Ouse Fen comes of age (what a tinca!)

Ouse Fen from the air and at boat level; a stunning Tinca (Tench) and a not-so-stunning stickleback

Amongst the headline figure of 104 booming Bitterns in the UK this year was the smaller milestone of the first successful nesting of Bitterns at Ouse Fen (Needingworth Quarry) in Cambridgeshire. The partnership project between Hanson and the RSPB will eventually become Englands largest reedbed as the habitat is created following gravel extraction.  A day spent reviewing progress was very instructive. The aquatic habitat in the developing reedbed compartments looked superb, with abundant aquatic weeds including the scarce Fan-leaved Water-crowfoot and Hairlike Pondweed.

A search for the now redundant Bittern nest was easy enough, thanks to excellent triangulation by the guys recording the birds activities during the long watches required to prove breeding. The typical small woven platform of reeds was constructed in what would have been about 60-70 cm of water at the time. All the feeding flights were from the nest location across to ‘Phase 1’, the oldest reed compartment. A further couple of hours of electro-fishing provided the reason for this. The relatively young area of habitat around the nest has few fish as yet, just a few sticklebacks, but the more mature compartment supports a respectable fish biomass of around 21kg/hectare, mainly Perch and Roach. This is above the 10kg/ha considered to be the threshold for successful Bittern breeding. So why did she not nest where the fish were? The simple answer is that the dry spring had reduced water levels in the reeds in ‘Phase 1’, thus reducing the security of any nest. However, the story is even more complicated. There were no booming males heard at Ouse Fen yet there was a nest. At nearby Fen Drayton, there were 2 boomers yet no nest. It is likely that the dry spring also affected the reedbeds at Fen Drayton and as a result the female moved across to the wetter Ouse Fen to find a secure nest location. Thus she found a mate, a nesting site and a food source in three different locations.

The electro-fishing also produced a few Tench; surely one of the most stunning of our freshwater fish with those golden scales. Less visually attractive, although interesting in a rather gruesome way, was the high parasite load of the Nine-spined Sticklebacks. The photo above shows one poor specimen with a bulging abdomen containing a Schistocephalus tapeworm (S. pungittii is the usual species in this stickleback). If that wasn’t enough it also has myxosporidium cysts (a protozoan parasite) to contend with.

Family parties of Bearded Tits (one of three) and Marsh Harriers were additional evidence that this new site is coming of age. A single Garganey, a few Little Egrets and several Hobby all helped round off a pleasant day in a boat.

The poly and the plover

Ringed Plover (on Scottish machair), Grass-poly and a view across Moore Lake, Fen Drayton.

The week ended with a leisurely stroll around Fen Drayton lakes, a series of gravel pits adjacent to the River Ouse. This is a good site for wintering waterbirds, notably including Smew and Bittern, and now also supports a couple of ‘booming’ Bitterns and breeding Marsh Harriers. Fen Drayton is one of the very few remaining sites for Grass-poly, now classified as ‘endangered’ in the UK. The usual site held just one plant that I could see, but as I wandered around, many flowering plants were visible in a new location. This is a plant that requires disturbed ground with winter flooding and a spring draw-down. A programme of spring rotovation of plots should see it survive here.

The site also supports a selection of typical gravel pit waders; LRPs, Ringed Plovers and Oystercatcher, as well as Lapwing and Redshank. It was good to see a pair of Ringed Plovers with chicks. After their surge into inland sites in the late 70s and 80s, this seems to be a bird that is withering away as a breeding species in the south-east. The inland birds are now much scarcer and those on the coast struggle against disturbance and sea level rise. The latest national survey reported a 37% decline, with the greatest decreases in inland areas.

The key for both the poly and the plover at Fen Drayton is the retention of some typical early succession gravel pit habitat. Yet natural succession is usually rampant at these sites, and before long, all you have is a tree-lined pool of dullness, unless some drastic management is undertaken. Fen Drayton’s greatest asset is its location in an active floodplain. Floods from the Ouse can put the whole site under water but this makes the habitats more dynamic. Combining this with the actions of large herbivores and some periodic ‘knocking about’ with big machines (to create habitat at the right level in the flood regime) should see the value of the site being maintained.

Apart from the Ringed Plover, it was fairly quiet on the bird front. Lots of moulting Tufted Duck and Pochard, a Hobby or two, Dunlin and Redshank on muddy islands, and a family of Water Rails.