Saturday, 28 June 2008

A baker's dozen


It’s that quiet time of the year, that brief midsummer lull between the sudden rush of spring and the boisterous climax to summer that heralds the arrival of a new generation of birds and insects. Admittedly there are a few young Blackbirds about, na├»ve and approachable, and the first of the late summer butterflies have been on the wing for a few days. However, a lot of the other more noticeable creatures (like birds and mammals) are quietly going about the business of breeding. This means that it is time to go in search of some of our less obvious insects, including the various bugs that feed on the lush summer vegetation. Some, like most of our grasshoppers and crickets are in their nymphal stages at the moment and look somewhat different from their final adult form. The same is true of the shieldbugs and many other insects. Others have yet to hatch, as evidenced by a cluster of 13 eggs, a baker’s dozen, neatly secreted on the underside of a leaf in the garden at work.

One of my colleagues brought the eggs in to show me, tiny off-white spheres which reminded me of the hundreds and thousands that used to decorate childhood treats. The 13 eggs were arranged so neatly, quite unlike the haphazard approach adopted by some insects. It was not entirely clear as to what sort of invertebrate had laid the eggs; were they the eggs of a moth or something altogether different? In order to find out, I kept the eggs and leaf in a suitable container, checked them daily and waited.

Just yesterday they hatched and I was delighted to see a little host of 13 tiny bugs sitting on top of the now empty egg cases. Over the last few days, a patterned line had appeared on the top of each egg, the shape reminiscent of the silhouette of a red kite in flight. Then the eggs darkened before hatching. I could see that these youngsters were baby shieldbugs, as nymphs they were more rounded in shape than they would be as an adult. Their pinkish-brown bodies strongly suggestive that they were, in fact, young green shieldbugs. The adult green shieldbug is the one that everyone seems to know, often encountered in gardens on ornamental shrubs, raspberries and even runner beans. The youngsters would remain next to the discarded egg cases until their first moult, after which they would attain a smart two-tone green and black livery. Rather than keep them in captivity, we returned them, and their leaf, back to the plant where they had been found. Perhaps we will see some of them again later in the year, or even next spring as adults.

Friday, 27 June 2008

Can you help House Martins?


The house martin is a familiar summer visitor, more suburban in habits than its country cousin the swallow. Although it once commonly nested on cliff faces, including the chalk cliffs at Hunstanton (where it last bred in 1967), the species has adapted to the opportunities provided by modern housing, choosing to place its characteristic nests under the eaves. However, it appears that the house martin is in trouble, a large-scale decline in abundance prompting researchers to place the species on the amber list of birds of conservation concern. House martin populations at the local level, for example colonies on a particular house or bridge, are known to fluctuate over time and this makes it difficult to untangle any underlying change in abundance that may be masked by such short-term fluctuations. It is clear, however, that even within Norfolk the characteristic large colonies at favoured sites have been lost. Saddlebow Bridge at King’s Lynn used to have a colony numbering 150 active nests but no longer. Nationally, the highest densities tend to occur in villages and small towns across East Anglia, so we should be concerned about the changes seen in Norfolk.

In an attempt to increase our understanding of how house martins are doing, and to add more records to Bird Atlas 2007-11, the British Trust for Ornithology (BTO) has teamed up with BBC Radio 4, to launch a house martin survey. Through the survey, researchers hope to establish where colonies of breeding house martins are to be found and to assess the size and success of individual colonies. If you have a colony nesting on your house, or are aware of one nearby, then visit the British Trust for Ornithology website (www.bto.org) to access the survey.

Despite its familiarity there is still a great deal that we do not know about the house martin. While we know that they tend to arrive here from the middle of May and depart again from September or October we know very little about exactly where they spend the winter. We know that they winter in Africa, south of the Sahara, and some 90 million house martins from Europe cross the Sahara each autumn. However, they are very rarely seen in Africa during the winter months. Unlike swallows and sand martins, they do not gather together in huge winter roosts in reedbeds or other accessible sites and it has been suggested that they actually winter above the thick jungles of central Africa. This, at least, might explain the paucity of sightings and recoveries of ringed birds. We also need to know more about the reasons for the decline seen across Europe. With luck, the new BBC/BTO project will shed some light on this particular mystery.


Thursday, 26 June 2008

Lingering Little Gulls bring hope of future breeding


There have been small numbers of little gulls reported from around the coast of Norfolk over recent weeks, a pattern that has become increasingly common throughout the last few years. The little gull is the smallest of our gull species and may be encountered at any time of the year. However, peak numbers are seen during the periods of spring (April to May) and autumn (August to November) passage, as individuals move between the breeding grounds and wintering areas that extend from the southern Irish Sea south to Morocco. There appear to be three distinct breeding populations and it is the westernmost of these, stretching from northwest Russia to southern Scandinavia, that is probably the source of our birds. Interestingly, even though this breeding population has been in decline, it has also shown an expansion westwards (bringing breeding birds into the Netherlands) and this may be why we are increasingly seeing little gulls lingering into the summer months beyond spring passage.

A more detailed examination of these lingering individuals seems to suggest that they are immature birds which presumably do not return to the breeding grounds until they reach maturity. Nevertheless, breeding has been attempted four times in Britain, with one pair getting as far as producing a clutch of three eggs at Rush Hill, Hickling in the summer of 1978. As with the other three nesting attempts, this one failed, most likely through predation. At breeding colonies elsewhere in Europe, little gulls often associate with other gull species, notably black-headed gulls, but they will also associate with marsh terns and avocets. Nesting sites are often alongside shallow freshwater pools, from which the gulls take a range of invertebrate food. Outside of the breeding season, the gulls become essentially coastal or marine in habits, switching to marine invertebrates and small fish.

Most of the birds that are here now will have arrived through the Channel and southern North Sea but observations demonstrate that other birds filter up the west coast of England and then make an overland crossing to bring them into the North Sea. Such overland crossings might seem unusual for a seabird but a number of species are known to use such shortcuts. In the case of the little gull, there are well-documented and annual movements overland, with birds gathering in the Camargue region of southern France before crossing continental Europe to reach the more northerly breeding grounds. The question of whether the species will ever breed here successfully will ultimately be determined by how well the breeding populations in the Netherlands and southern Scandinavia do. If they become well—established then I would expect to see a handful of pairs breed here over the coming decades.

Wednesday, 25 June 2008

Red-legs take up residence


Much to the delight of some of my office colleagues at the British Trust for Ornithology, a pair of red-legged partridges has taken up residence in the meadow outside our window. Despite my initial indifference to these introduced birds, I have to admit an increasing, if somewhat grudging, feeling of tenderness towards them. The two birds are full of character and behave like an old married couple, confident in each other’s company and without the need to indulge in any overtly showy behaviour.

The species has a fairly long, if somewhat patchy, history within Britain, having been first introduced by Charles II in 1673. This attempt, involving the liberation of a number of individuals at Windsor failed, as have so many other attempts since that time. However, an introduction near Orford (Suffolk) by the Marquis of Hertford in 1770 was successful, perhaps because he introduced the birds as eggs, brooded here under chickens but of French stock, and the population has increased and spread since then. It has always struck me as a bit odd that people should go to so much trouble in order to introduce a species that is widely regarded as being a poor sporting bird. Red-legs (or Frenchies as they are sometimes known) show a propensity to escape on foot when flushed for the guns.
Part of the difficulty in establishing the species here comes down to our climate. Red-legged partridges are birds of hot dry areas within their small native range (which takes in France and Spain, together with a small part of Portugal and Italy). As such they only seem to prosper on light, free-draining soils within areas of low rainfall. This may go some way to explaining why they seem to do well here in the Brecks. The wider distribution within Britain shows a close match to those areas where the average maximum July temperature remains above 19 degrees. Within suitable habitat, however, the red-leg is capable of great reproductive output, the female laying two separate clutches of eggs, one straight after the other, which are then incubated simultaneously – the female on one clutch and her mate on the other. The resulting chicks soon learn to feed on grass and weed seeds, with a small proportion of insects, something that makes them less vulnerable to the effects of pesticides than the native grey partridge. The numbers of red-legs probably peaked many decades ago, since when there has been something of a decline, initially linked to the poor-breeding success associated with hybridisation (with the introduced chukar) but now down to a decline in habitat quality. Our population of these birds is now the third largest of any country in the world, so perhaps I should hold them in greater esteem.

Tuesday, 24 June 2008

The river


There is something deeply soporific about the river. It moves with a nonchalant ease, the surface silky smooth and the presence of a current only betrayed by the steady underwater ripple of waterweed. Spared from the immediate effects of recent rain, the water is clear, the gravel bed visible for the first time in weeks. Such is the clarity of the water that the strengthening sun casts shadows within the water column itself, such that those cast by waterweed dance and toy with the stones on the river’s bed.

The air above the river is crowded with tiny flies, each catching in the sun’s rays which stream down through gaps in the overhanging trees. The trees themselves cast deep shadows, combining cool and heavy shade with the dry brightness of the sun and making it difficult for my eyes to adjust as I scan from one to the other. Here and there, patrolling dragonflies cruise in level flight before returning to a favoured perch to watch for rivals or a passing female.

Standing on the bridge, one of my favourite viewpoints from which to watch the river and the life that surrounds it, I can see a shoal of small fish. They seem to favour the shallows, or is it that they are easier for me to spot there? I can make out the red of their fins, suggesting they are rudd or, possibly, roach. It has been a while since I have seen any larger fish in this stretch of the river. Perhaps they have been fished out or, more likely, that they now favour one of the deeper stretches up-stream. There used to be a shoal of chub here, that I would see virtually every day but they, like the pike, are nowhere to be seen. Sadly, so close to the road, the river carries the scars of its human neighbours. An old mattress half covers the slowly rusting frame of a bike and other, smaller, items are scattered nearby. What on earth prompts people to treat the river in this way?

Leaving the bridge and moving upstream, the taint of Man is lost and the river regains its graceful elegance. The banks are thick with emergent growth and it is only because I am on higher ground that I can still see open water. Away from the traffic, the air carries a soft hum, the combined droning wingbeats of a myriad of insects. The pitch of this sound resonates within me and adds to the sense of somnambulance. It is a wonderful feeling to immerse myself in the life of the river in this way and I understand just why it is that I am drawn to her margins.

Monday, 23 June 2008

The Hairy Caterpillar


When it comes to avoiding the unwelcome attention of potential predators, caterpillars must surely take the prize for the most diverse array of defensive ploys. Some are coloured so as to blend in with the vegetation upon which they are sitting; others are more obvious but pretend to be something else, like a dead stick or a bird dropping. Then there are those caterpillars which make a point of really standing out from the crowd, adopting bright, warning colouration to deter predators with the threat of a toxic or unpleasant meal.

Fortunately, the adoption of these different approaches often provides a basis for identifying the caterpillar you have just found lurking on some plant. Just the other day, for example, I was shown a small caterpillar on the leaf of our new pear tree at work. The dark, inch long larva was covered in an amazing array of coloured hairs, arranged in groups along the length of its body. At the head end there were two elongated tufts of dark hair, sticking out in front much like the tentacles of a garden snail. There were also shorter tufts along the back of the caterpillar; four yellow tufts towards the head end, all in a neat row, and smaller red ones towards the rear. In addition to these, the whole body was covered in fine long hairs of just the kind that would no doubt irritate the skin of a predator or over-inquisitive child. This combination of features revealed the caterpillar to be the larva of a vapourer moth. The adult vapourer is a fairly common species, on the wing from July through into October, and likely to be found in woodland and gardens across the county. Unusually, the female vapourer is wingless and presumably attracts the male to her through the use of pheromones. The eggs remain unhatched right through the winter and it is only the following spring that the distinctive caterpillars appear to feed on a range of tree species.

The evolution of such defensive ploys has been driven by natural selection, with moths from different families going down different evolutionary pathways, some which have led to the adoption of cryptic colouration, others to the development of chemical deterrents and colouration that advertises their presence. Of course it is not just the caterpillars which are under threat of predation; adult moths also show a range of approaches to reduce the chance that they will be eaten. This evolutionary arms race, between predator and prey, is repeated across species and groups worldwide. It is a race that is still ongoing and the strategies that we see today should not be regarded as an end point, merely a step in an ongoing evolutionary process.