Friday, 28 June 2013

Lac de Grand-lieu

The Lac de Grand-lieu has been the origin of many of the colour-ringed Great White Egrets seen in the UK; a power-house for herons and another launching pad for UK colonisation.  A day on a boat with the warden Sebastien Reeber brought home what an amazing wetland this is.   The Lac de Grand-lieu has open water (the lake), reed, marsh extensive willow scrub, wet woodland and surrounding wet grassland.   The water is around 0.8-1.5 m deep in summer but rises by 2 m in winter to cover an amazing 6,000 ha. The levels fluctuate annually to maintain a huge dynamic wetland.

The summer extent of the lake is around 2,000 ha and of this 1,000 ha is covered in water lilies.  It took half an hour or so to pass through the lily zone, mobbed all the time by Whiskered Terns.  Sebastien listed the breeding birds: 2,000 pairs of Whiskered Tern, 1,000 pairs of Cattle Egret, 160 pairs of Spoonbill, 300 pairs of Little Egret, 160 pairs of Purple Heron, 250 pairs of Night Herons, 700 pairs of Grey Heron…….  In winter, there are 13,000 Shoveler, 5,000 Pochard...  The Great White Egrets have been established as a breeding bird since 1994 and have increased to a current total of 160 pairs over the last 20 years.

The wet grassland beyond the lake will typically be submerged by 1m or more of water during the winter before the levels drop in April/May.  The breeding birds include Black Tern, Black-necked Grebe and occasional Ruff amongst the more familiar waders.  The ‘yellow’ wagtails on site are an intergrade between flava and iberiae with a corresponding range of head patterns.  An endangered plant now restricted to just a handful of sites in the UK, Starfruit was abundant along the tracks, along with Lesser Water-plantain.

The wetland is however, far from being in a ‘natural’ state.  Invasive non-native species are a major problem, with some of the most obvious being Red Swamp Crayfish, Sacred Ibis, Black Swan, Coypu and Muskrat.  We don’t want any of these in the UK!   And yes, there are plenty of Ruddy Ducks.  The Sacred Ibis are being controlled.  Formerly 800 pairs, they are now reduced to around 300.  The Ibis are a major predator of the eggs of birds on the wet grassland but perhaps more of this later.

Red Swamp Crayfish is now abundant in the wetland – abundant to the tune of 2 tonnes of crayfish per hectare. Perhaps their most obvious effect is the devastating loss of submerged aquatic weeds.  RSC have previously spread throughout the Camargue, where they have been well studied since they were first found in the 1990s.  The RSC have a varied diet (insects, fish, amphibians, plants and seeds) essentially made up (80%) of organic matter and detritus.  Introduction of crayfish into ponds showed that after 10 weeks a density of 3 RSC m2 leads to an 80% decrease in the biomass of macrophytes and a 33% decrease in the diversity of macro-invertebrates.  On the plus side, they now form the major prey item for some wetland birds; 67% of the diet of Bitterns and up to 80% of the diet of Glossy Ibis, Spoonbill and Cattle Egret.  The Red Swamp Crayfish create a direct link in the food web between organic detritus and the top predators, providing the latter with a large invertebrate prey (primary consumer) in high numbers, increasing the overall amount of food available to them.   Even I can’t eat enough crayfish to get us out of this mess.
Above - lilies, Whiskered Tern, and nest.  Below: not-so-sacred Ibis, Starfruit, Flava/iberiae wag.

Saturday, 22 June 2013

Launching pads

Grumpy and his miserable friends go south to France.  Why?  Vin, fromage et saucisson?  Non. To gain a better understanding of the breeding habitat requirements of birds which have the potential to colonise the UK.... and also to look at the functioning of wetlands to the south of us which have a similar climate to that projected to occur in southern England.  We've looked at the essential 'landing pads' in the UK for colonising species, now its time to understand the places they came from: the 'launching pads' on the continent.

First stop, down to the Baie de Somme with our host Benjamin Blondel.  Just one hour south of Calais and what a difference to the UK.  Bluethroats in every bush, reedbeds full of Savi's Warblers, Icterine Warblers in the Alder-dominated mixed woodland and Black-winged Stilts on the muddy pools.  The first pool we looked at had more breeding Black-necked Grebes on it than the entire UK population - 60 pairs. Why don't we dry out pools completely each autumn as they do over there?  While Great White Egrets and Spoonbills are increasing, the small populations of Kentish Plover are struggling against human disturbance on the beaches and also from predation by Beech (!) Martens. But on the protected areas, great to see Kentish, Ringed and LRPs all together with chicks.

Scruffy, scrubby areas hold Melodious Warblers, Nightingales and Turtle Dove, the latter only having declined by about 20% in France.  We need more of this.  Dunes full of Lizard Orchids.  A dusk visit to botanically rich wet marshes with Spotted Crakes, long-legged Agile Frogs leaping and mechanically droning Mole-crickets.  With some useful ideas to bring back home, time to move on south....
Above: Stilt, KP, Lizard O.
Below: Black-veined White, Black-necked Grebes, Melodious Warbler.

Sunday, 16 June 2013

The results show

The end results of habitat management often seem to pass by in a flash in June.  This week offered the opportunity for a bit of speedy site checking to see how target species were faring.  A quick look at North Warren found 40+ spikes of Sand Catchfly, maintained in its favoured bare patches by a bit of raking and over-zealous cyclists.  Who says disturbance is bad?  A whizz by Minsmere produced a distant Savi’s Warbler and, more impressively, a Bittern booming out in the open in front of the Bittern hide, where else?

Fen Orchids are just coming into flower.  The remaining Norfolk plants are found on just four sites, with the majority now under the management care of the RSPB.  Since taking over the management of Sutton Fen, experimental cutting of 80 blocks of fen on a variety of rotations has seen the number of Fen Orchids rise, with an estimated 1,200 plants last year.  Other fen specialists such as the Round-leaved Wintergreen and Crested Buckler Fern seemed to be doing okay.  Shallow turf ponds have also been cut to re-establish early successional conditions and now have the Shining Ramshorn snail in residence.

Cranes are busy at their usual sites, with their usual mix of problems.  One pair had already lost two clutches and have probably given up for the year; others have young chicks in tow but still are a long way to fledging.  The breezy conditions were not ideal for Swallowtails but several were seen briefly at both Sutton and Strumpshaw Fens.  Much better was a ready to emerge pupa pointed out by the warden.

Thursday, 6 June 2013

Landing pad for Little Bitterns

Jig jig jag jag, ping ping, boom, woof....woof....woof”.  Singing Reed Warblers everywhere, Bearded Tits feeding young and I can hear six booming Bitterns around me.  A Great White Egret flies by and a hidden Garganey rattles from a nearby reedy pool.  As it gets dark a Spotted Crake whips from the ditch next to the path.  Where am I?  Another foreign jaunt?  No, in deepest England on a wetland that didn't exist 20 years ago.  Woof....woof....woof, Oh, did I mention the singing Little Bittern.

Where wetland colonists are concerned, a recent study has shown that the six species that have established populations in the UK since 1960 all started to breed in protected areas.  Furthermore we know that larger sites are more likely to attract colonising species.  Such areas have been described as ‘landing pads’ for colonising species, allowing them to become established before viable populations spread out.  Ham Wall (and the wider Avalon Marshes in which it sits) is living proof of this as we hopefully see the first stages of Little Bittern establishing a regular population in the UK. 
Climate change analysis predicts that Little Bittern may colonise southern England. Although populations on the continent declined sharply over recent decades, the cause of which is considered to be drought conditions in the wintering areas, some recovery has been evident in the last ten years.  The Ham Wall birds returned this year as usual in early May and feeding flights will hopefully be evident soon.  The Avalon Marshes ‘heron hot spot’ is rapidly taking over from Suffolk as the key area for (Great) Bitterns in the UK, with 35 boomers this year.  At Ham Wall there are 12 boomers with at least 5 nests pinpointed so far from feeding flights.  

Previously I had looked at the developing reedbed ‘rejuvenation’ area, which as predicted, attracted good numbers of waterfowl in its early flooded phase.  A singing Pied-billed Grebe, along with Ferruginous Duck and Ring-necked Duck, were the highlights.   There were still plenty of duck on show during my visit – Pochard, Tufted, Shoveler, Wigeon and a couple of Garganey - but high water has pushed any waders to the margins.  Just a dozen Black-tailed Godwits, a few Redshank and a Whimbrel were visible.  However, as water levels drop by late summer, the area should increasingly attract a good range of autumn passage waders and waterbirds.

below: rejuvenation area and reed-nesting Grey Herons