Saturday, 26 August 2006

Modern parenting in Norfolk

This time last week I was at the Birdwatching Fair at Rutland Water, manning a stand promoting the BTO Garden BirdWatch. While the weekend itself was very interesting, it did mean that I missed the largest flock of dotterel to have visited Norfolk in nearly fifty years. This attractive wader breeds high up in the mountains of Scandinavia, Scotland and Russia. As such, it remains a scarce visitor to Norfolk, passing through in small numbers during its spring and autumn migrations. At least two-dozen of these delightful birds, and possibly as many as three-dozen, spent much of the weekend frequenting fields near Choseley Barns, which sit on the hill inland of Titchwell.

With smart grey upperparts, a white band across the chest and a rufous belly, a dotterel in breeding plumage is quite a sight. Remarkably (for a bird) the female dotterel is more brightly coloured than her mate, a trait that hints at an unusual approach to courtship and parenting roles. It is the female dotterel that initiates courtship, performing song flights to attract a mate. Once courtship is complete, the female will usually leave the male to incubate the eggs and rear the chicks on his own, while she is free to search for another mate. The roles of the two parents are determined by the different levels of investment in the reproductive attempt. Since eggs are energetically costly to produce, and sperm is cheap, it is unusual for a female bird to have the upper hand in determining the parenting roles. In this case, however, the female seems to have overcome any handicap.

The dotterel used to be more numerous when on passage through the county. Stevenson, writing in the late 1800s, noted that the species used to be so numerous that there was considerable profit in hunting it. Within Norfolk, the birds were netted, providing a great delicacy for the table. Some authors considered that the dotterel was easy to trap, a fact that may have had some bearing on its Latin name of “morinella”, which means “little fool”. The vernacular name itself shares a linguistic root with “dotard” again suggesting that the bird was rather stupid.

The general pattern within Norfolk is for more dotterel to be seen during spring migration than during the autumn one but, interestingly, the largest groups tend to be encountered during the autumn. A record 47 were seen together at Terrington Marsh on 20th August 1959, while the largest group seen in more recent years was 17 near Docking on 26th August 1996. These small groups of dotterel are known as “trips”. It’s just a shame that I couldn’t make the trip back from Rutland in time to see them last weekend.

Friday, 25 August 2006

Rare caterpillar found at Holkham

The National Nature Reserve at Holkham is an excellent place to visit in search of wildlife. The combination of habitats (such as dunes, inter-tidal sands, grazing marsh and pine woodland) allows the development of many very different communities of animals and plants. During the winter months, visiting shore larks and snow buntings can be seen on the saltings, while huge flocks of geese crowd onto the grazing marsh. The summer warmth brings natterjack toads, spotted flycatchers and a whole host of buzzing, chirring and clicking insects. The heat also attracts the tourists and Lady Anne’s Drive soon hosts two long lines of parked cars, the sun on their windscreens creating the illusion of two great glasshouses stretching out towards the pines. Fortunately, Holkham is a large site and, with the lure of the sea, it is very easy to leave the crowds behind and lose yourself in the solitude that the reserve has to offer.

Bedstraw Hawk-moth, Holkham

Just the other weekend I visited Holkham with two friends whom I had not seen in a long while. It was the perfect venue for a gentle walk and provided an ideal opportunity to catch up on each other’s news. Skirting west alongside the landward edge of the pines, we soon reached the George Washington Hide and followed the boardwalk out onto the dunes. Common blue butterflies were on the wing in the shelter that the dunes provided, and alongside these were graylings, dark green fritillaries and dozens of gatekeepers. Scanning the ground for beetles and other invertebrates soon turned up our most interesting find of the day – a bedstraw hawkmoth caterpillar. This pale, straw-coloured caterpillar was a sizeable beast – the length and thickness of your middle finger. Complete with a series of ‘eye-spots’ along the flank and a red ‘horn’ on the rear end, I recognised it as a hawkmoth caterpillar, but one that I had not seen before. Photographs were duly taken and, once home, these were compared with various illustrations before my tentative identification was confirmed by a number of experts.

The bedstraw hawkmoth is a migrant species, albeit a fairly regular one, and so was not something that I would necessarily have expected to encounter as a caterpillar. Most records tend to come from the eastern counties and, in particular, from coastal localities. Adults usually occur between May and August and may be taken at light traps but the caterpillars themselves tend to be found from July to September. They feed on various bedstraws and willowherbs before overwintering just below the surface of the soil. At various times in the past, temporary populations of this moth have become established in North Norfolk. This latest record may suggest that one such population exists today at Holkham.

Thursday, 24 August 2006

House MArtins may linger

The familiar house martin, with its blue-black upperparts, crisp white underparts and white rump, has had something of a mixed summer. The extended period of drought will have made it difficult for individuals to find the mud needed to build or repair nests but, at the same time, the summer warmth has helped to support an abundance of aerial prey. Many house martin pairs will be on, or have finished, their second broods and it will not be long before some begin to depart our shores. Others may be further behind and a few, well ahead of the game, will have managed to squeeze in a third brood. The recent run of cold wet weather will have limited chick growth rates, extending the development time well beyond the usual 16-22 days. This means that some youngsters may remain in the nest through into the middle of October. The presence of such lingerers often prompts phone calls to my office from worried homeowners, fearful that “their” house martins will be unable to make the journey south. Such fears are usually unfounded unless the weather deteriorates and food becomes scarce. I suppose that this highlights our attachment to these delightful birds that have chosen to nest alongside us. Although the majority of nests are now on man-made structures, especially under the eaves of houses or under bridges, house martins would have once nested in cliff faces, like those at Hunstanton.

Not everyone welcomes nesting house martins and the mess that accumulates below the nest, but it is worth remembering just how far these transequatorial migrants have come and the amount of effort they have put into building their nests. Each nest takes about 10 days to build and is made up of over 1,000 beak-sized pellets of mud. On occasion, having just completed their nest, a pair will find themselves evicted by house sparrows. These more dominant birds muscle in and take over what is, for them, an ideal nest site.

Despite the apparent familiarity, we actually know surprisingly little about house martins. Although some individuals roost within the nest during the breeding period, it is thought that most roost at altitude on the wing (like swifts). We know even less about what happens to these birds during the winter months. Even though some 300,000 have been ringed in Britain, only one has been recovered in Africa, south of the Sahara, although local reports suggest that this is where they winter. Researchers believe that house martins spend the winter months feeding on the aerial insects that gather above the rainforest canopy. Since very few people live in such areas, perhaps it is no surprise that more individuals have not been found.

Wednesday, 23 August 2006

Audit of our bird populations

Last weekend saw the launch of “The State of the UK’s Birds 2005”; a now regular publication charting the changing fortunes of our bird populations. Published by the British Trust for Ornithology, RSPB and the Wildfowl and Wetlands Trust, in association with the four Country Agencies, the report provides a very valuable measure of how our birds are fairing. Knowing that a species is in decline can help to trigger appropriate research and plan effective conservation action. Similarly, evidence that a species has begun to recover from a previous decline is a valuable yardstick by which the success of conservation work can be measured.

The information presented in the most recent report comes from a wide range of surveys, some covering multiple species, others targeted at single species or particular sites. While figures for individual species provide the best level of detail for conservation efforts, they can also be used collectively to stimulate appropriate policies for Government. One way in which such an approach has been adopted is through the use of indicators, such as the “farmland bird indicator” which summarises what has been happening to farmland birds as a whole. This particular indicator demonstrates that, as a group, farmland birds underwent a period of sustained decline during the 1970s and 1980s, from which they have yet to recover. Government sees indicators as a useful tool for communicating the state of the natural environment and for measuring the success of Government policy. However, some researchers feel that they are of limited value, not least because they may hide all-important differences between individual species. Nevertheless, thanks to work carried out at the British Trust for Ornithology, the UK has led the way in developing these useful tools, demonstrating the role they can play in conservation.

One underlying message within the report was the essential role that volunteers play in collecting the information upon which much of our understanding is based. The report presents long-term information for many widespread species, monitored by volunteers who give up time each year to go out into the field and collect information in a standardised manner. Without these volunteers, we simply would not have the information that is needed. A similar message emerged from the recent run of programmes on BBC Radio 4 looking at “citizen science”. This American term is used to describe the way in which ordinary “citizens” participate as volunteers in projects organised by scientists to collect information at a scale that could not be achieved with paid professionals. If you are out in the countryside, collecting information on plants, bugs, birds or bees, then you are doing your bit of citizen science and helping the nation’s conservationists and researchers.

Tuesday, 22 August 2006

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.

Monday, 21 August 2006

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.