Dee Estuary News December 2003

The Dunlin (Calidris alpina) is one of the most numerous waders on the Dee Estuary during winter with typical numbers between twenty and thirty thousand, only eclipsed numerically by an occasional high peak count of Knot and Oystercatcher. At low tide they can be seen anywhere where there is a mudflat busily scurrying around, only pausing to sleep for a brief period at high tide. They often move around the estuary in small flocks of 50 to 200 birds or so, but sometimes gather in huge flocks ten thousand strong spectacularly flashing light and dark in the sun. They are best seen on the high tide roosts at Hoylake, West Kirby and Point of Ayr, or busily feeding off Leasowe, Caldy or Thurstaston.

Steve Round - Dunlin at West Kirby 9/10/03.

Counts in the last few years have been high with the 41,656 present in November 2000 the highest since February 1976. Figure 1 below shows how numbers have changed over the years from the start of the Birds of Estuary Enquiry in 1969 to the present time with what is now called the Wetland Bird Survey (WeBS). Note that the data quoted is of high tide counts (i.e. birds at high tide roosts) unless specifically stated.

Figure 1 - Dunlin on the Dee Estuary.
Five year moving average of peak counts - 69/70 to 02/03.

Figure 1 shows Dunlin numbers were high during the 1970's, dropped to about half during the 1980's before making a slow recovery until the present time. Up until the last few years the figures follow the national trend, except that the decrease during the 1980's was steeper on the Dee. There was a significant decrease between 97/98 and 00/01nationally, whereas numbers have gone up here.

The reasons for the decrease nationally during the early 1980's may well be linked to a series of mild winters, consequently birds tended to stay further east on the continent, especially the Wadden Sea. Locally the period between the mid 1970's and mid 1980's was one of a high amount of disturbance, particularly on the major roosts of Little Eye and West Kirby. The need to reduce disturbance at West Kirby was the reason for the formation of the Dee Estuary Voluntary Wardens in 1986. Better protection of roost sites through the wardening scheme and the purchase/ management of large tracts of the estuary by the RSPB, in particular the wader roosts at Oakenholt and Point of Ayr, is very likely to have been a major contributory factor in the rise in numbers since 1985. Another big factor is the establishment of the high tide wader roost at Hoylake, more often than not the largest roost on the estuary for both Dunlin and Knot. The rise in the level of the sand banks here over the years means that there is a significant area for the birds to roost on all but the highest spring tides. Mixed flocks of up to 50,000 or more waders - Dunlin, Knot, Bar-tailed Godwit, Sanderling, Grey Plover etc. - seen close to Hoylake promenade at Kings Gap, must be one of the most spectacular birding experiences in the country.

Three races of Dunlin appear on the estuary. The race which spends the winter here is C.a alpina, they breed in northern Scandanavia and northern Russia. Two other races pass through in spring and late summer/autumn, the majority are C.a schinzii, breeding mainly in Iceland and Greenland and spending the winter in West Africa. Much smaller in number are the birds of the race C.a artica which breed in north-eastern Greenland and also winter in West Africa. Birds of other races can occur but in such small numbers that they can be considered as vagrants. The above sketch by Tony Broome indicates some of the size differences among the different races - although it should be remembered that as there is a quite a wide range of size within each race, and also overlap between races, race identification is somewhat problematical!

The dark blue line in figure 2 below shows the current distribution of Dunlin across the year. The peak in November are birds of the C.a alpina race, these are present mainly from November to February. The secondary peak during August is of passage birds on their way to West Africa (C.a schinzii and C.a artica). Small numbers of C.a schinzii and C.a artica are also seen on return passage in April and May.

The distribution in the 90's was little changed from that seen currently, albeit with smaller peaks, but the line for the 1980's demonstrates just how low numbers had dropped during that decade. However, it is the 1970's distribution which is so very different from anything seen since. Although the winter peak is only slightly higher than the past few years, these high numbers were maintained throughout the winter, and even in October, currently a very poor month, large counts were made. But it is the spring and autumn passage which has changed the most. During spring (April/May) we had about four times as many birds and in the return passage (August) at least five times as many. So what has happened to these passage Dunlin? Have the populations of C.a schinzii and C.a artica crashed, or are they for some reason now avoiding the west coast of England on migration - perhaps passing further east, or is there another explanation? I have to confess I don't have the answers to these questions, but from limited data I can at least make what I hope are some interesting observations.

Interestingly Dunlin are not the only passage wader whose numbers have crashed on the Dee Estuary since the 1970's. The average (not peak!) number of Knot passing through in August during the 1970's was 22,102. The average for the past three Augusts (1999 to 2002) comes to a grand total of 18. The average number of Sanderling in May during the 1970's was 6,333 - with similar numbers in July. The average for the past three years in May is 19. The equivalent counts for the 1990's were also very low (unfortunately no counts were made in the 1980's in spring and autumn). Does this indicate some fundamental change in migration patterns, perhaps influenced by change in the weather? This does not appear to be the case - take Sanderling for example, there were 4,370 on the Ribble Estuary and 3,760 on the Alt Estuary in May 00/01, not untypical counts for sites only a few miles away from the Dee. The Ribble also had 50,729 Dunlin passing through in May 00/01, a far higher count than the winter peak that year. So there appears to be nothing wrong with the populations of passage waders, but for some reason they have forsaken the Dee. Possibly excess disturbance at the high tide roosts or lack of suitable food at these crucial times of spring and autumn passage may be to blame. Of course counts at high tide roosts do not tell the whole story, a trip to the estuary at low tide in May can often reveal several flocks of Dunlin 10,000 strong - but they are here one day and gone the next and never seem to stay over high tide. Presumably they end up on the Ribble Estuary.

This article wouldn't be complete without mentioning the nearby Mersey Estuary which in recent years has held the highest numbers of wintering Dunlin in the country. They typically have peak counts of around 50,000 compared with the Dee's 30,000. The pattern of counts over the years has been very similar to the Dee, except that in the 1970's the Dee usually had higher numbers. It could well be that the Mersey has become more attractive to them because of improved water quality and undisturbed roosting sites. Interestingly if the numbers of Dunlin on the Mersey and the Dee which currently overwinter are added together, the total comes to about 80,000, the same as was present in the 1970's. So locally at least, the C.a alpina race of Dunlin seem to be doing very well. Note that like the Dee the Mersey has only low numbers of passage Dunlin.

WeBS data were supplied by the Wetland Bird Survey (WeBS), a joint scheme of the British Trust for Ornithology, The Wildfowl & Wetlands Trust, Royal Society for the Protection of Birds and Joint Nature Conservation Committee (the last on behalf of the Countryside Council for Wales, Department of the Environment Northern Island, English Nature and Scottish Natural Heritage). WeBS data should not be used without prior permission of the WeBS Secretariat.