Sunday, 21 August 2011

Spiders lose out to a special wasp

When you think of spiders, you tend to think of them as having things pretty much their own way. They are top dogs, the fearsome predators that snare, snatch and trap insects and other invertebrates. When it comes to web-producing spiders and flying insects it seems even more one-sided, the insects blundering into the web where they are quickly subdued.

Spiders do not have it all their own way though; there is, for example, a group of wasps that specialises in hunting spiders. Belonging to the family Pompilidae, these spider-hunting wasps are smart species, lightening quick in flight and easily overlooked (despite their size). The wasps favour open ground, with short vegetation and areas of bare sand in which they can excavate a nest. These wasps do not form complex social nests like our more familiar yellow and black vespids, but are solitary in habits. The female hunts spiders, paralysing her prey with her sting and then delivering it back to a prepared burrow into which she will lay a single egg. Each nest chamber (or cell) will contain the egg and one paralysed spider, the latter providing a meal for the wasp larva that will soon emerge from the former.

This group of wasps contains 43 species nationally but, even with such a small number to get to grips with, they represent a serious challenge for the entomologist. Knowledge of their favoured habitats means that finding them is not that difficult. Their quicksilver movements, however, make them particularly hard to catch. Once in the net they have the tendency to sting (from experience I can tell you that it feels like a white hot needle and makes your finger joints ache) and once in a glass tube they can still be exceedingly difficult to identify. Fortunately, some of the more common species can be identified fairly readily, through a combination of body and leg colour plus structural features and wing venation.

One of the first of these wasps that I ever encountered was a species called Episyron rufipes (like many less familiar insects, these wasps lack a common name). It is widely distributed but favours heathland and coastal dunes. Active mainly from June through into August, it targets orb-web spiders (but will also take wolf spiders). Once it has paralysed a spider, Episyron will store it temporarily on a plant while the nest is dug, before retrieving it. Watch a suitable patch of sand and you might be lucky enough to see it deliver the spider to the newly excavated nest. That the orb-web spiders don’t have things all their own way makes me see them in a slightly different light; now they are victim as well as villain.

Friday, 19 August 2011

Swallows fatten up for the journey ahead

At this time of the year, many small birds will be getting ready to migrate south for the winter. Preparation is largely centred on the need to build up sufficient fat reserves to fuel the journey ahead, the amount of fat deposited varying in relation to the length of the distance to be covered. Thanks to the efforts of bird ringers we know a lot about the different strategies adopted by migrating birds. Some, for example, complete their journey in a single hop, while others break it up into a series of smaller flights each broken by a stopover to take on fuel. Many small birds will, for example, stop before crossing the Mediterranean, fattening up on its northern shores.

Now you might think that Swallows, being aerial feeders, would not need to fatten up. They can, after all, feed as they go, snacking on the aerial plankton of flies and other invertebrates with which they share the sky.  It seems, however, that this is not the case and that our Swallows also fatten up ahead of their journey. The latest research on this subject is based on some work with which I, as a bird ringer, was involved a few summers ago – you may even remember me discussing it in one of my previous columns. We’d spent a run of summer evenings netting Swallows at South Lopham, as the birds came into roost each evening in the reed beds there. It was a wonderful way to spend the evening, hearing the birds approach and then watching them drop down to roost. There was even the occasional sighting of a Hobby, it too fattening up ahead of its migration – though this time fattening up on the Swallows and martins! We were not working alone and it is only now, seeing the report in which the research was published, that you realise just how many other bird ringers were spending their summer evenings in a similar fashion, at other reed beds up and down the country.

As well as ringing the birds, we also took various measurements, some of which revealed the quantity of fat reserves being laid down. The results show that our Swallows fatten up in southern Britain, taking on sufficient reserves to allow them to travel to the Continent where, according to the work of colleagues overseas, they then take on more reserves in preparation for crossing the Mediterranean and the Sahara of northern Africa. It is amazing to think that our efforts have contributed, in some small way, to increasing our understanding of how these wonderful birds migrate, and the strategies that they use to undertake such an amazing journey to their wintering grounds in South Africa.

Thursday, 18 August 2011

Live traps reveal small mammals

The rich, waist high vegetation of the riverside meadow has proved a good hunting ground. The small mammal traps deployed here over the weekend have yielded good numbers of mice, voles and shrews. Other traps, set along a woodland edge, have proved equally successfully. The traps were being used as part of a workshop on small mammal identification and trapping, organised by the Norfolk & Norwich Naturalists’ Society to give more people the skills and confidence to get involved in wildlife recording. Small mammals are a group that I have worked on for many years, so it was rewarding to be able to pass on some of my skills to a new generation of small mammal enthusiasts.

The traps, known as Longworths, consist of a small aluminium nest box, attached to a tube that contains a trap door and a trigger. A small mammal entering the box, trips the door and then is caught and held until the traps are checked a few hours later. In the meantime, the mouse, vole or shrew has bedding and food available, which seems fair compensation for its inconvenience.

The woodland edge traps were dominated by captures of Bank Vole, neat creatures with rich, chestnut red fur in the adults. Also present in a smaller proportion of the traps were Wood Mouse and Common Shrew. The success of these traps was remarkable; with two traps set at each of the trap points we managed to catch in every trap and, incredibly, we even managed to catch two animals (a Wood Mouse and a Bank Vole) together in one trap. This high capture rate ensured that each of the dozen participants got to see and handle various small mammals. Handling your first small mammal, especially when the eyes of the rest of the group are on you, is not the easiest thing in the world but folk grew in confidence as the morning progressed. The riverside traps were dominated by captures of Field Vole, hardly surprising since the rank vegetation was classic Field Vole habitat and the matted vegetation dominated by their network of interconnected runs.

Each trap was emptied in turn into a large, clear polythene bag. This allows you to see what you have caught and then to manoeuvre it into the corner of the bag where it can be more easily removed. This involves holding the small mammal very gently through the bag with one hand, while the other is placed in the bag, your finger and thumb used to pick the animal up by the baggy skin at the back of the neck. This provides a brief opportunity to determine age, sex and breeding condition before release. It was a morning well spent.

Wednesday, 17 August 2011

Busy colony occupies roof space

Significant numbers of wasps are entering and leaving the roof-space above the kitchen via a tiny gap under the bargeboard outside. The volume of traffic hints at a busy nest, with numerous workers now mature and able to resource the demands of a nest at its seasonal peak. I don’t begrudge the wasps access to the roof-space, though I know that many people do. It is not as if I use the tiny space for anything other than the storage of a few boxes and pans. My curiosity has, however, prompted me to net one of the wasps in order to identify it; there are a number of similar-looking species in Britain, at least two of which will make their nests in houses and outbuildings.

It turns out that this is a Common Wasp, hardly surprising but worth knowing since the other species I have seen in houses locally (the Tree Wasp) tends to produce much smaller colonies. By comparison, an average-sized Common Wasp nest can produce some 10,000 workers, 1,000 queens and 1,000 males over the course of a season. I guess some readers will be less than impressed with this piece of newly-acquired knowledge.

These wasp colonies are annual affairs, with only the fertilized queens overwintering by seeking shelter in outbuildings, houses, behind bark and, occasionally, under stones. Activity picks up in spring and then gradually builds to a late summer peak, the colony finally collapsing in mid-November (or later if it remains mild). The colonies of Tree Wasps can last even longer. It is the nests that wasps produce that have long-fascinated me. Constructed from wood, which is chewed up to form paper, the domed structure can reach a fair size, holding up to 12,000 cells in which the queen lays her eggs. The resulting larvae are fed on a regurgitated mass of masticated insects and spiders, many of which would be viewed as garden pests by most gardeners. The adults, however, have a diet based around nectar, tree sap and honeydew. It is this preference for sweet things that has brought the species into conflict with us, the wasps arriving to spoil afternoon tea and other meals taken outside.

Wasps don’t have things all their own way though, with a number of other species known to exploit the nests and their larvae. Larvae of the small beetle Metoecus paradoxus, our soul representative of the Ripiphoridae, will invade wasp nests to feed on the developing wasp larvae. One of our hoverflies also rears its larvae in the nests of wasps, this time scavenging some of the waste produced by the colony. Later in the year I will take a look at the nest itself but for now I’ll leave it unmolested.

Why be fearful of owls?

There is a lot of superstition surrounding owls, much of which extends back through history to lands beyond our shores. With its large eyes and rounded facial disc, the face of an owl has an almost human quality. Maybe this is why so many people find them such appealing creatures. The largely positive view of owls held by most people in our modern western society is, however, balanced by a fair degree of superstition, and perhaps a little fear; after all, these are creatures of the night, associated with wild woods and lonely ruins.

At various times the owl has been the witches familiar, the messenger of death and the companion on our journey to the spirit world. Many of these superstitions have their origins in the Classical world of the ancient Mediterranean. The Romans, for example, feared the screech owl and thought that hearing one foretold a coming death (that of Julius Caesar being one of the most quoted examples). The owl was viewed as an ill omen by the Romans because it was thought that witches were able to transform themselves into the shape of an owl and then go abroad and do harm.

These ancient superstitions found their way into our literary heritage through the likes of Shakespeare, Chaucer and Spenser. Shakespeare lyrically described the owl as the ‘fatal bellman’, using it in several of his works to portray a coming death, while Spenser equally thought it a ‘hateful messenger of heavy tidings’. This perception of owls as birds of ill omen was intertwined with folk tradition. In Shetland, for example, it was said that a cow frightened by seeing an owl would give bloody milk, while in Sussex it brought misfortune to a household. Through association, the body of a dead owl was thought to ward off evil if it was nailed to the door of a building, a practice that continued in some rural parts of Europe until surprisingly recently. A related belief has it that the body of a dead owl, hung in this way, would keep hail and lightening away!

There is a certain ambivalence about owl folklore. The owl was sometimes seen as a force for good, an ingredient in a cure for whooping cough, for example. In Wales, the call of an owl was said to foretell that a girl would lose her virginity, while in Spain it revealed the sex of an unborn child. While our modern perceptions are mostly positive, owls are still feared in parts of Africa and the Far East, something that has disrupted attempts to conserve rare species. Superstition adds a cultural dimension to our association with owls, something that can be a force for good.

Monday, 15 August 2011

The richness of insect life is staggering

The malaise trap has had its first outing of the summer, ahead of some survey work planned for the coming weekend. The trap is designed to intercept flying insects and resembles a small tent with sloping sides that don’t come anywhere near to the ground. Insects fly into a central vertical panel (which does reach the ground) and, finding their way blocked, they then fly up to the highest point of the roof. It is here that there is a small aperture, allowing the insects through into a collecting bottle.  It is a simple enough arrangement and one which allows me to catch up with many insects that would otherwise be overlooked.

Large (and hence obvious) insects can be excluded from the collecting bottle by using a little piece of wire mesh. In this way you can avoid the capture of butterflies, day-flying moths and the larger bumblebees; these, after all, are species that can be found and identified with relative ease by using other means.

The trap seems to be particularly good at catching small flies and solitary wasps, many of which will take some time to identify. A few more obvious species are usually present in each haul. The other day, for example, there were three brightly coloured ichneumonid wasps, each yellow and black with an elongated body and legs. These were Amblyteles amatorius, their identification confirmed under the microscope, a species that is fairly common on umbellifers at this time of the year. The species parasitizes a number of different moth caterpillars, including some of those species that I am catching in my moth trap at this time of the year.

Also represented in the trap were several different species of hoverfly, some of which can be surprisingly abundant in the garden at this time of the year. There are in excess of 250 hoverfly species known from Britain, with a few new ones added over recent years. This sort of number, coupled with their relatively large size, often well-marked abdomens and subtleties in wing venation, means that this is a group that amateur naturalists often tackle once they move on from butterflies and bumblebees. There are several good publications available and I have been putting these to good use as I have taken more interest in hoverflies over the last couple of years. Unsurprisingly, the group shows a diversity of lifestyles; for example, the larvae of some species are pests of bulbs, while others feed on aphids, making them both a garden pest and a gardener’s friend. The smaller species are some of the more difficult to identify (or find) and this is where the malaise trap really helps, turning up things that I would otherwise miss.