Colour ringed Curlew report

OPSBirds

 

Colour ringed Curlews on the Lower Severn, May 2012 to April 2013,  by  John Sanders

Introduction

In the Septembers of 2010 and 2011 a total of 95 Curlews Numenius arquata were trapped at the high tide roost on the Severn estuary at Wibdon Warth, Tidenham, and fitted with unique combinations of five colour rings, plus one metal. I have used these colour ringed birds as the basis for a study, in an effort to discover more about the population that winters on the Lower Severn. The area covered extends from Lydney Harbour and Sharpness Docks in the north, down to the first Severn Road Bridge, and this report includes observations made during the period from May 2012 to April 2013 inclusive. In conducting the fieldwork I made about a hundred visits, which generated a total of 1,126 ring sightings.

Colour rings

Each bird was fitted with a yellow over white ring on the left tarsus, which identifies the scheme. In addition a single colour ring was placed on the left tibia (blue, lime or white), and two rings on the right, to identify each individual.

Picture 1. Curlew, white / black / red at Aylburton Warth on 23 October 2012. For ease of identification the colour of the left tibia ring is given first, followed by the two on the right, top over bottom.

 

All birdwatchers will know that Curlews are extremely wary, and difficult to approach when at roost. For this reason I have made myself a lightweight portable screen that I can set near the roost sites, in order to make closer observations and take pictures. But still a large measure of organisation and patience is required to guarantee success.

Picture 2. The screen set at the top of the shore at Aylburton Warth.

 

The furthest roost site is an hour’s walk from the car. The tide starts to run about five hours before high water, and the lowest mud banks are covered four hours before. It is at this time that the Curlews begin to move. Most fly to feed on higher banks, some fly up-river ahead of the tide, but a very small number already come into the roosts. For this reason, ideally, I have to have my screen set, and be in position, at least four hours before high tide. I then have to wait for another hour and a half, before all the mud banks are covered, and the main arrivals into the roosts occur.

Picture 3. Curlews arriving at the Aylburton Warth roost.

 

Roost sites

There are five main roost sites on this part of the estuary, but not all are occupied at every high tide.

Aylburton Warth, ST618994: this site is the one that is used most consistently, although it was largely deserted in late March and early April 2013, when it was noticed that no birds were feeding on the river off Lydney New Grounds. The birds did return in good numbers, however, later in April. At neap tides the birds roost on a small rocky outcrop on a promontory immediately south of abandoned salmon putchers. On higher spring tides they are pushed off the shore and rest on the salting immediately above. For this reason, observations are best made on the lowest neap tides, when it is a simple matter to set a screen under a high bank at the top of the shore. Birds tend to start arriving a little later than at the other roosts, about three hours before high water. Late afternoon and early evening tides are best avoided, since one is looking south into the sun. Disturbance is rare, with just the odd visit from farm workers, river authority staff, wildfowlers and birdwatchers. When the birds are disturbed they tend to fly a short distance to the south-west, and rest on the salting on a promontory at ST611991.

Guscar Rocks, ST595982: here there is a large expanse of seaweed covered rocks, with a vegetated mud bank on the landward side. Birds start to arrive four hours before high water, and it is usual for them to settle initially in the seaweed, where rings are difficult to read, before they move under the mud bank, where they are out of sight. For this reason it is best viewed on high tides of 10.5 metres and above (measured at Narlwood Rocks, mid-river off Oldbury Power Station), when the birds are pushed inshore on to the top of the bank. Setting a screen is more of a problem, since there is only a low ridge at the top of the beach, but there is always a lot of driftwood lying around, which can be arranged to provide extra cover. On the highest spring tides the birds rest in a field immediately above the shore, and when disturbed either fly north to a site below Plusterwine at ST602986, or south to Woolaston Grange at ST590980. Human disturbance is a little more of a problem here, and in addition to that mentioned for Aylburton Warth, dog walkers are occasionally seen. 

Wibdon Warth, ST571962: this is a stretch of salting, over a kilometre long, with a muddy shore below. The Curlews sometimes start to congregate on the beach about five hours before high water, and on the higher tides fly up on to the salting. Observations are difficult, but are best made on lower tides, when the birds remain on the shore. The main problem is that it is not possible to predict exactly where the birds will settle, they change their roost position from time to time. In addition, although there is a high bank above the beach, the mud below is soft and sticky, so setting the screen can be a very messy business. But when things go well this roost can be the most rewarding. As it is near the ringing site, the percentage of colour ringed birds is highest here. Disturbance is minimal, with just the odd wildfowler during the shooting season, farmworker and fisherman at Pillhouse Rocks to the south.

Pillhead Gout, Oldbury-on-Severn, ST597923: this is the main roost site on the east side of the estuary, and lies just south of Thornbury Sailing Club. It is used less frequently than the three sites already mentioned, presumably because there is more human disturbance. The occasional jogger appears in the early mornings, and there are many dog walkers, plus ramblers walking the Severn Way footpath, later in the day. The site consists of an earth sea wall, a narrow band of salting, and wide muddy shore. The site is exposed, there is no convenient bank under which to set a screen, so it is difficult to remain concealed, and make observations. Lower tides are best, which do not push the birds up on to the salting. When they are flushed they either fly downriver to Littleton Warth, ST584910, or across to Wibdon Warth.

Lagoon 3, Oldbury Power Station, ST604941: this is a settling lagoon for water pumped from the nuclear power station. Maintenance staff are often present during working hours, so this disturbance means that the roost is occupied most frequently at night at the highest spring tides. Despite the fact that there is a hide, observations can be difficult. They have to be made in the very early mornings, when the light is poor. Added to which, the birds are often at long range, and standing in vegetation. As there is no tide to move them they are invariably motionless, and standing on one leg, which makes the simultaneous sighting of both tibia and identification of each individual impossible. So, for a variety of reasons, this site tends to be the least productive. In former winters a few Curlews used to roost at lagoon 1, ST608952, but only one recent sighting has been made here. A very small number of birds feed on the shore at Shepperdine, ST608956, and at neap high tides remain to roost at the top of the beach.

Numbers

 

It has been impossible for me, working alone, to make simultaneous counts at the five roost sites, so I have been unable to produce accurate tallies for all the birds wintering in the area. The largest numbers that I have seen have been 'scoped from Guscar Rocks. Being about the mid-point in this part of the estuary it is possible to scan a large area. I estimated 800 from here in the third week of July, and 900 in mid-August. My highest counts were about 1,100 during cold weather on 20 January 2013 (when some duplication may have occurred) and 1,000 at low tide on 2 March 2013. The last count may well have included passage migrants, since a small number of winter visitors were known to have already left by this time. My best guess is that there are between 900 and 1,000 Curlews regularly wintering on the Lower Severn, which means that about ten per cent, or slightly less, should be colour ringed.

Early summer

This report begins in May, when all the breeding birds have left the estuary, and only a few remain. These are likely to be first-summers, since it is generally agreed that Curlews do not start nesting until their second-summers (third calendar years). I believe that it is important to make counts of these remaining birds, since they give some indication of the numbers recruiting into the wintering flock, bearing in mind that we have no way of knowing whether all the first-summers remain on the estuary. At least 30 birds were present during May 2012. This appears to be a low total, taking into account the fact that wintering birds number in excess of 900, and it would suggest that Curlews have to live to an average age of 30 years, in order to maintain a stable population. Counts will have to be made in future years in order to discover whether 2011 was a particularly unsuccessful breeding season.

Late summer return

The first adult colour ringed birds reappeared at the end of June, earlier than I had anticipated, and before I had started to make planned observations. I made two sightings in the last week of June, and then there was a mass return during the first half of July, when a further 59 birds were seen. Eleven more arrived at the end of July, and fourteen in the first half of August, bringing the total of ringed individuals to 86. After this no more new birds were seen up until the end of April 2013.

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week

25

26

27

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29

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June     July                                August          

Table 1. In the table above I have plotted the number of new ringed arrivals against the week in which they were first seen. Observations are always subject to the vagaries of weather and tide, as well as the behaviour of the birds, but it is evident that I did not start early enough, and it is likely that more arrived in late June than are charted here.

It was clear that these returning birds were in active primary moult, so it would appear that there is pressure on them to come back quickly to the relative safety of the estuary. Obviously it would be a disadvantage to continental breeding birds to undertake migrations when the efficiency of their flight feathers is impaired.

Picture 4. A Curlew in the final stage of wing moult on 11 September 2012. The outer primary, p10, is old, while all the other primaries are new.

 

It is easy to assume that the birds that breed closest to the estuary return first, while those from higher latitudes on the Continent come later, but so far I have no evidence to support this. Eight ringed birds have been identified on their breeding grounds, and the first of these to be seen was from The Netherlands on 27 June 2012 (see Picture 5). This was closely followed by two more from that country on 4 and 6 July. These birds breed on a similar latitude to Gloucestershire, at 51º and 52º N. Another bird, colour ringed and breeding in Germany, on a corresponding latitude, returned on 4 July, while two from Finland also returned during the first week of July.

Picture 5. Curlew, lime / orange / green, a Dutch breeding male that was first seen back at Aylburton Warth on 27 June 2012. Coincidentally, it was one of the first birds to leave the estuary, and was last seen on 30 January 2013.

 

Of those known to be nesting in England, one from near Banbury appeared on 4 July and another from North Yorkshire on 12th. The closest known breeding bird, from Coombe Hill, just north of Gloucester, also was seen on 12th. Clearly there is still a lot to be learned about what determines the timing of the autumn migration.

There is no easy way of identifying first-winters in the field, so when they appear remains something of a mystery. Only one was colour ringed out of a total of 95 birds. As the ringing dates were 26 September 2010 and 27 September 2011 this may well indicate that the main arrival is later than September, and that juvenile birds do not accompany their parents to winter quarters.

Roosts

During July a majority of the Curlews roosted at the three sites on the west bank, but from mid-August onwards there was a dispersal, when they started using the roosts on the east side. From October to December many flew into fields to feed over high tide, sometimes at a considerable distance from the river, which made ring reading difficult. At Guscar Rocks some frequented a pool just behind the sea wall, while at Wibdon Warth very few were seen on the shore during this period. The habit of feeding in fields stopped in January, apart from the times of very high spring tides, when the whole estuary was flooded and the birds were forced off the shore. It follows that the optimum times for ring reading are from late June to September, and again from February to April.

Feeding sites

Three main feeding sites have been identified on the Lower Severn, Oldbury Sands, Sheperdine Sands and Lydney Sand. At spring tides, not only does the water come in to a greater height, it goes out further at low water, and exposes a wider expanse of mud. The birds prefer to feed at these lower levels, and then move on to the higher banks as the tide comes in. A few fly upstream, ahead of the approaching water, so it is easy to assume that birds on this stretch of the estuary move freely between the mud banks, according to the availability of food. My colour ringing studies show that Curlews are extremely site faithful, it is the same individuals that return winter after winter to the Lower Severn. However, sightings at the five main roost sites further suggest that the site fidelity may be even more precise. It is as if there is an invisible dividing line extending from Guscar Rocks to Shepperdine, with some birds being seen far more frequently to the north, while others occur more often to the south, with an intermingling at the Guscar Rocks roost. The highest mud bank at the upper end of this section of the estuary is the northernmost part of Lydney Sand. The birds congregate here as the tide rises, and then most fly to roost at Aylburton Warth, with a few carrying on down to Guscar Rocks. At the lower end the highest point is on Oldbury Sands, and after gathering here the birds fly to Oldbury-on-Severn, Wibdon Warth and Guscar Rocks, with much interchange between these three sites. There are about 25 ringed birds that habitually roost at Aylburton, that are rarely seen at Wibdon and Oldbury, and conversely, 28 that frequent the Wibdon and Oldbury roosts which seldom, if ever, visit Aylburton. It is obvious that all colour ringed birds had to be at Wibdon Warth in order to be fitted with their rings. A little more analysis reveals that only four birds trapped out of a total of 66 on 26 September 2010 were what I would describe as ‘Aylburton birds’, yet at the second catch on 27 September 2011 the proportion was 21 out of 29. The reason for this disparity is that birds were deliberately disturbed from the roosts at Aylburton Warth and Guscar Rocks before the second catch, in order to drive them down to the Wibdon Warth roost, while at the first catch there was no disturbance, and 500 birds were said to have remained at Guscar Rocks. It is clear that in 2011 a majority of the trapped birds were those that had been forced downriver. There are important implications in a conservation context, in that it is possible that the loss of a single mud bank may mean the demise of the Curlews that perhaps habitually feed there. The idea that individual birds have favoured feeding sites is supported by observations made on the shore south of Shepperdine, one of the few places where it is possible to get relatively close views. About fifteen birds regularly frequent this area at low tide, and with them are often two ringed birds, for which many repeat sightings have been made. Similarly, another ringed individual is regularly seen feeding alone on the shore in the Pillhead Gout area. It is interesting to note that birds moved around the estuary more freely in April, when obviously there were lower numbers, and presumably less competition for food. There was a period early in the month when no birds fed on Lydney Sand, and none appeared in the roost at Aylburton Warth. What I regard as ‘Aylburton’ birds then arrived in the roost at Wibdon Warth. Yet on 20 April the situation was reversed. 290 birds fed at low tide on Lydney Sand, and later appeared in the Aylburton Warth roost, including several ‘Wibdon’ birds.

Spring departure

From January to April 2013 a total of 74 ringed birds were identified, twelve less than in the previous autumn. It is not known whether the missing individuals moved on to spend the early winter elsewhere, or whether a few mistakes were made in their identifications in the autumn. It is all too easy to make errors, particularly when the light is poor, and dark colours, blue, green and black are especially difficult to separate. It is known that a very small number, perhaps up to five, feed on the estuary between Purton and Fretherne, and roost on the Dumbles at the Wildfowl and Wetlands Trust, Slimbridge. Two were identified in this area in September (M.Smart, pers.comm.).

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January          February        March                  April       

Table 2. In the table above the first four months of the year are divided into weeks, and the numbers of last sightings for each week are plotted. Four birds were last seen in January. One of these was at Guscar Rocks on 30th, and back on its breeding ground in The Netherlands about two weeks later. Not all the Dutch birds left together, however, a second bird was last seen on 28 February, and a third on 7 April. A known German breeding bird was last seen on 14 March, and two from Finland on 7 April. Others from Banbury and North Yorkshire left after 5 and 6 March.

Picture 6. A German ringed Curlew, black / yellow / metal / yellow, last seen at Aylburton Warth on 14 March 2013.

 

Perhaps the most unexpected finding was that two thirds of the ringed birds, 50 out of 74, remained into April, the last being seen on 24th. It is possible that this indicates that a large proportion of the wintering population breeds at high latitudes in Scandinavia, since late April and early May is a time when Whimbrels and Bar-tailed Godwits are on passage up the estuary in relatively large numbers. Regrettably we do not have sufficient observations of ringed birds from these northern parts to substantiate this.

Conclusions

Despite the difficulties of close observation, much useful information has been gathered. For me the biggest revelation has been how late the majority of Curlews remain in spring, and how soon they return in the summer. For birds that breed in higher latitudes there seems hardly enough time for them to complete their breeding season, but clearly they have to be quickly back on the estuary to complete moult. It is obvious that juveniles do not accompany their parents to winter quarters, so it would be interesting to know how these are selected. Another unexpected finding was how faithful individual birds are to their feeding and roost sites. I have formed the impression that they are not adaptable, in the way that gulls are, for instance. This leads me to believe that any change in the environment, such as that brought about by the construction of a barrage, would adversely affect the numbers of Curlews wintering on the Lower Severn.

Acknowledgements

 

I gratefully acknowledge the help given by birdwatchers, who have sent me records of their colour ring sightings and counts of Curlews. During the year Peter Hazelwood sent many detailed counts and 22 records of ringed birds from the stretch of the estuary around the Oldbury Power Station. Mike Smart made five visits to the roost at Guscar Rocks, plus one to the Slimbridge area, and reported 88 ring sightings, while Andy Jayne recorded seven ringed birds, and Lewis Thomson one, on single visits.

 

Postscript

 

As I write on 22 June 2013 there are already Curlews returning to the Lower Severn, including six colour ringed. One of these is the regular bird from Germany, plus another new arrival from that country. Of the four Severn ringed individuals, one is known to frequent the shore between the Thornbury Sailing Club and Littleton Warth.  

All Curlew records received from the Lower Severn are appreciated, and Peter Hazelwood kindly collates those from the Oldbury Power Station area. Colour ring sightings are particularly important, and life histories of these birds can be supplied to interested observers.


Copyright John Sanders.