Autumn 'epidemic' hits common shrews

In my travels of recent weeks I have come across a number of dead common shrews, most of which have shown no external signs of injury. This late summer/early autumn phenomenon is well known and has been termed the “autumnal epidemic” by researchers. Despite the name attributed to it, this mass mortality is not the result of some disease but is, instead, the consequence of an increasing population and senescence of breeding individuals. Shrews are not long-lived animals and, following the breeding season, most of the breeding adults die, leaving a population almost entirely composed of animals born that year. Adult shrews show several, quite distinct, signs of ageing. Increasing numbers of grey hairs, often coupled with cessation of the autumn moult leave ageing individuals looking somewhat unkempt. Perhaps more important in terms of mortality, is the deteriorating condition of the teeth. Unlike the incisors of rodents, shrew teeth do not grow throughout life and so gradually wear down with use. With a diet composed of invertebrates, with their chitinous exoskeletons, and earthworms, packed full of soil particles, the teeth of shrews take quite a battering. As such, it is not unusual for the teeth of the oldest shrews to be worn down to the gum line. This makes feeding very difficult. Another important factor may simply be the sheer number of shrews around at this time of the year. High densities of these highly strung and territorial animals may lead to greater numbers of confrontations and increased stresses.

So why should this matter? Why should increased aggressive encounters, which incidentally only rarely end up in actual physical confrontations, lead to higher levels of mortality among older animals? Quite simply, shrews live on an energetic knife-edge. Small in size, they need a high metabolic rate in order to maintain their body temperature. This, in turn, demands a high food intake and requires that shrews spend most of the day feeding. If feeding time is lost to territorial disputes then an individual may simply not have sufficient time to find the food needed to keep itself alive. Since young males are dominant over older (post-breeding) males, the oldest males end up wandering about without a territory, getting into disputes and are thus prone to starvation.

This may all seem rather bleak, especially when you consider that only 20% of the shrews born will survive to breed themselves. However, the system clearly works, since shrews are one of the most successful groups of mammals, in terms of the habitats they occupy, their global distribution and the densities they can reach. I guess we should view the sight of a dead shrew, apparently untouched, as an individual that has probably enjoyed a full and productive life.

Visiting fritillary draws the crowds

There have been some very interesting reports over recent weeks of butterflies not often seen in our region. Some of these, like clouded yellow and camberwell beauty, are immigrants from the Continent, while others probably originate from closer to home. One particular individual, a silver-washed fritillary, put in an appearance at Natural Surroundings, near Holt, and attracted quite a crowd when its presence was put out on the pagers used by many Norfolk birdwatchers. (The middle of the summer is often a quiet time bird-wise and, in response, many birdwatchers take up a passing interest in butterflies and dragonflies). Given that this particular species of fritillary effectively became extinct in our region several decades ago, and that it is similar in appearance to the dark green fritillary (which does breed in Norfolk), there was understandable controversy as to whether this was simply a case of mistaken identity. In the event, the fritillary was well-watched by at least some observers capable of distinguishing the two species and its arrival occurred at a time when several other individuals were seen in Cambridgeshire and Suffolk. This species is largely restricted to the south and southwest of Britain and, with a powerful flight, it is considerably more mobile than our other fritillaries. During particularly hot summers individuals may be seen well away from the core range – a pattern matched by the butterfly’s response to climatic trends. The population has been shown to expand during warmer periods and contract during cooler ones.

Although I did not make the trip to see the silver-washed fritillary at Natural Surroundings, I did visit two of our dark green fritillary colonies this year, first at Winterton Dunes and then two weeks later at Holkham. Although a species of downs and grassland over much of its core range, the dark green fritillary seems to do very well on coastal sites, either on dunes or on undercliffs. Here the males spend much of the day on the wing, searching for females, and the patrolling flights are characterised by short bursts of rapid wing beats followed by periods of gliding. The females themselves perch low down in the vegetation and the males probably locate them by scent. Once mated, the female will return to perch within the vegetation, soaking up the sun’s warmth to help her eggs ripen. As such, although these butterflies are easy to spot when on the wing, they are difficult to photograph, except during the early morning and late afternoon when nectaring. A late July or early August visit to Winterton Dunes should guarantee views of dark green fritillaries and may just produce a clouded yellow or even a camberwell beauty.

Charity champions declining butterflies

This week has been designated ‘Save our Butterflies Week’ by Butterfly Conservation, a charity dedicated to the conservation of our various Lepidoptera (as butterflies and moths are known collectively). The week follows a new report into the changing fortunes of our butterflies, also published by Butterfly Conservation. The report, “The state of butterflies in Britain and Ireland” makes worrying reading, such has been the magnitude of decline for many formerly widespread species.  Norfolk itself has lost a quarter of its resident butterflies. Included among these are the large copper (which became extinct in 1864), small blue (1890s), chalk-hill blue (1971) and marsh fritillary (1970s). That we have lost so many species is a reflection of the changing land management and the loss of important habitats. The large copper, for example, was dependent upon areas of open fen, with high densities of the larval foodplant water dock, and as these disappeared so did the butterfly. Other species, like the high brown fritillary and the purple emperor have been lost as woodland habitats succumbed to modern management techniques.

For other species there is better news contained within the Butterfly Conservation report. The efforts of researchers, conservationists and volunteer lepidopterists have helped to secure the future of a number of species. The Norfolk Branch of Butterfly Conservation has been working hard to assess the habitat requirements of the silver-studded blue, a rare species, with a cluster of small colonies centred on the mid-Norfolk heaths. This delightful butterfly flies from June through into August and efforts have been made to see the number of colonies increase. The work, funded through the Heritage Lottery Fund has enabled the Norfolk Branch to hold training days, enabling volunteers to identify and monitor populations of this butterfly. The grant also funded a detailed study of all current and potential sites for the species within the county, with data collected on the condition of the heathland vegetation and on the densities of local ant populations (like a number of other blue butterflies, the silver-studded blue has a symbiotic relationship with one or more species of ant).

The result of all this work is a management plan for the species within Norfolk. The plan has established the size of the current silver-studded blue population and where it occurs. It has also highlighted sites where it is hoped that the butterfly can be established by moving a small number of individuals from existing sites with healthy populations. With luck, and quite a bit of effort, the future of the silver-studded blue should be secure. Now all we need to do is work on some of the other species that have declined over the years.

A good summer for immigrants

The two buddleias that stand on either side of our rather narrow garden have come into their own over recent days, with a succession of insects arriving to nectar on the tiny pink and purple flowers. Evening has proved the best time to stand and observe (and in some cases be observed by) the butterflies, bees and moths that bustle about from flower to flower. The most numerous of these visitors has been the silver-‘y’ moth, Autographa gamma, an immigrant from the Continent that arrives, and breeds, here each summer. Numbers vary from year to year but this summer they have been truly amazing, with in excess of fifty individuals on the buddleia each evening this week.

The silver-‘y’ belongs to a group of moths called the noctuids. These are typically stout-bodied, medium sized moths with forewings substantially longer than they are deep. Most of the 400 or so British species are brown in colour but the silver-‘y’ can be identified readily by the presence of a conspicuous and unbroken metallic silver ‘y’ mark on the upper surface of the forewing. Unlike most other moths, the silver-‘y’ has an early evening feeding flight that brings it out alongside the day-flying butterflies and presents the casual observer with an ideal opportunity to get good views as it feeds.

The many butterflies have included large numbers of red admirals, another species that arrives as a summer visitor but which, thanks to global climate change, seems increasingly able to overwinter here as well. In fact, it has always been something of a mystery as to why the red admiral was unable to survive our winters. After all, the species overwinters successfully further east in colder parts of Europe. It may well be the combination of cold and wet that caused problems. These striking red and black butterflies have a powerful flight, with deep regular wing strokes. They also seem to be fairly inquisitive and of the butterflies feeding on my buddleias, they are the only one to approach right up to me as I stand by the bushes and watch. In with the red admirals are half a dozen or so peacocks, the occasional comma butterfly and, on one evening only, two painted ladies. The painted lady is another immigrant, this time from much further south, from the warmer climes of southern Europe and North Africa. Perhaps its origins explain why it seems to spend more time basking in the sun than the other species. I have been hoping that the presence of these familiar immigrants will also mean the arrival of one or more rare visitors: a clouded yellow butterfly or maybe a hummingbird hawkmoth. We shall have to wait and see.

Robber of the dunes

At first glance, Norfolk’s coastal dunes seem an inhospitable environment in which to live. Shaped by the wind and baked by the summer sun, there seems little opportunity for wildlife. However, a diverse community of plants and invertebrates makes a living from the dunes, overlooked by those who come in search of sun, sea and sand. A weekend trip to Winterton Dunes highlighted the popularity of the beach; the car park quickly filled and the shrieks of children soon settled in with the rhythm of the sea. Intent on gaining a spot on the beach, few tourists venture into the dunes that stretch inland. These particular dunes are a Site of Special Scientific Interest, reflecting some of the special animals to be found here. Among these is a group of flies that require the hot, dry conditions that the dunes provide. Known as robberflies, virtually all of the 28 species found in Britain have a strong southerly or coastal distribution. These robust flies depend on the warmth to carry out rapid attacks on unsuspecting victims.

Dune Robberfly

On Sunday I was lucky enough to watch what I believe to be a dune robberfly (aptly named, I know) as it pounced on a smaller fly. The robberfly had been sitting alert, seemingly scanning its surroundings. The large compound eyes curve around almost to the back of the head providing excellent all round vision and allowing the fly to spot potential prey. The facets that make up most of the compound eye are fairly small and it is thought that this gives the fly great visual resolution. Unusually for a fly, the facets at the very front of the eye are much larger, and the front of the eye itself is rather flat, and this provides excellent binocular vision, something that is essential if the fly is to make an accurate strike at potential prey. There was an instant where the fly seemed to tense, before springing from the ground to make a darting flight at its victim. The victim was caught unawares and, held in the sharp piercing mouthparts, hardly managed a struggle before succumbing to the poisonous saliva that the robberfly had injected. The robberfly returned to its perch and I was able to take several photographs which, I hope, will enable me to confirm my tentative identification.

At about 25mm in length, the robberfly was impressive. Despite its size and the sharpness of its mouthparts, the species is harmless and rarely bites, even when handled.  But it is exactly the sort of fly which would not be well received by those on the beach. It is a good job that they choose to ignore the dunes and the wildlife they contain.

Bedstraws delight

It would be fair to say that I am not much of a botanist. Although I can recognise a fair number of different plants, I have never put the time into learning how to identify them in the way that I have birds or beetles. As such, I have a great admiration for those botanists, like Alec Bull and Gillian Beckett, who have studied and mapped the distribution of our county’s flora (some 2,400 species) over many years.  My efforts tend to be opportunistic, in that I will sometimes spot a plant that catches my eye and then set myself the task of working out what it is. Over the years I have got to grips with a number of species, such that my morning walks in the forest are brightened by knowing the names of the various plants that flower alongside my regular routes. At the moment the forest rides are bordered with the greenish yellows of wild mignonette, the bold purples of viper’s bugloss and the dense yellow spikes of lady’s bedstraw. This last species is related to cleavers (known by some as “goosegrass” and by others as “sticky willie”), whose stalks and seeds catch on the dogs and take ages to brush out.

The yellow flowers of lady’s bedstraw have a strong scent of honey but, when dried, are said to smell of newly mown hay. The name itself comes from the old habit of including the plant in straw mattresses, notably those of women about to give birth. This was not the only use that lady’s bedstraw was put to.  Compounds within the plant have coagulant properties and it was used as a styptic to halt blood flow from wounds. Lady’s bedstraw was also reportedly used as a substitute for rennet in the production of cheese but modern attempts to use the plant in this way have proved unsuccessful.

Another species of bedstraw that holds an interest for me is wall bedstraw. This tiny plant (just five to 10 centimetres in height), is not exactly striking. In fact, it is very easy to overlook, even when you know where it grows. It is a nationally scarce annual found on bare ground and old walls, including the 12^th Century nunnery ruins in Thetford ­– hence my knowledge of the species. The plant has always been scarce and over recent decades has declined even further as the open, infertile soils it favours have been ‘improved’ through the process of agricultural intensification and the old walls have been demolished or repaired. The choice of sites shows how intolerant of competition the species is and careful management will be needed to retain it within a changing environment. 

Watch out for hummingbirds

Over the last few weeks I have received a number of telephone calls from excited householders, each describing how a tiny hummingbird has been visiting their garden flowers. These “hummingbirds” are not birds at all but a species of migrant moth that arrives in varying numbers each summer from continental Europe. Known as the hummingbird hawkmoth, after its habit of hovering to feed at the tubular flowers of a number of wild and cultivated species, it is easy to see why people think that it is a bird. The rather squat body is greyish-brown in colour, with darker markings at the rear and a neat row of cotton bud white spots along its flanks. The orange-brown wings are visible close to the body, but become a blur towards their tips; such is the rapidity of the wing beats. As such, the hummingbird hawkmoth is unlikely to be confused with any other moth, although the two – now rather scarce – bee hawkmoths share a passing resemblance.

It is during August and September that the largest numbers of hummingbird hawkmoths are reported from across the country. Although they can appear in any month, most reports fall between April and December. A small number of records from late in the winter presumably refer to individuals that have attempted to hibernate in houses or outbuildings, a behaviour that is well-known within the main part of their range. The arrival of these moths is usually associated with warm weather pushing up from the south. This weather also brings with it many other invertebrate migrants, including familiar species like the painted lady butterfly, silver-‘y’ moth and several species of hoverfly. In certain parts of the eastern Mediterranean, the hummingbird hawkmoth is considered a bringer of good tidings, a belief that did not go unnoticed by one writer who reported that a small swarm of these moths was seen flying low over the water from France to England on D-Day, 1944.

This day-flying moth tends to be on the wing on warm and sunny days. However, if it is exceptionally hot, then the moth will become torpid, not emerging until late in the afternoon once the temperature has dropped.  Individuals have been shown to return to the same flowerbeds over a period of several days, suggesting that they possess the ability to map the location of good nectar sources. Plants like phlox, buddleia, petunia and red valerian are favoured, although the female moths will also visit various bedstraws in order to deposit their eggs. So do keep an eye out for this delightful little moth over the coming weeks and do let me know if you have one visiting your garden this summer.