Saturday, 22 August 2009

Our passageway diner

The passageway that separates our end of terrace cottage from the neighbours is littered with the wings of moths. Most are from a common and familiar species around here, the Large Yellow Underwing. In addition to the moth wings there are tiny dark brown droppings, some on the floor and others caught on the rough plaster walls. It is evident that something has been perched on the walls feeding on moths caught above the garden; that ‘something’ is a Brown Long-eared Bat. Every now and then I catch sight of the bat, either hawking around the trees or perched on the wall, mid-meal.

You might imagine that bats have it all their own way when it comes to moths; that the bats simply fly around, detect and then catch the moths, thanks largely to their echolocation system. Well, the arms race that is evolution has equipped the moths with one or two tricks that can help them to evade capture.

Most bats use echolocation, through which a series of high frequency sounds are produced by the bat, which then listens for the returning echo as the pulse of sound bounces back off an object. Different bats use echolocation in different ways but some general patterns can be seen, largely centred on the habitat in which the bat is foraging. One group of bats that hunts out in the open uses a technique known as aerial-hawking (this group includes the Common Pipistrelle and the Noctule), while another group uses a technique known as ‘flutter-detecting’, which relies on the Doppler effect. A third group of bats, whose members tend to glean food from the surface of leaves, adopts a different approach.

Many moth species have specialized hearing organs which can detect the echolocation calls made by bats from up to 30m away. Since most bats can only detect moths present within 5m, this gives the moths time to turn away from the approaching bat. If, however, the bat is much closer to the moth when it is first detected, the moth will make a sudden and pronounced dive towards the ground. A few moths respond to the calls by making clicking sounds of their own which, when heard by the bat, cause it to turn away.

Such adaptations appear to give the moths an advantage so it is perhaps unsurprising that, despite what you might think, moths usually only form a small proportion of a typical bat’s diet (up to 40% in the case of the Brown Long-eared Bat but as low as 5% in the case of the pipistrelles). Some bats do, however, specialise on moths (e.g. Barbastelle) but these tend to take smaller moths, which lack the anti-bat defence systems possessed by most larger moths.

Friday, 21 August 2009

Forever blowing bubbles

It’s the small things within the natural world that truly fascinate me, the ones that you have to get down on your hands and knees to really appreciate. As a child I would spend a greater part of the summer holidays on my hands and knees, watching insects as they went about their business. Although I have considerably less free time these days, I still find the time to watch insects, from the hoverflies perched in the early morning sun and busy cleaning their large eyes, through to the spiders that emerge from between the fence panels at dusk. These spiders are dark, almost black in colour, quite flat in shape and, for this reason, somewhat sinister in appearance.

One of the things that really fascinated me one summer many years ago was cuckoo-spit, the tight mass of bubbles secreted onto a grass blade by a tiny insect known as a frog-hopper. The name comes from their somewhat frog-like appearance; with a squat body shape and large head, coupled with a propensity for leaping, these small insects are very much like miniature frogs. The frog-hoppers, of which there are several species, belong to an order of insects known as the Hemiptera (the bugs). Included within this order are the familiar shieldbugs, water boatmen, aphids and various plant-hoppers.

The old name for the frog-hopper was ‘cuckoo-spit bug’, a reference to the frothy fluid used by the nymph for protection from desiccation. The spittle is produced by the secretion of a fluid through the anus; this is mixed with a secretion from special abdominal glands which is thought to help the spittle to maintain its coherence, even in wet weather. A special valve in the abdomen blows air into the spittle, effectively producing the bubbles. It is thought that the spittle also acts to protect the pale yellow nymph from predators and parasites but it fails to deter a Solitary wasp, called Gorytes, which drags nymphs from their bubble sanctuary to deposit them in the larval cells of its own developing brood.

While the nymphs are rather pale and unobtrusive, certain adult frog-hoppers are quite striking beasts. One of our larger species, Cercopis vulnerata, is red and black and can be found quite readily between April and August. The red and black acts as a warning colouration, suggesting to potential predators that this species is either poisonous or has an unpleasant taste. By comparison, the more familiar species, our Common Frog-Hopper, is rather understated, favouring greens and browns and relying on camouflage as a defence against predators. It is a spectacularly variable species, with many very different looking colour forms. Of course, to appreciate this you have to get your knees dirty!

Thursday, 20 August 2009

Ladybirds play the villain

While it made a change from reading seemingly endless headlines about the credit crunch and the ‘will they, won’t they’ story of the ashes, recent coverage of the ladybird ‘invasion’ was nonetheless disheartening. In many ways it simply reinforced my view that we seem incapable of tolerating the natural world around us whenever it steps over the line beyond being simply a convenient backdrop to our daily lives.

The spectacle of many millions of 7-Spot Ladybirds concentrated along the coast of Norfolk should be something to marvel at, a mass of coleopteran life rarely seen in this country. Instead it was viewed as a threat, something that might keep the tourists away, curtail the summer and further deepen our economic gloom. Alarmist headlines, such as the Independent’s ‘Ladybird invasion hits Norfolk’, were matched by comments like ‘it was quite horrific’, ‘it ruined our visit’ and ‘there was absolutely no escape from them’. Still, at least we didn’t stoop to the levels seen in 1976 when far larger numbers were present, with cartoons showing ladybirds with teeth and, unbelievably, Nazi swastikas in place of spots. At worst, the presence of so many ladybirds was an inconvenience.

As was the case with the 1976 outbreak, the origins of the ladybirds involved this summer are somewhat unclear. Were they, as some have argued, immigrants blown in from the near Continent or were they home-grown ladybirds, the product of a good breeding season? I favour the latter explanation, in part because it follows the well-reasoned case made by the late Michael Majerus when explaining the origins of the 1976 occurrence. A successful breeding season last year, followed by a fairly mild winter and, importantly, a superabundance of aphids during the early part of the year would have enabled the 7-Spot Ladybird population to reach better than usual levels. A sudden decline in the aphid population, possibly weather-related, would have left the ladybirds devoid of a food supply, prompting them to move on. Those reaching the open sea would have turned back, to congregate on the coast where they covered boats, fence posts and other surfaces.

Look at photographs of these gatherings and you will notice that virtually every last individual is identical, such is the lack of variation seen in this particular species. This may seem like a fairly obvious statement, but many ladybird species show a great deal of variation in their appearance. Melanic forms are common and the number, shape and size of spots can vary considerably from one individual to another; counting spots will not necessarily help you to identify the species!  Rather than moan let’s celebrate a bumper year for this normally popular insect.

Wednesday, 19 August 2009

Great Spotted Cuckoo

The discovery of a Great Spotted Cuckoo at Gramborough Hill in north Norfolk on 23rd July was a cause for much excitement among certain of my birdwatching friends. Although not a really rare bird, with fewer than 40 individuals recorded in England it was of sufficient interest to draw the crowds. Similar in size to the more familiar Common Cuckoo, this vagrant to our shores is arguably a more attractive bird. Individuals of all ages show a general pattern of dark upperparts (spotted with white) and paler underparts, although in juveniles the upperparts are a very dark grey, almost black, while in adult birds they are a soft silvery-grey. Juveniles also show a rusty-brown wing panel which, coupled with the white belly and lemon yellow throat make for a rather smart bird.

The Great Spotted Cuckoo’s breeding range extends from Spain and Portugal east across southern France to Iran and south to South Africa. Southern populations tend to be resident, while those from further north are migratory, with many individuals wintering south of the equator (though a few remain in southern Spain). The pattern of records from Britain is interesting, in that the first overshooting spring migrants can reach us as early as February, with others reaching us from March through to the end of May. This pattern reflects the very early spring migration period exhibited by the northern populations of this species. However, there is also another cluster of records from late July through into the autumn, typically involving young birds (juveniles or first-years) rather than adults.

Like most other cuckoos, the Great Spotted Cuckoo parasitizes the nests of other birds and within the European part of its breeding range it tends to specialise on the Magpie. Adults feed on caterpillars, often favouring large or hairy species and those that are gregarious in habits; the Processionary Moth appears to be a particular favourite. Habitat-wise within Europe, the Great Spotted Cuckoo makes use of scattered Cork Oak and Stone Pine woodland, but it can also be found in almond and olive groves.

This is only the seventh record of the species in Norfolk, the last being a spring bird found at Waxham in March 1999. The first was a bird shot on the denes between Yarmouth and Caister in 1896 and this individual is now on display at the City of Birmingham Museum. One of the other Norfolk birds is also on display, this time at Castle Museum in Norwich, and involves an individual found dead in the North Dunes at Winterton in August 1958. The species appears to be increasing its breeding range so it seems likely that Norfolk will host more of these rather attractive birds in the future.

Tuesday, 18 August 2009

An unintended nature reserve

There is a small patch of grass on the way to work that was, for many years, mowed to the level of a bowling green. Neatly manicured as this was, it had little in the way of wildlife value, serving only to support the local travellers, kids making doughnuts on their motorbikes and the flock of feral geese who largely live off the handouts delivered by local residents. Then, last year, the area was left uncut (the more cynical might suspect that this was done as a deterrent against the travellers) and allowed to develop into an area of wildlife-rich rank grassland. Cut towards the end of last summer, it has again been allowed to develop largely untouched this year.

A narrow margin of cut grass along the edge and a cut path through the middle, tip a hat to the fact that its future is still very much under our control; this area is being managed but it is being managed for wildlife. And that’s a good thing because this small block is now alive with invertebrate life. There is the steady high-pitched reeling call of Roesel’s Bush Crickets, a species whose range expansion across the county has been a striking feature of recent years. There are the softer calls of various grasshoppers, the soft drone of dozens of different species of fly and the sight of grassland butterflies rising and falling just above the grasses.

It is not just grasses that are present; there are flowers and, along the back edge, where a strip of woodland separates cuts down towards the river, there are nettles and umbellifers in profusion. Right in the middle is a patch of thistles which are now setting seed. A steady stream of fluffy seeds lifts up from the plants, caught on the breeze to be deposited elsewhere, a short-lived flurry of botanical snow. Splashes of yellow highlight small clumps of Ragwort, the flowers visited by small hoverflies and even smaller beetles. The occasional Hornet patrols lower over the grasses, its ginger-yellow waspish form purposeful as it seeks prey. On the umbellifers there are even more insects and, additionally, many snails. These snails are pinky-yellow in colour, with dark brown spirals and a dark lip, though there is much variation in colour between them. The snails are secure within their shells, sealed to reduce water loss on these hot, late-summer days.

Each time I pass this patch of grass I wonder for how much longer it will remain. The contractors, with their strimmers and ride on mowers, are sure to return and the grass will be levelled. Let’s hope the cut comes at the season’s end and that the meadow will return next year.

Monday, 17 August 2009

A trip to see Gannets

It is the smell that you notice first as you approach the seabird colony at Bempton in North Humberside. Walking down through the traditionally managed meadows, there is little other sign of the impressive limestone cliffs that hold this stretch of coast high above the sea. These cliffs are home to many thousands of nesting seabirds, from Kittiwakes and auks through to the Gannets we have come to see. The cliff-top path follows the line of the coast and every so often it becomes a promontory, affording views of the cliff itself as it folds back into the mass of land. Such viewpoints provide a wonderful opportunity to observe the nesting seabirds, some just a few feet away, perched precariously on the narrow ledges. Others glide past at head height, wings rigid as they effortlessly slide in and across the prevailing wind before dropping down to the ledge on which their nest is placed.

Many of the early nesters have finished breeding and just a handful of nesting auks remain, the rest now at sea with their young. The colony is dominated by Kittiwakes and Gannets and the air is full of their calls, especially the onomatopoeic ‘kitti-wake, kitti-wake’ call of the slender Kittiwakes. The Gannets are less vocal, perhaps because this is a low density colony for them – the narrow ledges, interspersed with sections unsuitable for nesting, limit where the Gannets can nest and so you do not get the densely packed colony structure more typical elsewhere. Bempton is also unusual for the fact that it is Britain’s only mainland Gannet colony and one of just three located on our east coast. The colony itself was founded in the 1920s, most probably by birds dispersing from the Bass Rock colony which lies to the north. For decades, just a handful of pairs nested at Bempton but then, in the 1970s, the colony suddenly started to increase in size; by the mid-1980s it had reached 780 pairs and by the mid-90s it had reached 1,631 pairs. The most recent census, carried out in 2000, numbered the colony at 2,552 pairs. The sudden growth in the colony also brought with it other changes, namely an earlier onset to breeding and increased breeding success, both thought to be the result of the increased social stimulation that comes from having more birds within the colony.

How this breeding colony will fair over the coming years is less certain and will very much depend on the availability of nesting ledges and the abundance of fish stocks in the North Sea, with overfishing and climate change worrying factors that may yet have a part to play in the future of Bempton’s Gannets.