’t really know but the tendency for young birds to disperse some distance away from where they were born, coupled with the capacity to produce large numbers of young has established this species in our avifauna. Perhaps the collared dove is not as stupid as it first appears.

Monday, 1 May 2006

Norfolk's Tiger

Last week, drawn by the sun’s warmth, I made a visit to Thetford Warren Lodge. This splendid example of a dry lichen heath has been somewhat spoilt by litter, left by some of Thetford’s more ignorant inhabitants. Fortunately, such people are also lazy and once you move away from the car park, the litter ceases and the heath is relatively untouched.  I had come in search of a number of creatures, all of which I hoped would also have been brought out by the sun. Common lizards were much in evidence; some proving to be approachable as they basked but many others disappearing rapidly into the vegetation. Bare patches of sand were alive with spiders and solitary wasps, while queen bumblebees and brimstone butterflies were on the wing.

Alongside these were green tiger beetles, a common and widespread species of heath and moorland, and one of my all-time favourite insects. The green tiger beetle is one of five related species to be found in Britain. All the others are rather scarce and localised in their distribution, with only one of them occurring in Norfolk. This is the dune tiger beetle, found at a handful of sites along the North Norfolk coast and a species I have yet to see. Both species breed in early spring, their larvae inhabiting vertical burrows in the sand from which they ambush passing prey.

Tiger beetles are difficult to approach, especially on open ground, as they take flight readily and have excellent vision. As I approached, each one would rise up into the air and make a short buzzing flight before dropping to the ground some 15 or 20 feet further away. In flight, they sparkle jewel-like, as the light catches their extremely shiny purple and green undersides. On the ground, and viewed from above, they are equally striking: leaf-green wing cases, marked with pale cream spots, cover the abdomen. The green extends forwards across the thorax and up onto the head, where two large eyes sit above a cream white face and a fearsome pair of jaws. Long-legged and well-equipped, this is a predatory species that relies on speed to catch smaller prey. Interestingly, tiger beetles digest their prey externally by excreting the enzymes needed to break the prey down before sucking up the resulting semi-fluid remains. To be able to do this tiger beetles have highly specialised mouthparts, with some parts fused together to form a powerful sucking pump. I was surprised when I discovered this fact and, if honest, also a little disappointed. I’d always imagined that such a fearsome predator would use its large jaws to tear its prey apart in much the same manner as its mammalian namesakes.