Saturday, 8 April 2006

Springtime buzz

A visit to Thetford Warren Lodge the other lunchtime not only produced my second Brimstone butterfly of the year but also my first bumblebee. Although queen bumblebees that have overwintered successfully can be seen on warm days earlier in the year (as early as February in species like the familiar buff-tailed bumblebee), the cold weather of recent weeks has held things back somewhat. Queens of the six most common and widespread species are likely to have emerged from hibernation by now and will be searching out early flowers in order to replenish energy supplies. Over the coming weeks, the flowers of both red and white deadnettle will prove attractive to many species. However, if the weather deteriorates (and at night) the queens will seek shelter, often in deep leaf litter.

One pattern that you may begin to notice over coming weeks is for queens to fly very low, just above the contours of the ground as they search out a suitable place for their nest. Many species will seek out disused small mammal burrows in which they will lay their first batch of eggs. These eggs, usually 8-14 in number, produce the first generation of offspring, smaller females that are usually infertile and who serve as a worker caste, helping to raise later broods as the colony begins to grow in size. One quite common species, known as the common carder bee, is more adaptable when it comes to nest site selection and may be found using compost heaps. Not all bumblebees make their own nest. There is one group of bumblebees, known as cuckoo bumblebees, which will invade an already established nest, often killing the incumbent queen in the process, prior to laying their eggs.

Some of our bumblebee species emerge later in the year, including the rather localised red-shanked carder bee. There are records of this species from the north Norfolk coast and Breckland but it appears to be in decline. This species, like many other carder bumblebees, emerges in late May or early June. This means that there is quite an extended period of emergence across our 25 or so species, something that provides plenty of scope for the interested naturalist. Those individuals on the wing at the moment face many challenges if they are to successfully found a colony from which fertile males and females will be produced later in the year. Mortality rates of queens are very high (perhaps 80%) because of predation, inclement weather and a nematode worm. The queens that you are seeing on the wing now will be busy getting into condition before producing the larvae which will form the basis of their colony over the coming months.

Friday, 7 April 2006

Roads take heavy toll on toads

Heading out of Thetford on Sunday I came across the sad sight of a dozen or so common toads squashed on the road. The corpses were all concentrated along one twenty-foot stretch, a crossing point used by the toads on their annual migration to a local breeding pond. Over the coming nights it seems inevitable that other corpses will be added to the sickening toll, as more toads reach the road and attempt to cross. One of the reasons why common toads seem so susceptible to motor traffic is their habit of crawling a few feet and then resting for several minutes before continuing. Although the scene will be repeated on other stretches of road close to breeding ponds, there have been efforts to reduce mortality levels through the erection of warning signs and the designation of registered toad crossings. It is hard to estimate the impact of traffic-related mortality on the toad population but, from the handful of studies that have been carried out, it appears that many populations can compensate for the annual losses since the populations remain stable over time.

We have our ‘own’ toad that frequents the patio and borders on warm summer nights and frightens our over-inquisitive dogs by adopting a defensive posture. I do not know where this particular individual breeds but it may use the pond next door, where toad spawn was in evidence last year. Our own smaller pond is deemed unsuitable and just supports the local frogs (spawn first appeared on 26th March).

Common toads are only dependent on water for breeding and outside the breeding season spend little time in the water. Instead, they search for prey in gardens, rough grassland and even woodland. This means that your best chance of viewing these creatures is during late March and early April when they gather at the breeding ponds. The smaller males outnumber females at these gatherings because not only do they mature at a younger age but they arrive first and remain longer at the ponds. Sometimes there may be ten males to each female, which means that competition for a mate can be intense. Males attempt to grasp a female and position themselves so that they can fertilise her spawn. Other males try to break the embrace and manoeuvre themselves into position so it can all get rather chaotic, with over-enthusiastic individuals grasping at other objects including goldfish, vegetation and other males. Unlike frog spawn, toad spawn is formed in strings and is wrapped around the vegetation. The emerging tadpoles, like their parents, use toxins to deter would-be predators and this may be why they are successful in ponds containing fish, where frogs usually fail.

Thursday, 6 April 2006

Good times for Norfolk plover

For a birdwatcher, there is something very significant about the arrival of the first summer migrants. Over the past two weeks some of our earliest migrants have put in an appearance, including the stone curlew, a largely nocturnal plover-like bird. The species is the only European representative of a group of birds known as ‘thick-knees’, characteristic of open habitats in dry or arid zones. With its hunched shape and overly large eyes (essential for hunting surface-dwelling invertebrates at night) the stone curlew is one of the most enigmatic birds to be found in Britain. It is also one of our most threatened, with two main breeding populations – one on Salisbury Plain and the other in Breckland. As such, it has been the focus of highly targeted conservation efforts, which have put the species in a far more favourable position.

Spring arrivals typically occur from late March, as the birds return from wintering grounds in West Africa and it is not long before individuals will be on breeding sites. Stone curlews favour areas with a very short sward or significant amounts of bare ground. Here, in Norfolk, arable fields planted with spring-sown crops, like carrots and sugar beet, are used. The birds are very difficult to see as they spend the daylight hours concealed in short cover, keeping a watchful eye over the area around the nest. However, they become more active at night and their eerie penetrating calls may give away their presence locally. The chicks leave the nest soon after they hatch and generally remain in the vicinity of the nest where they are fed on invertebrates by both parents. There have been occasions where over-enthusiastic birdwatchers have disturbed nesting pairs and for this reason (and because the species has strong legal protection) it is best to visit the Norfolk Wildlife Trust’s reserve at Weeting if you want to see these amazing birds.

The core breeding areas are all that remain of a wider breeding range following a population decline earlier last century. The recent upturn in fortunes is due to the RSPB/English Nature Stone Curlew Recovery Programme, supported by local landowners. Through the programme, fieldworkers locate and mark the locations of nests, alerting landowners to their presence to reduce losses to agricultural machinery. Other elements of the work encourage appropriate habitat management through financial incentives and collect information on population size and breeding success. The aim is to have 300 pairs nationwide by 2010, with some recolonisation of the previous breeding range. Thanks to the combined efforts of the conservation bodies and the landowners, this target should be reached, something that may provide a useful example for the conservation of this species elsewhere in Europe.

Wednesday, 5 April 2006

Common Moths undergo pronounced decline

It is about this time of year that I first put out my moth trap; a powerful light positioned above a collecting box. This enables me to record the many different species of moths that are nocturnal visitors to my garden, which would otherwise go unnoticed. The special mv bulb emits a light that is particularly attractive to moths, and those that collide with the bulb drop into the box below, ready to be identified and released the next morning. At the height of summer, following a warm, dark night, there may be dozens of moths in the trap.

Moth trapping is more than just an increasingly popular hobby, because the information collected can tell us about changes in the distribution and abundance of many of the 2,500 moth species to be found in Britain. Systematic recording using moth traps is at the centre of a new study published by Butterfly Conservation and Rothamsted Research. The Rothamsted network of light traps has been operating since 1968, collecting information from an average of 83 traps per year at sites across a range of habitats, from gardens and woodland through to moorland and coastal sites. An analysis of the trapping records, funded by the Esmée Fairbairn Foundation, has highlighted some staggering changes to the populations of many of our ‘common’ moths. The study charted what happened to 337 moth species over the period 1968 to 2002 and showed that two-thirds of the species had undergone a decline. The populations of 75 species had declined by over 70%, with an additional 57 species showing declines of over 50% and 60 species showing declines of over 25%.

Perhaps the most alarming thing about these findings is that virtually none of the declining species was previously thought to warrant any conservation action – these are species regarded by many as being ‘common and widespread’. Some of the biggest declines have hit familiar garden species, like Heart and Dart, Garden Carpet, Scalloped Oak and Common Wainscot. Since those species that use gardens tend to be generalists, the fact that they, too, are in decline should be sounding alarm bells. The reasons for such a general decline across species are likely to lay with habitat loss and a decline in habitat quality, with increasing pesticide use, light pollution and global climate change also playing a part.

Although the study did reveal an increase in a small number of moths, these were typically introduced species or those expanding their range northwards from the Continent. The results reinforce what many of us have noted over the years, a general reduction in the number of moths seen in car headlights while driving about at night and smaller catches in our traps.

Tuesday, 4 April 2006

Climate and the elegant wagtail

The lengthening days and spring sunshine make my riverside walk to work all the more pleasurable.  The background chorus of singing birds lifts the spirit, something that is further enhanced by the sight and sound of grey wagtails, newly returned to this stretch of the river. At first glance the name ‘grey wagtail’ seems a confusing one; for, although this elegant bird sports a slate-grey back, it has a striking yellow breast and sulphur yellow undertail coverts. However, once you have seen a yellow wagtail, ­the choice of name seems a fair one. A sometimes-used local name for the grey wagtail is ‘water wagtail’ and this provides an accurate distinction between the two species. While the yellow wagtail is associated with lowland wet grassland, the grey wagtail is strongly tied to fast-flowing streams and rivers.

Although widely distributed across Britain, the grey wagtail has its stronghold as a breeding bird in the uplands, traditionally only moving into the lowlands in winter. Over the last hundred or so years the species has been noted increasingly as a breeding species across lowland Britain, nesting among tree roots or under bridges above water on faster stretches of river. The grey wagtail remains scarce in the east of England, with the first successful breeding attempt in Norfolk not recorded until 1923 (at Taverham Mill). Today some 20 or 30 breeding pairs are reported annually from within the county. One of the main reasons for this change in breeding range is thought to be climate change. As a year-round invertebrate feeder, the grey wagtail is susceptible to extended periods of cold winter weather, during which food is hard to find and mortality rates increase. The two severe winters of 1961/62 and 1962/63 are known to have had a big impact on the grey wagtail population. Since then, the run of mild winters has enabled the population within lowland England to increase.

Breeding starts very early in the year, with eggs laid from late March or early April, and this certainly ties in with the activity I have been seeing over recent days. The presence of the birds close to where they nested last year is revealed by the distinctive flight call, a metallic sounding ‘tzitzi’. The birds themselves can often be seen in flight, bouncing and undulating their way up- or down-river.  However, it is when seen on the ground that they are at their most elegant. The long tail is proportionally longer than that of our other wagtails and seems in constant motion as a bird walks about picking up small spiders or flies. On occasion one will fly up to catch an insect in flight, just like a flycatcher only with more balletic poise.

Monday, 3 April 2006

Nuthatches construct a secure home

Over the last few weeks, a pair of nuthatches has taken ownership of the nestbox outside my office window. They must have found the box, situated high on the trunk of a beech tree, to their liking because this is the third year in succession that they have used it. Over recent days the male has been vocal in his proclamation of ownership. His song, made up of a series of piping notes, has been delivered from a high perch and periodically answered by another pair nesting nearby. The ringing call is a feature of early spring across much of southern Britain, where deciduous trees provide nesting and foraging opportunities. Later into the season the birds fall silent and attract less attention.

This particular nestbox is of a modern design, made from a mixture of concrete and sawdust (known as ‘woodcrete’), a combination that not only provides superb insulation but also offers protection from the unwelcome attentions of nest predators like grey squirrel and great spotted woodpecker. Seemingly not content with the nestbox as provided, the nuthatches have been busy gathering mud to cement the removable front panel to the rest of the box. This is common practice amongst box-nesting nuthatches, which will sometimes cement the box to the tree. Famously, one nest was found to have had nearly 4kg of mud added to it! Nuthatches often use mud on natural cavity sites, primarily to reduce the size of the entrance hole down to about 30mm, thus preventing access by larger species that might evict the occupants. The nest itself is a simple affair, the cavity lined by flakes of bark and dead leaves, within which the eggs can be hidden when no bird is on the nest.

Many readers will be familiar with this striking bird, with its seemingly overlarge head and long chisel-like bill. Blue-grey above, with a dark eye stripe, the underside is a mixture of buff and rusty brown. Perhaps the most obvious characteristic is the way that the nuthatch forages “head-down” on a tree trunk. Other tree-climbing birds work their way up the trunk, their bodies supported by strong feet and stiff tail feathers. While the nuthatch retains the strong feet, it has a soft tail, not used for support. Insects taken from cracks and crevices form the bulk of the summer diet, but during autumn and winter seeds are also taken. These may be inserted into a crevice and hammered open with the bill, to be eaten straight away or stored for another time. Judging by the continued use of the nestbox by my office, this particular pair has a territory with a good supply of both seeds and insects.