Saturday, 10 September 2016

The rejuvenation of Leighton Moss

Leighton Moss wetland rejuvenation from graham white on Vimeo.

Leighton Moss was traditionally the stronghold of the Bittern in north-west England and, along with Minsmere, was one of the sites that provided much research knowledge when the Bittern population was at a very low ebb in the 1990s.   However, whilst Bitterns are now surging upwards elsewhere on the back of this research, they continue to decline at Leighton Moss. Yet the site continues to support one of the best densities of Eels and other fish, the Bittern’s main prey, that we have come across.  So what is going on?  The issue is that Leighton Moss is an old and ‘littered’ reedbed. Recession of the reed margins away from silted pools and ditches is making the fish less available to a foraging bird. The problem of reed recession in old reedbeds is well known.  The formation of toxic by-products by the reed litter under anoxic conditions in a high, stable water regime, reduces reed shoot density and vigour.   Reed re-colonisation into anoxic sediments is known to be poor.  This issue is compounded at Leighton Moss by high grazing pressure from an increasing population of Red Deer and by Greylag Geese.

However, when reedbeds start to die-back due to the ageing processes described above, or grazing (by geese or deer for example), the extent of healthy reed can be increased by lowering water levels to expose mud and allow new plants to germinate and establish.  Periodic drying out can be used to vary the proportions of open water and swamp vegetation and effectively rejuvenate the habitat.  Such a pattern of management has been used in the Oostvaarderplassen in The Netherlands.  A mosaic of reed and open water habitat is maintained by a combination of fluctuating water levels and grazing by Greylag Geese. This results in a periodic expansion and recession of reed.  During high water levels, the geese graze back the margins of the reedbed.  During low water levels, the exposed mud is colonised by reed, other swamp plants and ruderals.  When water levels rise again, the seeds of the ruderals provide food for wildfowl and the new reed shoots form an expanding reedbed.

So in 2014, water levels were lowered on Leighton Moss to reveal acres of gloopy mud, with the aim of drying the site during the summer months for a number of years.  In addition, a small cull of Red Deer began, with the aim of reducing their grazing impact.  Such actions that result in changes to the accepted norm are not always popular, but an appropriate quote for this situation might be: ‘If you want things to stay the same, you have to change them’.

By August 2016, significant changes had taken place.  The gloopy muds had consolidated and vegetation was spreading across it.  Reed was both germinating in the damp mud and spreading out by extending runners from the established clumps.  Early colonizing plants such as Golden Dock and speedwells were abundant, providing a massive boost of seed for wintering wildfowl.  The lowered levels bring other changes: more waders in front of the hides and Spotted Crakes appearing in the reedbeds.  Another year or two of lowered water is probably required before returning to a more normal water regime.  We should then see a boost in many species.  However, whether Bitterns will return to their former glory is uncertain.  The reedbed will be healthier but it is still an old, rather dry reedbed, and as such will support a different, but just as important, range of wildlife.

Below:  the same view, 2014,2015 & 2016

Tuesday, 16 August 2016

A tern around at Rye Meads

Common Terns at Rye Meads from graham white on Vimeo.

As regular readers will recall, the colonization of rafts designed for terns by Black-headed Gulls is good news for gull fans but can be less positive for the terns.  After the first colonization of rafts in the Lee Valley on the Herts/Essex border in 2008, Black-headed Gulls had risen to a total of 306 pairs at 3 sites by 2015.  Meanwhile, Common Terns had declined to a low point of 58 pairs in the Valley in 2012 with a slight recovery since.  Further analysis of individual colony total and productivity in the Valley has helped clarify the situation.

There are four main colonies in the Valley: at Rye Meads, Amwell, 70 Acres lake in the Cheshunt gravel pit complex and at Walthamstow Reservoirs. The combined populations peaked at 117 pairs in 2007 but then began a steady decline to a low point of 58 pairs in 2012 (a very poor year generally due to bad weather) before rising slightly to 67 pairs in 2015.  In general terms, the populations at Rye Meads and Walthamstow have declined, whilst those at Amwell and Cheshunt have remained stable or increased.  The decline at Walthamstow has been particularly severe, with just four pairs in 2015 in contrast to 42 pairs in 2007.  This decline is likely to be due primarily to the poor condition of the rafts.  With no maintenance being undertaken in recent years, the rafts are now covered in vegetation.  In addition, increasing populations of breeding Herring and Lesser Black-backed Gulls at Walthamstow are known to predate the terns. There are no Black-headed Gulls nesting at Walthamstow.  If Walthamstow is excluded from the total counts, the total at the other three sites has shown a more moderate decline from a peak of 75 pairs in 2007 to 63 in 2015. The majority of the population decline in the Lee Valley can be attributed to the situation at Walthamstow Reservoirs.

So has the colonization of the Lee Valley rafts by Black-headed Gulls had any impact on the Common Terns? Firstly, analysis of colony data shows that the population and trend of Common Terns in the Lee Valley overall is not dissimilar from the national situation.  Also, there is no significant impact on the terns productivity after the arrival of the gulls, and furthermore the productivity of terns in the Valley remains higher than the national average.
There is some evidence that the terns actually prefer to nest amongst the gulls, and certainly don’t avoid them, probably gaining some protection from predators.  However, the major impact appears to be that with increasing numbers of gulls, there is simply less space on the rafts for the terns, and this pressure is increasing each year.   With Black-headed Gull nesting earlier, much of the available space may be taken by the time the terns settle. Common Terns appear to be nesting later in recent years with unfledged chicks frequently remaining on the rafts long after the gulls have departed.  The reasons for later nesting are unclear but food supply may be suspected.
As the 2016 breeding season approached, we suggested that new rafts were constructed at all the colonies.  Unfortunately this happened only at Rye Meads, where a new 6 x 6m raft was constructed to create more space for breeding Common Terns. The existing rafts were cleaned and positioned in March for the Black-headed Gulls to colonise.  By contrast, the new raft was positioned very late, in the last few days of May as Common Terns were beginning to settle. 

The results?  Black-headed gulls increased in numbers at all 3 sites to reach a total of at least 466 pairs (from 306 in 2015). Common Terns slumped to their lowest Lee Valley count for many years, with a total of just 57 pairs. However, at Rye Meads, numbers increased to 28 pairs, the highest since 2008, with the new raft holding the majority of pairs and fledging good numbers of young.  Let’s get building some more rafts next year.

Below: rafts at Walthamstow are overgrown and falling apart, with just a few pairs of terns nesting.

Sunday, 24 July 2016

Another silent spring?

As spring passes to summer, we begin to gather data from nature reserves as to how birds have fared during the breeding season and to assess how conservation work for key species is progressing.  On a reserve that has ground nesting birds, such as waders, gulls or terns, an assessment can often be made very quickly and at an early stage, before the final figures are collated.  A productive site has a characteristic sound and feel: the noise of birds interacting with each other or defending their nests or young.  Lapwing or Redshank quickly rise to scold and chase passing crows.  ‘Kleep kleep-ing’ Avocets tell you that they have something left to fight for.  Usually the initial impression gained from a quick eye-ball of the habitat and a listen to the action tells all.

However, the incoming data will not be good on all sites.  There may be few chicks surviving, with overall productivity being low.  In fact, this is a widespread issue.  A study of the breeding success of key species from the Waddensea region of The Netherlands/Germany showed that 18 out of 29 were in decline, with poor breeding success being the important driver of decline. Three species, Avocet, Arctic Tern and Oystercatcher, had such poor breeding success that they were rarely able to reproduce themselves and local populations were in deep trouble.

When trying to determine the cause of low numbers of birds or poor productivity, two or three key issues are likely to hold the answer.  Poor weather in varying forms can cause problems.  In cold, wet conditions chicks will take longer to fledge or die of starvation or cold.  By contrast, dry conditions may also make food hard to find. Mitigating the effects of poor weather is often closely linked to habitat quality; a crucial factor that is well within the control of the land manager.  Wet grasslands in particular are frequently in poor condition; the sward is either too long or overgrazed, or the site often too dry.  Water control structures, if not maintained, can be leaky and in a poor state of repair.  Such problems are usually obvious to the trained eye.   Finally, and often the key issue when everything else looks to be okay, is the problem of predation.  This is the issue that is most often ignored as it asks some difficult questions for the conservationist.

For me, a trip to a range of French nature reserves a few years back was particularly illuminating.  Arriving one evening prior to an arranged reserve visit the following day, we wandered out to discover and enjoy a gloriously large, noisy, smelly, productive colony of Sandwich Terns and Mediterranean Gulls.  Next day, at the nature reserve, just a few Avocets loafed silently on a deserted island.  Apparently, when acquired, the reserve had thriving Avocet and Stilt colonies, but gradually the local foxes targeted the reserve and the birds moved on.  Reserve managers argued about controlling the foxes and finally agreed.  But just two foxes had been killed that year.  And what of the adjacent productive site?  It was owned and controlled by hunters who routinely undertake control of foxes.  An uncomfortable truth is revealed. Top reserves often seem to have plenty of staff asking for your membership or serving you coffee but not so many happy to understand and tackle the difficult issues of predation or habitat management.

Nature reserves surely have an obligation to deliver good breeding success for the key associated species. Yet now isolated in an impoverished countryside, such key reserves are a magnet for predators.  Too many nature reserves are in poor condition and are clearly acting as a ‘sink’ for the species.  Even worse, habitat is created to attract key species but the issue of predation ignored as it is considered too difficult for the organization to handle.

So how do we tackle the problem of high levels of predation on nature reserves yet distance ourselves from the likes of intensive game bird operations?  Killing anything has to be the option of last resort.  First must come habitat design and management, to reduce the impact of predators.  Many of our older traditional reserves desperately require a design re-think and tweaks to improve the situation may be relatively easy to implement. Secondly, non-lethal methods of dissuading predators need to be considered.  The best options here may be predator-exclusion fences (see below).  Ok, they may look a bit ugly and we need to improve design and positioning but their impact can be substantial.  Finally, if we come to consider lethal control (and this is essentially foxes), it needs to be based on a clear understanding of the ecology of both predator and prey.  It also needs to be undertaken effectively and professionally. We do not want to, nor is it possible to, completely remove predators but the aim should be to create a window of opportunity for key threatened species to reproduce where the options for safe nesting are limited.  An ineffective predator control programme achieves nothing.

Is it another silent spring on your local reserve?  Think it through and start the journey to noisy, smelly success.

Below: predator-exclusion fence set within a ditch at Wallasea (ditch yet to be flooded).

A short video on a predator exclusion fence trial at Titchwell - here

Saturday, 16 July 2016

The Albufera of Valencia

A work trip to a conference in Valencia in Spain provided the opportunity to learn about and see the Albufera of Valencia; a shallow lake in excess of 2,000 ha located to the south of the city.   Little more than a metre deep, it is of huge importance for its wildlife, supporting around 90 breeding bird species and up to 50,000 wintering waterfowl.  Around 8,000 years ago, the Albufera was open to the Mediterranean Sea but gradually became cut off by sandbanks.  Originally some 30,000 ha in extent, it slowly reduced in extent through both sedimentation and its margins being progressively claimed for agriculture, principally rice production.

There have been major issues with water quality, notably nutrient enrichment, since the 1980s, mainly as a result of spreading urban development from the city and the surrounding agriculture. The formerly clear waters, full of aquatic waterweeds, have given way to algal dominated turbidity. The population of 1,000 or so pairs of Coot declined to just a few tens of pairs as the vegetation disappeared. As the floating vegetation nesting platforms of Whiskered Terns disappeared, so did the terns.  To try to resolve this problem a partnership of organisations came together to form the LIFE+ Albufera Project, funded by the EU.  Between 2007-2012, three areas of rice fields close to the lake were converted to wetlands designed to enhance water quality.  These 'tancats' were carefully land-formed and then planted with a range of emergent plants such as Phragmites and Typha that will extract the nutrients from the water as it flows through.  Water flows down a series of terraces before being pumped back up into the lake. One of these areas, the Tancat de la Pipa was created in 2008 on the northern side of the lake. Its construction was supported by the Ministry of Agriculture, Food and Environment and is managed by SEO Birdlife and Accio Ecologista Agro.  Early results show that significant improvements have been made.

I was shown around the site by the excellent guys from SEO Birdlife (the Spanish Ornithological Society, notably Pablo Vera, who has undertaken many of the bird studies in recent years.  Despite some declines, the Albufera is still a fantastic place for wildlife.  The bird list is impressive, with many species doing well. For example, latest estimates suggest 60 pairs of Purple Heron, 400 of Squacco Heron, 200 pairs of Glossy Ibis, 50 pairs of Little Bittern, 600 pairs of Gull-billed Tern and 1,000+ pairs of both Common and Sandwich Tern.  Around 40 pairs of Red-crested Pochard, a local priority species, breed annually. There have been re-introductions of both Marbled Teal and Red-knobbed Coot, a nest-building pair of the latter causing great excitement on my visit.

A short video here:  The Albufera of Valencia