Tuesday, 22 November 2016

Lakenheath Fen - Crane powerhouse

The fact that Lakenheath Fen RSPB reserve was a carrot field some 20 or so years ago is often repeated.  The detail that has gone into the design of the site and the management for key species is less often appreciated.  This is no airy-fairy, lets see how it develops type of reserve, beloved of some sections of the conservation community.  This is a targeted and planned development that now delivers for Bitterns, Cranes, Bearded Tits, Marsh Harriers, Water Voles, Otters and a range of specialised wetland and Breck plants and invertebrates.

This week, I attended the ecological review for the year and undertook the annual audit.  There were a minimum of 5 booming Bitterns, with at least 3 successful nests. We reviewed the results of the fish survey throughout the reserve and discussed the habitat management. The detailed management undertaken for Bitterns that has led to a remarkable recovery in the UK is well known, but perhaps the work for Cranes less so.  Cranes have also had a good year at Lakenheath Fen with three chicks fledged from the 2 nesting pairs.  But this is no chance event, rather the result of considerable effort by site staff.

The case of Cranes is an interesting one.  Recently released figures show a total of 48 pairs in the UK this year:  a combination of 27 pairs of (mainly) wild birds in 4 population centres and 21 pairs of the ongoing re-introduction scheme in Somerset. At least 16 young were reared this year. The population in the Fens is doing well, contributing 7 of these fledged youngsters, whilst the Broads contributed 4 and Somerset just one.

There have been several re-introduction schemes of various species of cranes around the world.  None have been totally successful.  They have suffered from various problems such as imprinting on humans, lack of migratory instinct, poor productivity and the cranes generally failing to think and act like wild cranes, particularly when it comes to predators! The well publicised nesting attempts of some of the released birds where they clearly fail to recognize good nesting habitat or exhibit typical crane behaviour is a good example.  Lets hope these issues can be resolved.

So how has Lakenheath become the UK crane powerhouse? The birds first appeared in 2007.  DNA analysis of feathers suggested a Finnish/Russian origin.  Over 10 years, 23 clutches of eggs have been laid, with 11 chicks fledging: an average of 0.55 young per pair.  The fledging rate was poor at first but is increasing in recent years; the last 5 years average 0.8 and a remarkable 1.5 for the last 2 years.  Detailed study of the Cranes by site staff since they first appeared at Lakenheath, notably by Norman Sills, has led to a good understanding of their requirements.   Habitat management seeks to create ideal conditions for both nesting and chick rearing.  However, it is predation, mainly by Foxes that poses the major threat to the youngsters.  Habitat manipulation can help with this but ultimately some control of Foxes is required.  To ensure success, the latest technology of trail cameras and thermal imagers is used in conjunction with a skilled marksman.

The reedbed management aims to create structural diversity for the benefit of the key species.  Cutting and grazing are the key management tools and also aim to enhance viewing opportunities for visitors.  No airy-fairy stuff this – rather it is determined and targeted conservation effort that brings results.

Photo below - Dartmoor ponies eating reed and typha rhizomes


Wednesday, 2 November 2016

Tipping the Balance - the story of Black-winged Stilts in 2016

Black-winged Stilts 2016 from graham white on Vimeo.

There have been repeated nesting attempts by Black-winged Stilts in the UK in recent years.  While spring influxes of stilts into Britain have largely been driven by dry conditions in south-west Europe, at least part of the most recent surge of records is also likely to have also been due to an increase in the nearest breeding populations, particularly in northern France. It is also clear that some individual stilts are now returning to Britain in successive years. Although breeding attempts by stilts in Britain are no longer just one-off events, their overall success has been poor and colonisation is by no means assured.  In 2016, we decided to try to do something about this.  This blog covers the events of the year, with a fuller account being published in British Birds in November 2016.

Following the failed breeding attempts by stilts in 2015, we decided to be proactive and try to maximise the chances of successful breeding in 2016.  But where should we focus our efforts given that stilts could turn up anywhere? We agreed that Cliffe Pools was the obvious site to work on, given that stilts had nested there in the two previous years.  Our plan was to develop a habitat so good for Black-winged Stilts that we would draw them in from all around.  Little did we realize the impact this would have!

The aim was to encourage stilts to nest in a location that contained not only good chick-rearing habitat but that we could also protect from predation. Foxes and Badgers are a known threat, but we also wanted to avoid a similar situation to 2015 where stilts nested on islands where the chicks were predated by Black-headed Gulls.  So, beginning in autumn 2015, management work started to try and encouraging stilts arriving in spring 2016 onto a key pool.  This included cutting and grazing vegetation to turn it into prime stilt habitat, installing a predator exclusion fence around the margins and manipulating water levels to provide ideal feeding conditions as the likely time for stilt arrival approached.  We sat back and waited.

The first stilt, a female, was recorded in Sussex on 12th April.  The following day, she was on the pool at Cliffe.   A pair of stilts arrived on 17th and a fourth bird the following day.  Two birds left on 19th April but three days later the remaining pair was joined by a further four stilts – a male and three females.  A pair were recorded at Manor Farm in Bucks on 24th but relocated to Cliffe the following day.  Stilts had been recorded at only three other sites in Britain during April yet 8 or 9 birds had passed through or settled at our pool at Cliffe!  Photographs showed that two of the males present, and which subsequently nested, had near-identical head and neck markings to the two males that had nested nearby in 2015.

Following a delay during a period of cold weather, two pairs of stilts began nesting inside the fenced area, with a third pair nesting on a lagoon outside out of it. The first pair began incubating on 1st May. Based on plumage, the male is assumed to be one of the males that nested at Cliffe in 2015.  A wardening scheme was quickly organised as soon as this first pair began nesting, and the frequency of Fox control was increased.  This pair diligently took turns to defend the nest against a wide range of other bird species. As the hatching date approached, the number of Black-headed Gulls feeding in the area of the stilt nest began to increase. The gulls were feeding on chironomid larvae, made more accessible by falling water levels, so we brought in a pump to raise water levels slightly. This successfully reduced numbers of Black-headed Gulls in the vicinity of the stilt nest, in the run up to hatching. 

The first egg hatched on 25th May, and a second two days later. The adults successfully defended the two chicks against potential avian threats before the effects of bad weather intervened. At three days old, the youngest and smallest chick succumbed during a period of torrential rain and cold winds. The first chick continued to grow well, and became increasingly independent, frequently exercising its wings.  However, on 13th June, about ten days from fledging, came another period of torrential rain, this time overnight. Sadly, there was no sign of the chick the following morning.

The second pair began nesting on 3rd May, but failed five days after the start of incubation, when rather bizarrely a male stilt was seen removing and dumping the eggs from the nest. This pair then relocated across the Thames to South Essex, a reserve where stilt habitat had been factored into the design and stilts have turned up every year after the initial wetting-up.  However, we were concerned about the predation risk at this site and we waited to see where they would settle. After much to-ing and fro-ing around the site, the stilts eventually settled to nest on 31st May, on an island on Pitsea scrape. A poor choice!  This scrape is close to a landfill site, and consequently surrounded by high densities of Foxes and large gulls. The reedbeds surrounding the scrape meant it was impossible to install a temporary predator exclusion fence.  A wardening post to watch over the stilts was again immediately arranged, but the nest failed five days later. Frustation!

The third pair started nesting on 14th May but unfortunately just outside the fenced area.  Both birds would chase off any potential avian predators, including even quite distant Marsh Harriers, commuting back and forth over the site, and posing no obvious threat. Consequently, the eggs were frequently left unguarded. The pair lost their eggs on the night of the 17th May.  Although the cause was uncertain, a Fox was seen close to the predated nest the following day.

This pair subsequently reappeared in north Kent, where they were first seen at Higham Marshes on 7th or 8th June and began nesting there on 11th June. The nest was in an area where Fox control was already being carried out, and where other waders had enjoyed a successful breeding season. We again organised wardening of this pair. The incubation period proceeded uneventfully until, on 4th July, just before the eggs were expected to hatch, all four eggs were predated by Carrion Crows.

As discussed above, the establishment of Black-winged Stilt as a regular breeding species in Britain is by no means assured, despite the projected changes in climate.  In particular, there are very few ideal breeding areas in Britain – areas that have appropriate feeding and nesting habitat, and where predation levels are sufficiently low for stilts to fledge young.  The key measures to increase the breeding success of pioneering pairs should involve:
  • creation and management of ideal feeding and nesting habitat, 
  • management of water levels to provide optimum feeding and nest protection, 
  • minimising nest predation, particularly by Foxes, through the provision of predator-exclusion fencing or by lethal control, and 
  • preventing disturbance by humans, and the theft of eggs, by rapidly implementing wardening schemes.

The RSPB reserve teams in North Kent and South Essex, together with our contracted wardens, worked supremely hard to try to deliver stilt success in 2016. Despite what may seem as a failure with the loss of all five nesting attempts, we have all learnt a huge amount about how to manage for this colonizing species and this will no doubt tip the balance fully towards success in future years.  Plans are already afoot for 2017!

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.