Thursday 29 August 2019 – Breeding Water Rail?

An indifferent morning’s ringing, not entirely unexpected with fresh NW winds the night before, saw 33 birds of 17 species trapped with 30 new, 2 re-traps and 1 UK control Reed Warbler. New birds: 2 Blackbird, 7 Blackcap, 1 Blue Tit, 1 Bullfinch, 3 Chiffchaff, 1 Dunnock, 1 Great spotted Woodpecker, 1 Pied Flycatcher, 1 Reed Warbler, 1 Robin, 2 Sedge Warbler, 1 Song Thrush, 1 Tree Pipit, 1 Whitethroat, 4 Willow Warbler and 2 Wren.

Things tailed off quickly towards noon and I was taking the net down at the southern end of Marsh Ride when a juvenile water rail scuttled across in front of me. Fully grown, but with a brown bill, white throat and buffish belly it was the first time I have seen this plumage. I am assuming that this is indicative of breeding on the reserve for the second year in a row.

The area around the southern end of Marsh Ride is the wettest part of the reserve in the summer, which makes me wonder what else would breed if the reedbed were wetter. Those of you who know me will be well aware that I have a bee in my bonnet about the way historical drainage works at SML have adversely impacted on the reserve by lowering the water table, introducing gradients from north to south and from east to west, which have accelerated the natural succession from reedbed to wet woodland. In 1991, an independent environmental survey at the Ley concluded:

In 1945 South Milton Stream took a more central course down the upper section of the valley to disappear into the reedbed less than one third of the way down. By 1989 the drainage pattern had been altered with the new course of the main channel taking a more northerly route and extending another 500 metres down the length of the ley before the flow disperses into the reeds. The construction of this ditch has contributed to the drying out of the adjacent marsh . . .

. . . The health of the reedbed is dependent on a high water table, particularly in the growing season from April to August, the very season when water availability may be reduced. It should be remembered that any measures to speed up the water flow through the the Ley by ditching,  the deepening and widening of ditches, or their clearance will tend to depress the level of the adjacent water table. The north ditch has been cleared in the past to enhance the removal of effluent and to separate the flow from the main body of the reedbed. In the absence of excessive nutrient loading, a better strategy might be to divert the stream water into the reedbed at a point higher up the valley and allow the natural reedbed to treat any excess nutrients in the water. Mass flow of water through the reedbed is preferable to unimpeded water movement through a ditch system.”

Despite this professional advice, in 1994, just three years later, major ditching work took place and South Milton Stream, which used to be about 0.5m deep, meandering along the northern edge of the reedbed, was straightened, widened and deepened to about 2m.

South Milton Stream – From this in 1991
To this in 1994

At the same time in 1994 works were completed on “improvements” to the public footpath across the reedbed, which was frequently impassable during the winter months. In essence, a 1m high, earth dam was constructed across the width of the reedbed using spoil excavated from the ditch. It was recognised at the time that this would impede the natural flow of water down the Ley and lateral drains were installed. Unfortunately, in the absence of a hydrologist, no allowance was made for impact of the ditching works on the water table and these drains are high and dry apart from periods when the marsh is already at its wettest.

The public footpath – From this in 1991
To this in 1994

Although suspected by many, adverse impacts have been hard to prove but measurements of hydrostatic pressure made in 1996 show the wettest area at the bottom end of South Milton Stream, where the ditch discharges into the reedbed, instead of the expected gradients down and across the Ley. The wetter area at the end of transect 2 is caused by underground seepage and is now the major source of freshwater into the central reedbed. It is also the area where Water Rails have bred and last year’s Spotted Crake was heard.

Average piezometer readings of hydrostatic pressure in 1996 (blue = wetter areas)

The impact of all these alterations on the reeds themselves has also been measured and the results from a series of transects across the reserve to measure reed growth clearly illustrate an adverse effect on the biomass of reeds with a north-south gradient across transect 1 and significantly reduced growth across the whole width of transect 2 caused by the deepening of South Milton Stream and construction of the footpath respectively.

Location of transects used to determine biomass of Common Reed (Phragmites australis) in 1996

Biomass of Common Reed (Phragmites australis) g-1m-2 across three transects at South Milton Ley in 1996

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