Saturday, 26 July 2008

The cricket season has started

Last weekend, at Knetishall Heath, I heard the steady reeling buzz of a Roesel’s Bush Cricket. To me, this signalled the start of the cricket season, prompting me to get out in search of other crickets and grasshoppers. Earlier today I found myself on the warren, not far from Brandon, in the company of Paul Stancliffe, another birder turned grasshopper enthusiast, and his two young daughters.

They say that as you get older so your hearing deteriorates and some of the first sounds to be lost are the high-pitched calls of grasshoppers and crickets. This is something that Paul had noticed the previous year, unable as he was to pick out the Roesel’s calling in the grass just outside of work. Fortunately, I can still hear the calls but quite probably not as well as Paul’s daughters. Paul had a trick up his sleeve though, a bat detector, which captured the buzzes and reels of the different grasshopper and cricket species and broadcast them at a frequency he could hear. By using this it was possible to distinguish the different calls, from the distinctive pulsating sound produced by the Striped-winged Grasshopper through to the strengthening buzzing chirps of Mottled Grasshopper which ended abruptly.

Working our way around the warren we encountered Field, Striped-winged and Common Green Grasshopper in the areas of longer grass. All three species were calling well and we netted a few to check on the identification features. Grasshoppers and crickets are a good group to get into; although there are not that many species, the variation in size and colour within a species ensures that identification is testing enough to keep you on your toes. In the areas of shorter breckland sward we encountered Mottled Grasshopper, smaller and more delicate than the other species we had so far seen but equally variable in colour (12 distinct colour forms are recognised). Some of the Mottled Grasshoppers were beautifully patterned, a mixture of soft greys and browns and perfectly camouflaged even on the bare ground.

The various grasshoppers go through a series of moults, increasing in size before they become full adults. We netted a fair few individuals that had not yet reached adult size, highlighting the fact that it was still early in the season. Other species that we might expect to find, such as Slender Ground-hopper, will not reach adult size until the start of August, so a return visit may be needed to pick these up. Searching for grasshoppers also tends to reveal a myriad of other small creatures, from beetles and shieldbugs through to solitary wasps and ants. It is tempting, though challenging, to tackle some of these more difficult groups.

Friday, 25 July 2008

Butcher bird takes up residence

The presence of a Red-backed Shrike at Sea Palling over the weekend is a noteworthy occurrence, not least because this particular individual has been present for a good few days. Older readers may recall the Red-backed Shrike as being a regular breeder at scrubby sites scattered across the county. However, the shrike is now effectively extinct as a breeding species in Britain thanks to a long-term decline first noted in the late 1800s.

The Red-backed Shrike‘s demise was first noted in the north of its British range such that, while populations in northern counties were being lost, the bird was still being described as ‘tolerably common’ within East Anglia. This changed as the Twentieth Century saw a period of relentless decline. A survey in the 1960s put the breeding population at 253 pairs but by 1971 this had fallen to 81 and by 1980 there were just 23 pairs remaining. The final pair was last confirmed breeding in 1988 at Santon Downham in the Brecks and a pair probably bred nearby in 1990. Since then, breeding has been confirmed on just two occasions, although there is a handful of reports each year of singing birds seemingly on territory in suitable nesting habitat. These days most records refer to birds seen on passage, passing through East Anglia during spring or autumn and seemingly of birds from breeding populations in Finland, Sweden or Norway.

The Red-backed Shrike gets its local name of ‘butcher bird’ from its habit of impaling prey on the thorns of prickly shrubs or the barbs of barbed-wire fences. These grisly collections of prey may be used as larders since they can sometimes be found close to active shrike nests. Favoured prey items include larger insects (like butterflies, day-flying moths, beetles and dragonflies), together with small birds, small mammals and lizards. It is thought that a massive decline in the availability of larger insects lies behind the losses, possibly the result of agricultural change, increasing use of insecticides and a decline in habitat quality. One other factor implicated in the decline has been the illegal and morally repugnant activities of egg collectors. The colourful and highly-variable eggs of the Red-backed Shrike made them especially popular with egg collectors. As the birds became scarcer so their eggs became all the more sought after by collectors. The problem was exacerbated by the suggestion that the British shrikes were a distinct race, thus widening their attractiveness to egg collectors operating elsewhere within Europe. It is also possible that other difficulties may have faced the shrikes on their African wintering grounds. Whatever the ultimate cause of the decline it seems that things here will not improve in the short term.

Thursday, 24 July 2008

A dark-breasted Barn Owl

I received a report last week of an unusual visitor to the county, in the form of a dark-breasted Barn Owl. While our ‘white-breasted’ Barn Owls will be familiar to most readers (Norfolk being a particularly good county for them), birds of the dark-breasted race remain very occasional visitors from Continental Europe. More correctly known as ‘guttata’, this dark-breasted race breeds from Germany, eastwards to the Ukraine and Bulgaria. Populations to the west of this range are ‘white-breasted, belonging to the race ‘alba’. Birds of the dark form tend to be more heavily marked, with more spotting on the breast and around the facial disk. In addition, they show a soft warm ochre wash across the breast which gives them their name.

One of the reasons why we don’t receive more such visitors is because Barn Owls are rather sedentary birds, their young rarely dispersing far from the nest. An analysis that I carried out of ringing data for the BTO’s Migration Atlas revealed that the average distance moved by young Barn Owls here in Britain was just 12km. These youngsters dispersed away from the nest over a four or five month period and after this made little additional movement. The relatively sedentary nature of our Barn Owls contrasts with what is seen on the Continent, where youngsters can move considerable distances. Interestingly, the average distance moved by these Continental birds has been shown to vary from year-to-year. In some years there is a pronounced degree of dispersal, with the young on the move from late summer and not settled until November. Such years have been given the name ‘Wanderjahren’ and are thought to result from the combination of a particularly successful breeding season followed by a sudden crash in the vole population ­– voles are the main prey species and their numbers vary in relation to the availability of softer grasses and herbs.

There have been some rather dark-looking Barn Owls caught at breeding sites here in the East of England over the years and some of these birds may well have been mixed race birds. However, proving this can be difficult because the different races are well-represented in collectors aviaries and some of these do escape or are deliberately (and illegally) released. That dark-breasted individuals do reach our shores has been proved thanks to bird ringing, with a handful of ringed youngsters from Continental populations found here over the years. This most recent report also involved a ringed bird, this time coming from the Dutch population. In this respect it is similar to a Dutch-ringed individual found dead in North Norfolk back in October 1999 by Mike Crewe. This bird was stuffed and mounted and sits watching over me as I type these words.

Wednesday, 23 July 2008

Breaking the journey north

It’s a short stop on a long journey north, made possible because the quarry at Bishop Middleham is just a four-minute drive from the A1. Stopping the car in a small lay-by, a half-hidden sign directs us through a gap in the hedge and into a belt of woodland. After 100 metres or so of walking through the damp, heavily shaded woodland cover I wonder if we have stopped in the wrong place. The map I had looked at before setting off suggested that the quarry butted right up against the road. However, with the path suddenly turning and dropping downhill I am reassured, guessing that we are on the right track. Sure enough we soon emerge into sunlight, entering an industrial amphitheatre that is carpeted with a short rich sward and surrounded by rising walls of Magnesian limestone. The stone was quarried from here up until the 1930s and was used for buildings, agriculture and for various industrial processes. It is this history of quarrying that has shaped the community of plants and animals that thrive on these thin, magnesium-rich soils.

Large areas of the quarry floor are carpeted with the golden yellow blooms of Common Rock-rose and other lime-loving herbs, including eight different species of orchid. Among the Pyramidal, Bee and Common Spotted Orchids are the tall flowering spikes of Dark-red Helleborine (an orchid by another name). This colony of helleborines is thought to number some 2,000 or so flowering spikes, making this the largest colony in Britain, perhaps holding more plants than all the other populations combined. The plants themselves are surprisingly tall and erect in habit, with thick almost succulent leaves arranged in two rows up the stem. We are a little early in the season and the attractive dusky-red blooms are not yet open, the supporting flowering spikes hanging bent like the curve of a shepherd’s crook.

It is not actually the flowers that we have come to see, but a small butterfly – the Northern Brown Argus. This species occurs from the Peak District northwards and is on the wing from June-August. The population at Bishop Middleham belongs to a distinct race, known as salmacis and first described in 1828. It is more commonly referred to as the Castle Eden argus, a reference to this part of Co. Durham where it is found. Although the weather is not ideal, with thick shower clouds rolling through and blocking the sun, our fortunate timing coincides with a sunny spell and we are rewarded by really excellent views of the butterfly and some photographs to take away. If we’re passing this way again then another visit may well prove to be in order.

Tuesday, 22 July 2008

A taste of Africa

The Spoonbill is one of those birds that always reminds me of Africa; with its long, spatula-tipped bill, it seems just too exotic to be a truly British bird. It is, however, a species that was once a regular part of our breeding avifauna, with nesting colonies present in the Broads up until the middle of the 17th Century or thereabouts. Since that time it has become something of a scarce visitor, occasionally turning up in spring or autumn in small numbers and never straying far from the coast.

However, things have changed a little over the last few years with more birds arriving each year, the result of an expanding breeding population within the Netherlands. There have even been breeding attempts made here in East Anglia, though these are thought to have been unsuccessful. Thankfully, the Spoonbill has become a British breeding bird again thanks to a pair in the northwest of England which successfully fledged two young in 1999. With increasing numbers of birds arriving here we can expect to see more young Spoonbills fledging from nests over the coming years.

There is a link to Africa for our visiting Spoonbills because, even though they are birds associated with the 1,300 or so Dutch pairs, these birds are migratory and spend the winter in West Africa, favouring suitable habitats across Senegal and Mauritania. While things seem to be going well for these particular Spoonbills, breeding populations further east across Europe (in Russia and Hungary, for example) are in trouble. It is thought that land drainage and increasing levels of disturbance are behind the declines observed in these populations, prompting conservationists to highlight the difficulties that the species is facing and to direct efforts towards safeguarding their future.

Over the last two weeks there have been good numbers of Spoonbills around our coasts, with half a dozen birds loitering at the Norfolk Wildlife Trust reserve at Cley, and others on the east coast. This seems to be a feature of visiting birds, with individuals often lingering at favoured sites for days, sometimes weeks. Slightly smaller than a Grey Heron, with their white plumage and characteristic bill, they should be unmistakable. Mind you, with the current explosion of our Little Egret population, the Spoonbills will not be the only large white birds out on the coastal pools and lagoons at the moment. The spatula shape of the bill is less obvious in flight, so look for the way in which the bird carries its head. In the Spoonbill the head is carried with the neck extended forward, while all the egrets retract their neck in the same manner as that seen in Grey Heron.

Monday, 21 July 2008

A white-letter day

It has not been much of a summer for our resident butterflies and the cool, often wet, conditions will have undoubtedly caused problems for many species. While a single poor summer, or even a short run of poor summers, may not prove catastrophic, it can exacerbate the impact of long-term declines brought about by other factors. The poor conditions tend to have an impact on butterfly survey work as well, limiting opportunities to get out into the countryside to locate populations of some of our more under-recorded or localised species. Three such species are on the wing now; the White Admiral, the Purple Hairstreak and the White-letter Hairstreak. Of these, it is the White-letter Hairstreak that we should, perhaps, be most concerned about.

This is a species that has undergone a significant decline since the 1970s, in part a reflection of its association with the various species of elm. The widespread outbreak of Dutch Elm Disease in the 1970s resulted in the loss of some 10 million elms, and with them, the removal of the favoured larval foodplants.  The spores of this fungal disease were carried by bark beetles and transferred from tree to tree as the beetles moved about to seek food or lay eggs. At the time, experts predicted that the loss of the elms would be extremely damaging, suggesting that the future of the White-letter Hairstreak hung in the balance. However, the predicted sudden and catastrophic loss of the butterfly did not happen, largely because the butterflies took to feeding on the regrowth brought about by the elm’s ability to sucker readily; they also exploited some of the more disease-resistant varieties of elm. As a result, colonies still cling on across a large part of central and eastern England albeit at a greatly reduced level.

Our understanding of how the White-letter Hairstreak population has changed comes from regular monitoring work, carried out annually across a series of sites through the UK Butterfly Monitoring Scheme. What the scheme doesn’t give us, however, is a clear picture of where the remaining colonies are located. This is where we, as amateur naturalists, come in. Over the next few weeks, the hairstreaks will be on the wing and can be spotted making their erratic spiralling flights around the tops of elm trees. Here they will feed on aphid honeydew. They will also descend to nectar on bramble or thistles, so it is worth staking out a group of elms with nectar sources below. You won’t see the dark uppersides of the wings because the butterfly perches wings closed, but you might just see the orange stripe and white ‘W’ on the underwing. If you do, make sure you let Butterfly Conservation know.