Tiny passage of globally threatened bird

Recent weeks have seen the now annual handful of records of the globally threatened Aquatic Warbler from sites along the south coast of England. This cracking little bird, which resembles the more familiar Sedge Warbler in its general appearance, is something of an autumn speciality, a rare passage visitor to these shores.

It has been several years since I saw my one and only Aquatic Warbler and I can still picture the bird sitting quietly in the bottom shelf of a mist net (used by licensed ringers to catch birds for ringing) in an East Sussex reedbed and the look of joy on my colleague’s face as he reached to extract it from the net. The entire world population of this warbler, estimated at just 18,000 pairs, breeds in central and Eastern Europe, favouring extensive areas of river valley marshland, sedge bed and damp hay meadow. Drainage and agricultural improvement have led to a significant decline in the breeding population, which is why it has been given such a high profile conservation status.

At the end of the breeding season Aquatic Warblers migrate southwest across Europe, heading for wintering grounds situated in West Africa, south of the Sahara. The precise wintering grounds are unknown and more work is needed to work out where these are in order to determine if conditions there are deteriorating for the birds. If, during the early stages of the autumn migration, the birds encounter winds from the east, then they may be displaced across the English Channel to reach sites spread along the south coast of England. In exceptional autumns they may be found further north, with several Norfolk records and birds reaching as far north as Northumberland. There are even a few spring records, though these are even less common than autumn ones.

One of the things that makes the Aquatic Warbler such an interesting bird, aside from its rarity here, is that the male Aquatic Warbler has taken promiscuity to unprecedented levels! Females tend to be rather elusive, living skulking lives within the complex structure of vegetation in which they make their home. When a male encounters a female he copulates with her, remaining in copula for up to thirty minutes; in most birds, by contrast, copulation lasts just a few seconds. The male does this to prevent other males from mating with the female at the critical time and to introduce sufficient sperm to swamp those of any previous matings. In order to deliver this quantity of sperm, the male Aquatic Warbler is particularly well-endowed. Having mated successfully he will then seek out other mates, leaving the female to rear the young all on her own.

A festival of harvestmen

Scrabbling around on your hands and knees may not be the most dignified of occupations but it does mean that you can get down amongst the vegetation, a useful thing when looking for invertebrates. This morning I took the opportunity to search the grounds at work for woodlice and harvestmen, two groups for which there are a few species we have yet to find this year. The woodlice have been fairly well covered, thanks to a concerted effort early in the year, but there are additional species that I suspect may well be present in the BTO’s Nunnery grounds. So far, we have found three different species of harvestman which, based on a Norfolk fauna of 18 or so species, leaves others unfound.

Harvestmen are probably familiar enough to most readers. The combination of a small round body and long legs hints at the relationship between harvestmen and the closely related spiders and mites. These are arachnids but not ones to be frightened of. Writing in 1977 Savory decribed them as ‘the comedians among Arachnida: animals with rotund bodies ornamented with little spikes, with two eyes perched atop, back to back, like two faces of a clocktower, with ungainly legs insecurely attached, with feeble jaws and an undying thirst…’ Savory’s description is spot on, though to really appreciate this you have to view a harvestman through a handlens or binocular microscope.

Odiellus spinosus

As well as being interesting creatures, harvestmen have an interesting name and one that deserves further mention. There is a clear association with agriculture in the name, and a seasonal timing as well. Although many can be found year-round and others are most commonly encountered in spring, they are most abundant in late summer – harvest time. While we call them harvestmen the French refer to them as les faucheurs (the reapers). Writing in 1634, Dr Thomas Mouffett knew them as ‘shepherd spiders’, explaining that English shepherds thought that a field with lots of harvestmen made excellent sheep pasture. The association with sheep is still retained in the nomenclature used in harvestman classification. The order in which harvestmen are placed for classification is called ‘Opiliones’ and has its origins in the Latin ‘opilio’ which means shepherd.

Harvestmen are predators, feeding on a wide range of small, often soft-bodied, invertebrates. They tend to live on the ground, particularly the more robust species with their shorter and stouter legs, but some may be found on brambles and nettles. The species I was searching for this morning, successfully as it happens, was one of the stout ground-dwelling species, hence my being on my hands and knees and searching through the debris at the base of an old wall.

The river at night

Even on an overcast night there is still just enough light to make out the river as it flows inky black through the wood and on into town. It is the movement of the river that reveals its presence, the ever-changing nature of the surface film, which ripples and catches the eye. Other than the gentle murmurings of the river itself there is a reassuring stillness that envelops you, drawing you in with soporific ease. I don’t come down to the river that often at night unless I have dropped a car back at work after an evening talk. Rather than take the short route home, I sometimes follow the meander of the river through the woods and up to the bridges that mark the point at which the ancient trackway once forded the river.

I am not the only creature out at this late hour. Occasional sounds reveal other inhabitants pushing through the vegetation; muntjac startled at my presence, or small mammals flitting between the safety of their burrows and the patches of fruits or seeds on which they feed. It is the bats, however, that draw me here. I can just about make out their high-pitched calls as they make feeding passes up and down the river, presumably catching midges and moths as they go. More often than not, I will have my bat detector with me. Switching it on, the high-pitched calls are transformed into a pattern of beats and pulses that I can hear more clearly. Adjusting the dial across different frequencies I can make a stab at their identification.

From earlier visits, with more advanced detectors, I know that many of these bats will be Common and Soprano Pipistrelles, with good numbers of Daubenton’s Bat and the occasional Brown Long-eared. The Daubenton’s fascinate me, their series of echolocation calls reminiscent of a jazz drummer beating out a rhythm that slows and then accelerates with consummate ease. These small bats feed low over the water, typically between five and forty centimetres from the surface, taking insects from the water’s surface through the use of their feet and tail (contact of the prey with the tail membrane triggers a ‘strike’ with the feet). Since suitable prey are most readily recognised on calm water, these bats favour the more sheltered, slow moving parts of the river, away from surface vegetation and out of the wind. This particular stretch of the river, as it meanders through a block of alder woodland, seems ideal. It is only when I reach the bridge, with its solitary street light, that I can see as well as hear the bats and watch them as they skim the dark water for food.

Getting in a final round

There is a chill in the air this morning as we arrive in the pre-dawn gloom. The pits feel strangely still, a sign perhaps that summer has reached its end. We are here, among the reedbeds and lagoons, to set our nets for birds on the last of the summer’s netting sessions. Every ten days or so throughout the summer we have set our nets in fixed locations, with the aim of catching and ringing warblers, tits and finches as part of a wider project. All over Europe there are volunteers doing what we are doing, catching and ringing birds for science and conservation.

The nets are made of a lightweight material, the mesh sufficiently loose and dark that you soon lose sight of the nets against the vegetation. Strung throughout each net is a series of horizontal threads, which produce ‘shelves’ within the net. A small bird, flying into the net, drops into the pocket formed by the shelf and is held gently until we extract it for ringing. It may seem a bit undignified but it does no harm and provides important information on bird movements, longevity and survival rates.

It takes nearly and hour to set all of the nets and, just before light, we go round them again to open up for catching. Already there is some activity, though little in comparison with earlier in the season when the site was packed with breeding birds. The soft calls of Reed Bunting and Pied Wagtail can be heard from one of the reedbeds. These are roosting birds that have come together to spend the night in the relative safety of the reedbeds. With a chill in the air we check the nets every fifteen minutes, removing the birds we have caught and placing each in its own cotton bag.

Once back at the cars, where we have set up our ringing station, we process each bird in turn. Each is identified and then the appropriate ring for that species is selected and fitted. The rings are made of a soft, lightweight metal alloy and each carries a unique number and the address of the British Museum. If the bird is caught again, or found somewhere else, then we will know its history. We examine the plumage to work out its age (most birds moult in a predictable way that enables us to differentiate between old and new feathers) and its sex, before assessing weight and body condition. All done, the bird is released. Ringing provides a unique opportunity to see birds in the hand, to really appreciate their form and structure, and I feel very privileged to contribute to our understanding in this way.

Ladybirds get their own atlas

It is at this time of the year that I start to notice ladybirds around the house. Most of these are the Harlequin Ladybird, a non-native species whose arrival in 2003 resulted in a rapid and worrying colonisation of most of England and much of Wales. That we have been able to chart the spread of this species is largely down to the organisers of two national ladybird survey schemes: the Harlequin Ladybird Survey (www.harlequin-survey.org) and the UK Ladybird Survey (www.ladybird-survey.org). Because most ladybirds are large, colourful and recognisable (as being a ladybird), the surveys have been able to tap into people’s enthusiasm for these insects. Submit a record of a ladybird, perhaps with a photograph and perhaps of a species you do not recognise, and you’ll receive an identification and have your record added to the ladybird dataset.

Eyed Ladybird

The efforts of all this hard work have just been published in a new ladybird atlas, which not only reveals where the different species occur, but also gives pointers to their identification, seasonality and habitat preferences. Flicking through the atlas reveals the presence of many ladybird species in Norfolk that are easily overlooked. In fact, it might surprise many readers to discover that there are 47 ladybird species in the UK (and more than 4,500 worldwide). Roughly half of our ladybird species are instantly recognisable as being ladybirds, with brightly coloured and spotted elytra (the hard wing cases that form the back of the ladybird). The remainder are typically small, dull in colour and readily dismissed as being some other form of small beetle, unless you know what to look for.

The arrival of ladybirds in the house suggests that the season is changing and that the ladybirds are preparing for the tough times ahead. The unfavourable winter weather and scarcity of food, prompts ladybirds to enter a dormant state, often in aggregations of a dozen or more individuals. Most enter this period of dormancy towards the end of September, remaining in this state until the warming days of spring.

Many of our ladybirds are associated with particular plants, where they feed on aphids and scale insects. Knowing these preferences should allow you to seek out individual ladybird species, from the gorgeous Orange Ladybird, which favours Ash and Sycamore, to the narrow-bodied Water Ladybird associated with reeds and other waterside vegetation. Others can be swept from low vegetation with a net or sought on the trunks of Larch.

For a budding naturalist, the ladybirds form an excellent group. There are not too many species; identification is not that difficult and you can quickly make a very valuable contribution to our knowledge of the distribution and habits of these beetles.

Sad strandings

Last month’s news that a Sowerby's Beaked Whale had washed up at Thornham was the second recorded stranding of this species in as many years. Unfortunately, although alive when found, the animal had to be euthanased. The individual found stranded on Blakeney Point on 13^th August 2009 was more fortunate, being rescued and helped back into the sea before being guided away from the shore. These were the second and third records of the species for Norfolk, following one at Happisburgh in 1952.

Sowerby’s Beaked Whale has one of the most northerly distributions of all of the beaked whales, occurring in the temperate North Atlantic, south to the Azores. Most reported strandings come from the coastlines of the North Sea, giving rise to the name ‘North Sea Beaked Whale’, which is sometimes used. The presence of this small whale in the North Sea is surprising given that this is a deep-water species, feeding on small fish and squid taken from depths of 400m or more. The relatively few live sightings come predominantly from the waters off NW Britain & Ireland, Iceland and the Faroes. Quite why so many strandings occur around North Sea coasts is unknown, but it may be linked to the whales becoming disoriented when they enter this relatively enclosed body of water. Although strandings have been reported in all months of the year, there is a peak in late summer/early autumn, matching the two recent Norfolk strandings, and perhaps suggesting that individuals are moving south across their range at this time.

The stranding of any whale is a sad event and seems to trigger a great deal of interest from the wider public. Perhaps it is because of the romantic image that we have of these leviathans, creatures so different from ourselves that they arouse interest and (in most quarters) respect. I remember the crowds that made the long walk out at Heacham to see a beached Sperm Whale in the early 1990s, and the look of wonder in the eyes of many of the children who had been taken to see the unfortunate creature. Despite our respect, and indeed the legal protection afforded to the whales and dolphins that use our waters, our activities still pose a threat to these graceful mammals and their environment. In addition to the obvious threats from oil and gas exploration, over-fishing and the deployment of military sonar, there is the insidious threat from the vast quantities of plastic that end up in our oceans. If we can build upon the general interest in our whales and dolphins then perhaps we can educate people and get them to think about their personal impact on the wider world.