Chattering from the reedbeds

A rather special warbler has recently put in an appearance on the Nunnery Lakes reserve in Thetford. I use the term “put in an appearance” rather loosely because the bird in question, a cetti’s warbler, has been rather elusive. Skulking within the thick vegetation, this particular individual has only revealed its presence through its characteristic song – a series of staccato notes and disyllabic alarm calls. These have an explosive quality and are far carrying – very useful when faced with a drab brown bird that prefers to remain low down within reed beds and damp willow carr.

The cetti’s warbler is the only British representative of a group of Asian warblers, known simply as the bush warblers. This particular species is a relatively recent addition to our breeding bird community. At the turn of the Nineteenth Century, cetti’s warbler was restricted, within Europe, to the shores of the Mediterranean. It then began a northwards range expansion that extended the breeding range to the Loire Basin in 1927, the Seine in 1932 and the Netherlands in 1968. The first confirmed breeding record from Britain was in 1973 and the species was first recorded in Norfolk in the same year, when an individual was found dead in the middle of Norwich city centre. This particular individual happened to be carrying a ring, showing that it had come from the newly established Belgian population. The bird is still held at the Castle Museum in Norwich.

A national survey carried out in 1996 put the British population at around 500 singing males, about 10% of which were recorded in Norfolk. Since then numbers have continued to increase and it is thought that the Norfolk population is now in excess of 200 singing males, most of which are located in and around the Broads at sites like Strumpshaw, Surlingham and into the Yare valley. This increase has only been possible because of the run of mild winters that we have been experiencing. Unlike other warblers that feed on insects, cetti’s warbler is a year-round resident and does not migrate to warmer climes in the winter. This means that it is extremely susceptible to periods of bad weather, when it is unable to find sufficient invertebrate food. We know, from past experience, that a particularly cold winter can dramatically reduce the numbers of cetti’s warblers in Britain. As such, the continued presence of this species within Norfolk, at the levels we have seen in recent years, is going to be very much dependent upon the pattern of future weather. Global climate change appears to be producing increasingly mild winters but it will only take one very cold winter to knock the population right back.

Where have all my birds gone?

Over recent weeks, increasing numbers of garden birdwatchers have contacted me to ask where all their birds have gone.  Familiar species like robin and blackbird have deserted gardens, leaving birdwatchers concerned that something untoward may be happening. Fortunately, this exodus is an annual event and one that forms part of the regular seasonal pattern of garden use. Autumn is a time of plenty, when Nature’s larder is packed full with wild fruits, seeds and nuts. As such, many birds will shun the food provided at garden feeding stations, in favour of beech mast, rose hips, berries and haws. The extent of this sudden switch in where to feed can be seen very clearly from the weekly records collected by the 16,000 BTO/CJ Garden BirdWatchers. Examination of the results for a species like blackbird (www.bto.org/gbw) shows an abrupt decline in the use of gardens, something which extends over several weeks. Once the supply of wild fruits is exhausted, the blackbirds return to gardens and the Garden BirdWatch reporting rate increases again.

You only have to wander out into the Norfolk countryside to see that many fruit or seed-producing plants have produced a bumper crop this year. Crops of acorns and beech mast are particularly large; that for beech being the largest since 2002. With such an abundance of food, seed-eating birds (such as coal tit, chaffinch, brambling, jay, nuthatch and woodpigeon) are likely to make very little use of garden feeding stations over the course of the winter months.  Work carried out by David Glue of the British Trust for Ornithology has demonstrated this link between seed availability and the use of gardens by coal tits.  During winter, when insect food is in short supply, coal tits switch to feeding on beech mast. When there is little beech mast available, more coal tits move into gardens to feed on peanuts and sunflower seeds. When there is a bumper crop of beech mast, they can remain within their favoured woodland habitats.

Many trees vary their production of seeds from one year to the next, using environmental cues to synchronise the size of the crop across large areas. This results in a run of years when seed crops are quite small, followed by the occasional year when they are very large. This large crop is known as a “masting” event and the strategy itself is designed to reduce the amount of seed taken by seed predators. If trees produced a similar crop each year then seed-predator populations would increase and the amount of seed eaten would be high. By having a series of small crops, followed by one big one, the trees effectively swamp the predators during the masting year, insuring that more seed survives to germinate.

Barking deer

From close by, the short, sharp-sounding bark of a muntjac deer unnerves the dogs. The deer has not heard our approach and can only be 20 metres or so away within the dense vegetation that borders this forest ride. More distant barks are ignored but the proximity of this one is enough to halt the dogs in their tracks. The series of single barks, each separated by a few seconds of silence, suggests that we are close to a female in season, possibly with a male in attendance; it is time to steer a different route and to leave these stocky little deer well-alone.

The dogs are certainly familiar with the muntjac that seem to do so well in and around Thetford Forest. The small hunched form of a muntjac is frequently seen crossing a forest ride and more distant barking is commonplace throughout the year.  The muntjac is an introduced species, originating from southeastern China, and first released at Woburn Park early in the Twentieth Century. Two different species were originally introduced, the Indian muntjac and the Chinese muntjac, but it is the Chinese muntjac that has become established. Standing about 48cm tall at the shoulder, these are small deer, short-legged and stocky in nature. The males have simple antlers which, in mature males, curve backwards and terminate in a hooked point. When alarmed, the short tail is raised and held vertically, displaying the conspicuous white underside – a useful identification feature. The very small hoofprints, with those of the rear feet registering upon those made by the forefeet, are also characteristic.

Muntjac breed throughout the year and young can be born in any month. After birth the female comes into season and will be pursued by a male for some time prior to mating. Each male utilises a home range that encompasses those of several females. Ownership of an area is signalled through scent marking, a behaviour utilised by many other mammal species for social communication. The males, and in particular dominant males, use scent marking to a greater degree than the females. In front of each eye is a large scent gland, which is frequently opened and sometimes everted (resembling a black grape) during courtship. Frontal glands, positioned on the forehead, are used to mark the ground and glands between the digits of the hind foot leave scent wherever the animal wanders. Such communication helps to signal the social status of individual males and is often enough to deflect potential aggressive encounters. These do happen on occasion and may sometimes be directed towards other species seen as a threat, including foxes and dogs – another good reason for me to steer my dogs away from the courting couple.

Moorhens enjoy good breeding season

There seem to be plenty of moorhens about on the river at the moment. The ratio of adults to youngsters suggests that it has been a particularly good breeding season and this bodes well for the winter ahead. It also suggests that the mink that had previously been present on this stretch have now gone, thanks to a combination of trapping and the return of the otters. There is something very comical about the moorhen. When seen out of water, the charcoal grey body appears to have been glued onto the thick yellow-green legs of a larger bird. The moorhen seems to emphasise this, as if unaware of someone else’s practical joke, by walking with a sprightly high-stepping gait. Alongside this there is the regular flicking of the short tail, like a nervous twitch, which again adds to a rather unusual demeanour.

Despite their comedic appearance, moorhens are rather interesting birds. For example, the moorhen holds the dubious distinction of being one of the first birds to fall victim to a firearm within Norfolk. Two great Norfolk nature writers, Gurney and Stevenson, both cite a record of one being “kylled wt the gun” in the mid-1500s. Indeed, the moorhen has sometimes featured as a bird for the table (and still does in other parts of Europe).  The dark meat was used in some rural parts to supplement the limited rations available during World War Two. Interestingly, some authors suggest that the bird was skinned rather than plucked prior to cooking. Certainly the moorhen is a popular component in the diet of a number of predators and the species appears especially vulnerable when nesting.

Although most of our resident moorhens remain on their breeding territories throughout the year, they are forced to move during periods of severe winter weather, when waterbodies freeze over and food becomes hard to find. Moorhens from breeding populations in Denmark, Sweden and the Netherlands are known to winter here and large numbers may regularly gather at favoured sites. Back in the mid-1990s we used to have a couple of dozen moorhens gather at the bottom of The Nunnery lawn in Thetford. These birds would feed on the edge of the lawn or venture into the wet woodland which separated the grounds from the river. We made an effort to trap and ring the birds – a job which involved careful handling because of the extremely sharp claws sported by these birds – and were rewarded several months later by the report of one that had made its way to Shetland. Other individuals (not ringed by us) have been reported from Denmark, Sweden and the Low Countries. There is certainly more to this bird than you might imagine.

Hornets aplenty

Just off my regular route to work, there is a large hollow tree by the river. The cavity is at ground level, large enough to accommodate those of slender build and heavily charred thanks to the fondness of the local children for all things pyrotechnic. At the back of the cavity, the pale off-white form of a large hornets nest provides a striking contrast to the charcoal black trunk. The nest extends down, marked with horizontal bands of cream, brown, yellow and white, that delineate the many layers of chewed wood pulp that have been processed by the hornets and formed into the paper soft structure. Even now the nest is active, with up to a dozen hornets around its entrance at any one time. Each magnificent individual is close on three centimetres in length, with a warm cinnamon-brown tinge to the wings, pronotum and legs. These are the largest and most majestic of our eight species of social wasp. In my experience they are also the most docile, being extremely good-natured around the nest. The presence of the nest explains the large number of hornet sightings along the river over recent weeks, as workers strike out in search of food.

At this time of the year such a nest could harbour several hundred individuals. Certainly, the steady stream of returning individuals and departures of others suggests as much. Most navigate between me and the entrance to the cavity, as I try to position my camera so as to get a shot of the nest. Others, just the odd one here and there, give me the once over, perhaps curious as to the object that now blocks their usual approach to the nest. It is amazing to be so close to such a large nest; those I have seen in previous years have tended to be far smaller.

The hornet is a rather uncommon and sometimes localised species, though widely distributed across the southern half of Britain. Most nests are, like this one, located in tree cavities, although they may be found within buildings or, occasionally, in cavities beneath the ground’s surface. Like the other social wasps, the hornet has an image problem and has become one of the most feared of British insects. This is a real shame, not least because it preys on a wide range of insects that are typically classed as garden pests. They may occasionally become a nuisance by preying on honeybees or by ring barking lilac shoots to exploit the sap but, left alone, they are one of the good guys. The question of just how long this nest will be left alone remains to be seen. With a footpath close by, the cavity is tempting for adventurous children.

Become a plover lover

Mixed flocks of lapwing and golden plover will be a familiar sight to those who venture into the Norfolk countryside at this time of the year. On dull autumn days, the hunched silhouettes of these medium-sized wading birds can be seen on freshly ploughed fields or areas cleared of sugar beet. Both species make less extensive use of mudflats than other wading birds and, as such, they can turn up almost anywhere across our largely agricultural county. While some of the lapwing will be birds that have bred locally during the summer, perhaps at Holkham or within the Brecks, most will be drawn from breeding populations elsewhere, including Fennoscandia and the Low Countries. All of the golden plover will be winter visitors – the species does not breed in Norfolk – drawn from upland areas spread across northern Europe.

Because the wintering populations of these two species make use of inland agricultural habitats they are not well-monitored through existing monitoring schemes, such as the Wetland Bird Survey (WeBS); paradoxically, though, Breydon Water supports an internationally important population of lapwing. The use of inland habitats means that the size of their wintering populations has to be determined through a special wintering plover survey. This is organised by the British Trust for Ornithology (BTO) and, with concerns of a potential population decline and a suggested shift in wintering range, the latest survey is taking place this winter. Most of our wintering lapwing arrive between late September and the end of November, with peak numbers of golden plover noted in November. Later into the winter, birds may become more mobile as freezing temperatures force them to move south or west to find areas free from frost, where they can feed. Both species prefer to feed on areas of permanent pasture, rich in earthworms, a habitat that is scarce within Norfolk. Instead they make use of winter cereals, oil seed rape and sugar beet stubbles, feeding at night on beetles and earthworms.

Sizeable mixed flocks can literally ‘carpet’ fields but smaller flocks, of 50 or so birds, often go unnoticed. If you are out and about in the countryside this month, make an effort to scan the fields for these two plovers or offer to take on one of the BTO’s survey squares. While the bulk of the survey work will be carried out by BTO volunteers, visiting the randomly selected survey squares, information is also needed from other areas. Records of lapwings or golden plovers seen anywhere within the county are especially welcome and can be submitted on a casual records form. Copies of the form can be obtained by downloading it from www.bto.org/goto/winter-plovers or by phoning the BTO on 01842-750050.