I visit this area fairly frequently, and I’ve been looking regularly over the last few days (mainly in search of colour-ringed Curlews), at the top of the latest high tide cycle (with high tides of around 9.0 metres at Sharpness). I generally arrive at the lowest point of the tide (typically three hours or so before high tide) and stay until the tide is high. Past experience, plus observations over the last few days, lead me to think I understand how Curlew use the area, at least at this time of the year.
The main feeding sites (as named on the O.S. map) are: Oldbury Sands; Sheperdine Sands; and Lydney Sands. Certainly at Sheperdine and Lydney Sands, the main feeding area seems to be the sandier areas. Sheperdine Sands (the area off the Oldbury tidal Lagoon) is the first to be covered by the tide; birds feeding there either move upriver to feed further on Lydney Sands, or move to roost at Guscar Rocks. Birds feeding at Lydney Sands go to roost at the tip of Aylburton Warth, once their feeding areas are submerged. Birds from Oldbury Sands, which are the highest and last to be covered, head straight across the river to roost on Wibdon Warth. From a vantage point at Guscar Rocks, it’s possible, if the light is good, to keep an eye on what’s happening at Wibdon and Aylburton. It seems that very few Curlews roost (at this time of year at least) on the east bank – perhaps because the seawall is very close to the water’s edge so that there is little open saltmarsh available; also the Oldbury Power Station inland lagoons are generally dry at this time of year, so Curlew don’t go there at this time of year.
Physical conditions are different at the three preferred roost sites on the west bank. A “warth” (good Gloucestershire wetland word! The “Dumbles” at Slimbridge is the same) is an area of flat saltmarsh which may be flooded by the highest tides; crucially, where the flat top meets the river’s edge, there is a steep earth bank or cliff, whose lower part shelves down to the water. On low high tides, the lower sector of this shelf is not covered by water, so the Curlews can roost just above the water level. On a “high” high tide (say 8.5 metres or more at Sharpness), only the vertical earth wall is uncovered, so the birds must either jump up on to the flat surface of the warth, or go elsewhere. As their names imply, there are warthes at both Aylburton and Wibdon, so birds there have to move when the high tide is over 8.5 metres. At Guscar Rocks, there is no earth cliff, and the shore shelves more gradually, so birds from Aylburton and Wibdon often move, as the tide rises, to Guscar. Birds may also move according to local disturbances, e.g by farm workers, dog walkers, or fishermen.
On Friday, Saturday and Sunday last, the Curlews gathered at all three of the above-mentioned west bank sites on the rising tide, but just before high tide they all moved to Guscar; usually the Aylburton birds arrived first at Guscar, then the birds from Wibdon came upriver; this year because of the heavy rainfall, the hay has not yet been cut at Wibdon, so the grass is long and not really suitable for roosting Curlew; Aylburton on the other hand is grazed, so grass there is shorter. In general the flock sat at Guscar; on the weekend they were often disturbed by dog walkers, in which case they moved back to the flat tops at Wibdon and/or Aylburton. On Monday, with a lower high tide of only 8.3 metres, only a few came to Guscar, with good numbers staying at both Aylburton and Wibdon. In the past two Septembers, we have canon-netted birds on the very highest tides (generally 9.5 metres or more) at Wibdon, because that is practically the only area not inundated by the very highest tides, and birds take refuge there if disturbed from any other sites.
So you have to look at the Curlews in the area as a single population, moving back and forth between roosting sites, according to daily variation in tide and disturbance. It doesn’t matter that much where you go to read rings, as all birds from the area are likely to turn up at any of the three main roosts. I got what I believe was a complete count of 910 birds on Friday; I don’t think this was unusually high, just a case of the birds being in the same place and in a reasonably countable situation, rather than spread all over the place. Even so this is nowhere near the international threshold figure of 8,500, and is short of the GB threshold of 1,400 (though if you add in Slimbridge birds, you get pretty close); the colour-ringing has shown there is some exchange between the Lower (Wibdon/Guscar/Aylburton) and Slimbridge (Upper) Severn basins in Gloucestershire; and the five-year mean for the whole Severn down to Bridgewater Bay and Cardiff, as given in the 2009/10 WeBS report, is 3,218.
Finally, as some other observers have noted, Curlew is nowadays about the only wader that uses the area in significant numbers. It seems to me pretty clear, as shown by the ringing, that the area is regularly used for post-breeding moult by a good number of adults; not yet clear whether all of them stay on to winter, though a fair number obviously do. In winter, when birds disperse more widely dispersed over wet fields along the river, it is more difficult to pin them down and obtain an overall complete count. The days when you would find flocks of several hundred varied Calidrids in winter or on post-breeding passage are long past, as noted in BTO publications about wintering waders moving away from west coast estuaries. In the last few days I have seen a single Whimbrel (the same bird every day probably), two different Dunlin (an adult and a juvenile), a lone Oystercatcher, single Common Sandpipers, and (on one high tide only) a little group of three Turnstones, no doubt passing migrants.