Saturday, 18 October 2008

Watch out there's a stoat about

The Stoat is one of those animals that I only see occasionally, perhaps running across a track or working the base of a hedgerow in search of small mammals or other prey. As such, I tend to take notice of where and when I see Stoats, noting them in my field book to report at the end of the year. It seems that I am not alone in this; look at the Norfolk Mammal Report (available from the Norfolk & Norwich Naturalists’ Society) and you will see the species features more often than others that you would consider to be more common across the county. Does familiarity breed contempt and are we ignoring the commonplace?

There is something mesmerising about a Stoat that is working an area in search of prey. Ever alert, it searches opportunistically, pausing every now and then to scan for other predators and would-be prey. I have only seen a Stoat tackle prey on a handful of occasions, usually a vole or mouse, but on once a Rabbit considerably larger than the Stoat in size. Stoats are reputed to mesmerise their prey by dancing but this is unlikely to be the case. The few incidents where observers have reported such behaviour may, in fact, be observations of a Stoat suffering from an infestation of a parasitic nematode, which causes skull deformity and spasms.

Although the Stoat is one of the smaller European carnivores, it is not the smallest and shares the British part of its wide range with the even smaller Weasel. The two species can be told apart fairly readily; the Stoat larger, with a long black-tipped tail; the smaller Weasel with a shorter tail and no black tip. Additionally, in Stoat, the chestnut back and flanks meet the cream belly in a straight line, whereas in Weasel this line is irregular and sometimes spotted.

There is one other aspect of Stoat ecology that fascinates me and that is its breeding system. Both male and female Stoats are territorial, with each male holding a territory within which there may be one or more smaller female territories. During the breeding season each male will adopt one of three strategies, the choice dependent upon his age and social status. Older dominant males adopt a roaming strategy, expanding the size of their territory massively. They then roam around the territory in search of females, spending a few days with each before moving on. Younger males stay within their smaller territories and the youngest males remain fully transient, unable to hold a territory of their own. Although mating occurs from April to June, the females delay implantation, so the resulting young are not born until the following spring.

Friday, 17 October 2008

Bush crickets make the most of autumn sun

I try to make the most of any late-autumn warmth by getting out into the field, treating each warm and sunny day as if it might be the last one before the onset of winter. While a bright morning might start with a nip in the air, it often warms as the sun rises and the small numbers of summer insects that remain can be seen on the wing or crawling across the surface of some fading leaf. The warmth of last weekend drew me to the coast and to some of the small lanes that drop down towards the beach at places like Kelling and Weybourne.

One particular stretch of low hedgerow, dense with bramble and in full sun, was alive with the sounds of calling bush-crickets. It is late in the season for these insects and many of the Breckland populations have fallen silent over recent weeks, possibly due to the handful of night frosts that we have already experienced. Here on the coast, however, there were good numbers of Dark Bush-crickets repeating their short chirping song at irregular intervals. The song is high-pitched and while I can still hear it, my birdwatching companion no longer can. With patience I was able to locate a couple of the calling individuals and point them out. Each robust and dark individual, about two centimetres in length, was calling from just within the bramble cover, producing its call by rubbing its modified wings together.

Typically, in male bush-crickets, the base of each wing is modified, with a tooth-bearing rib present on the underside of the left forewing. This is rubbed against the edge of the right forewing, next to which is a modified area, known as the ‘mirror’, which amplifies the sound that is produced. Only the Oak Bush-cricket lacks this adaptation; the male instead tapping out his drumming call with a hindleg onto whatever he happens to be standing on.

Dark-bush Crickets are robust creatures and have a long season, lasting through until the first frosts of late October or November. However, other bush-crickets are more strongly influenced by temperature and so I was a little surprised to see and hear two calling Roesel’s Bush-crickets in the same stretch of hedgerow. Roesel’s Bush-cricket is a species that has expanded its range northwards over recent years, a result of global climate change. First recorded in Norfolk in 1997, its colonisation of the county has been extremely rapid. Hot summers can boost numbers and while the 2008 summer may not have been the best, the species has clearly been fairly successful. Let’s hope they continue to call for a few more warm and sunny mornings.

Thursday, 16 October 2008

The first geese

The first of the winter’s geese are here and the morning sky echoes with their calls, as long skeins move between overnight roosts and daytime feeding grounds. These are Pink-footed Geese, newly arrived from breeding areas in Iceland and eastern Greenland, and their numbers will continue to grow over the coming weeks. Virtually the whole of the Icelandic and Greenland breeding populations winter in the UK, some 250,000 or so birds and representing at least 85% of the World population. The only other breeding population can be found on Svalbard and individuals from there winter in The Netherlands and, increasingly, Belgium.

These Pink-footed Geese will have arrived in Scotland several weeks ago, the arrival there continuing through into the middle of October, before most filter south through staging areas to favoured wintering grounds. The bulk of the population winters either in Lancashire or East Anglia, and Norfolk itself is a very important county for this winter migrant. The latest WeBS report, published by the British Trust for Ornithology, shows that the Wash and the North Norfolk coast currently supports some 59% of the UK wintering population.

The geese are attracted by the combination of undisturbed roosting sites and daytime feeding areas. Initially, the geese wintered and fed on saltmarsh, feeding on grasses and herbs on the short saltmarsh sward. More recently the birds have taken advantage of the food available on areas of arable land and pasture, with sugar beet tops and waste potatoes a favoured food. Such choice does bring a small amount of conflict with landowners if the geese move from the harvested beet fields to feed on growing crops elsewhere. This can happen if the geese suffer high levels of disturbance when feeding on the beet, so landowners often grow the sugar beet away from footpaths and busy roads; this benefits both the landowner and the geese, which may go some way to explaining why the population has increased over recent years.

To me, it is the movement of pink-feet between roosting and feeding sites that is the most evocative part of the Norfolk winter. To hear an approaching flight of geese, which first appear as a distant smudge on the skyline but which turns into distinct skeins as they approach, is truly magical. Equally magical is the sight of a huge flock, many hundreds strong, feeding across one of the larger coastal fields. To scan across these with a pair of binoculars reveals an army of individuals all feeding on the waste tops of beet. In some ways it is a shame that they are only here for part of the year but I suppose that if they were here all year round then their magical charm would become commonplace.

Wednesday, 15 October 2008

On the move

When you think of bird migration you tend to think of summer or winter visitors, such as Swallow, House Martin or Redwing; birds that are only with us for a small part of the year. This means that many of the birds that are on the move during autumn or spring go unnoticed, simply because they happen to belong to species that you would normally see throughout the year. For instance, birds as familiar to us as Song Thrush, Chaffinch and Goldfinch all either have a migratory component to their British breeding population or to a population elsewhere within wider European range.

While most of our breeding Song Thrushes and Chaffinches are rather sedentary in their habits, some winter abroad in France, Spain, Ireland or Portugal. At the same time, Britain also receives Song Thrushes and Chaffinches arriving here to overwinter, or passing through en route further south during the autumn months. A visit to the Norfolk coast soon after dawn may well reveal some of these visitors. On Sunday morning, for example, I saw and heard good numbers of Chaffinches and smaller numbers of Song Thrushes passing overhead at Kelling Quags, together with other autumn migrants – including Brambling, Siskin and Meadow Pipit.

Both Meadow Pipit and Chaffinch are diurnal migrants (migrating during the day), and the birds I saw may well have been continuing journeys initiated in Scandinavia over recent days. Some of these birds, notably the Brambling, will have made a direct crossing of the North Sea but others, like the Chaffinch, will have avoided the crossing altogether by taking a longer route around the North Sea coast, through the Low Countries and then making a short hop across the English Channel. Such a convoluted journey will have required some nifty navigational skills along the way, given the change in compass direction needed at different stages of the journey.

Another interesting aspect of Chaffinch migration (at least for the birds leaving the northernmost part of the breeding range) is the difference in timing between males and females, and adults and young. Females Chaffinches tend to move before males (and migrate further) and the same is true of young birds, when compared with adults. In the case of the Song Thrush, individuals from different parts of the breeding range show variation in their movements; those from the northernmost populations tend to move the furthest south, with some birds even reaching North Africa. What amazes me about all these movements is not simply that such small birds can undertake such large movements but how individuals of the same species can behave so differently depending upon where they were born, what sex they are or whether they are a young bird or not.

Tuesday, 14 October 2008

An exceptional occurrence

By all accounts it has been something of a poor year for butterflies, with many species present in lower numbers than those usually seen. The poor weather over much of the summer contributed to difficult times for butterflies and there were several species that I either failed to catch up with entirely or of which I saw very few individuals. As such, a recent trip to Holkham proved to be a rather exceptional event thanks to the presence of a single White Admiral butterfly. I had gone to Holkham to look for a Radde’s Warbler that had been seen there earlier in the day. While I failed to catch up with the Radde’s Warbler, the presence of a fresh-looking White Admiral, close to where the warbler had previously been seen, more than made up for my disappointment.

This is a species of butterfly that I normally see much earlier in the year, with Knettishall Heath a favoured local site providing sightings of newly emerged individuals throughout June and July. The White Admiral is univoltine, which means that it has just a single generation each year. The graceful adults are on the wing from June to August, depositing eggs on the leaves of honeysuckle from which emerge the caterpillars. Each caterpillar begins its journey towards adulthood in late summer but then hibernates through the winter to resume its growth the following year. Given that the single generation of adult White Admirals is over by the end of August, my sighting of a newly emerged adult at Holkham posed an interesting question – what was it doing on the wing so late in the year? Was this an adult that had simply emerged spectacularly late or had a caterpillar completed its growth and transformation within a single season.

Searching through my numerous books on butterflies finally revealed the answer; I discovered that a small second generation of adults might very occasionally occur in those years when the first generation is on the wing exceptionally early in the season. It seems that a handful of individuals must have been on the wing in late May or early June, with some of the resulting caterpillars completing their stage of the life cycle without the winter break. If any eggs were to be laid as a result of these very late season adults then the resulting caterpillars would presumably enter winter hibernation at a much smaller size than normal, something that might reduce their chances of surviving through to next spring. Whatever the fate of this particular late-season individual I was glad to have seen it at a time when the butterfly season is just about coming to its end.

Monday, 13 October 2008

Riding out the storm

The onshore gales that are a feature of October bring with them birds more often encountered on the open ocean. If you are brave enough, and well prepared with several layers of windproof clothing, then you can spend a profitable morning seawatching, scanning the horizon for passing birds with your telescope. Many remain distant and difficult to identify as they are repeatedly picked-up and then lost from view, rising and falling in flight to dip below the crests of growing waves. In amongst the more commonly encountered species may be scarce birds like Great and Sooty Shearwaters; it is for these birds that many seawatchers brave the elements, hunkered down out of the worst of the wind.

However, for me it is the more common Gannets that tend to catch my eye. These striking birds, with their dazzling white adult plumage and black-tipped wings, have always fascinated me. I have been fortunate enough to see them on one of their breeding islands and to marvel at their two metre wingspans close-up. The breeding colonies themselves are boisterous affairs, as neighbouring pairs squabble, and the associated noise gives rise to the feeling that you are in the middle of a troublesome football crowd. Grounded, Gannets are somewhat ungainly but when on the wing they exude the grace and power of a heavyweight plunge diver.

Many of the Gannets passing east along the North Norfolk coast in autumn are young birds, fledged from one of our northern breeding colonies and now following the impulse to migrate south. Gannet chicks invariably leave the nest carrying extra fat reserves and so, having made the first short flight down from their island colonies, they land on the sea and find themselves too heavy to take off again. As such, their journey south begins with the bird swimming. Once some of the fat reserves have been used up, the youngster is able to take to the wing and begin its journey proper, passing down either the Atlantic or North Sea coasts and into the Bay of Biscay. These young birds show the strongest migratory urge and many migrate as far south as Senegal or beyond. Although Gannets typically do not breed until their fifth or sixth year, many do return at least some distance north the following spring. In fact, as the birds get older more of them return to the breeding colony, such that by the fourth year many males will set up a small nest site and attempt to recruit a mate. The Gannets that are passing our shores now are a mixture of ages (and plumages) and well-worth seeing. While you may see one on a still day, it is the rough ones that boost the numbers passing close inshore.