MOVEMENTS OF AQUATIC WARBLERS THROUGH BRITAIN Ian Lewis
The Aquatic Warbler (Acrocephalus paludicola) has the dubious distinction of being the most threatened passerine species in Europe (Grimmet 1988). It is a rare but regular autumn visitor to southern Britain, and this paper looks at various aspects of its natural history especially its occurrence in Dorset, with particular reference to birds trapped at the Keysworth Estate near Wareham.
The Aquatic Warbler superficially resembles the Sedge Warbler (A. schoenobaenus). Juveniles in autumn differ from the corresponding age of Sedge Warblers in the more tawny overall colouration, a blackish crown divided by a prominent pale central crown stripe widening into a ginger patch on the fore-crown, the 'tiger striped' mantle, streaked flanks and rump, and pointed rectrices (Walbridge 1991, Lewington et al 1994). Adults in autumn are much paler than adult Sedge, the ground colour is a creamy white and the black of the crown and mantle has often faded to a dull chocolate brown (pers obs.). However in some cases abrasion can remove the pale fringes to the feathers giving an overall dark appearance (Walbridge 1991). There is only one consistent difference in measurements between the two species, this being the difference between the longest and shortest rectrices which is 8-12 mm in Aquatic and 4-8 mm in Sedge (Svensson 1992).
Like Sedge Warblers, but unlike the vast majority of passerines, Aquatic Warblers do not undergo a post juvenile-moult prior to migration (Cramp and Brooks 1992). Adults trapped in autumn are heavily abraded, to the extent that the rectrices are often reduced to mere shafts. This abrasion is considerably more advanced than in adult Sedge Warblers.
Sedge Warblers are known to commence the moult of remiges and rectrices on arrival in Africa, suspend the moult and complete it prior to the northbound migration in the spring (Cramp and Brooks 1992). If Aquatic Warblers undertook the moult of remiges and rectrices immediately after arrival in winter quarters then these feathers would be up to 10 months old when the bird was trapped the following autumn, explaining their abraded state. The specialised breeding habitat of the species could also be a contributory factor.
Population, Distribution and Migration
Historically the breeding range has been given from Belgium and Holland eastwards in a narrow band to western Siberia, with isolated populations in Italy, Hungary and the former Yugoslavia (Harrison 1982), but it was probably only ever a sporadic breeder in western Europe (de By 1990). Today both range and total numbers of the species have declined markedly. The bulk of the population, an estimated 4-7000 pairs, occurs in eastern Poland and western Russia (Collar et al 1994), but recently a substantial population has been discovered in Belarus which might double the known world population (Anon 1995).
As the scientific name attests (paludicola is Greek for marsh lover), the bird breeds in wetlands, specifically stands of wet sedge, a habitat that has suffered extensively from drainage in western Europe, accounting for the serious decline of this species.
Autumn migration commences with the departure of adults from the breeding grounds in July. Plotting the mean arrival dates at various sites indicates a gradual migration westwards across Europe to major stopping-over areas in Holland, Belgium and northern France, prior to moving south to the Iberian peninsula and then Africa. There is a puzzling lack of records from northern, central and eastern Europe indicating that the migration is on a very narrow corridor. The occurrence in Britain is mainly on the south coast. For a species with an east to west migration the lack of records in East Anglia is surprising. Virtually nothing is known about the winter distribution, with a mere handful of records from Mauritania, Senegal and Mali. However the dates of these records may indicate a westward displacement within the Sahel zone during the course of the winter. Similarly the spring migration is poorly documented, but the relative abundance of sightings in Tunisia and Egypt indicate a route into Europe considerably further to the east than that taken in autumn. The fact that there are only two records in spring from Britain testifies to this.
Thus the overall pattern is of a migration route shaped like a number 8, surely unique amongst European passerines.
Occurrence in Britain
The Aquatic Warbler is a scarce but regular autumn migrant to Britain. Whether the species should be considered a regular autumn migrant or a drift migrant is open to discussion.
De By (1990) analysed 395 records of this species over the 20 year period up to 1982. 251 (63.5%) were ringing records and 74.5% of these came from counties along the south coast. Although this undoubtedly shows a real preference for the south coast it must be pointed out that several large reed beds occur in this area and consequently attract intensive ringing efforts. This is clearly the case at Keysworth where there were no records prior to the commencement of a major ringing initiative in 1991, but 63 since then.
In recent years there has been an increase in records, which led in 1982 to the British Birds Rarities Committee removing the species from the list of those it considers. For example in 1991 there were 60 records and in 1992, 42 (Evans 1992). This increase is probably due to an increase in the number of observers, combined with several large scale ringing efforts, in some cases using tape lures.
In many years within the south coast counties there appears to be a bias towards the west, with a number of sites such as Marazion in Cornwall being particularly favoured (Pattenden 1988). This however may in part be an artefact, as the species favours clumps of schoenoplectus sedge which are easily viewed at this reserve. In contrast the cessation of ringing and the closing of the central part of Lodmoor RSPB reserve to the public in 1983 has drastically reduced the number of records emanating from this site. A similar situation occurred at Radipole with the cessation of ringing in the late seventies, and in the past at Portland Bill most records of the species were in standing crops, a habitat that hardly exists there these days. Thus it is clear that it is impossible to draw firm comparisons between counties and individual sites with so many variables.
It is however possible to compare two sites where intensive ringing efforts occur. At Keysworth in Dorset and Icklesham in East Sussex ringing occurred on a daily basis throughout August and September over a number of years and tape lures were used at both sites, (although the length of netting at Keysworth was less than that used at Icklesham). The numbers trapped at Icklesham and Keysworth are shown below:
By considering the ratio of Aquatic Warblers to Sedge Warblers the effect produced by the differences in ringing intensity between the two sites can be removed. The table below shows the number of Aquatic Warblers trapped as a percentage of the total number of Sedge Warblers trapped.
During 1995 there were many records of Aquatic Warbler from a wide area of southern England, an indication that weather conditions may have produced a more easterly bias. This was coupled with a reduction in the length of net used and level of tape luring at Keysworth due to a combination of reduced manpower and equipment availability. In other years the more westerly site attracted the greater numbers.
De By (1990) explained the lack of records from eastern Britain as a reluctance of the species to cross large bodies of water, such as the North Sea. However if the Straits of Dover were the main point of entry into Britain the number of records in Kent and East Sussex should be much higher than in the south-west. It is also hard to see why a nocturnal migrant would follow such a route as it is well established that mainly diurnal migrants use such topographical features as land marks.
Perhaps the more logical explanation is that the species main stopover areas are along the coasts of Belgium and Normandy and Brittany, and that the numbers arriving in Britain each year are a refection of the extent to which southerly and easterly winds have encouraged the birds to cross the English Channel.
The Effects of Weather
The correlation between wind direction and the arrival of the species at Keysworth was demonstrated in 1992, a year when wind direction in August and September was noted on a daily basis.
In the early part of August an area of low pressure remained to the north of Britain producing a westerly or north-westerly airflow. On the 6th high pressure with weak easterlies settled over the south, three birds were trapped the following day. From the 7th to 22nd the wind remained mainly in the east and a further 9 birds were trapped. On the 13th a front lay over southern England with a low to the north and high pressure to the south. Any birds migrating from the continent encountering these conditions would probably make landfall as indicated by the 4 Aquatics trapped on the morning of the 14th. From 23rd to 11th of September the country was under the influence of a strong westerly airflow and no more Aquatic Warblers were encountered. By the 12th a high pressure formed briefly over Holland and the last individual of the year was ringed.
There have been two recoveries of foreign ringed birds in Britain. Two birds of the same brood, ringed in Poland on the 11th June 1990 with consecutive ring numbers, were retrapped within an hour of each other at Chew Valley, Avon and Helston, Cornwall on the 25th August of the same year (Mead and Clark 1991). The occurrence of both of these birds is in accord with the scenario outlined above.
Dates of Occurrence
On an annual basis between one and 26 individuals have been reported from Dorset. In the 25 year period between 1971 and 1994 there were 264 records of which 181 (68.5%) have been trapped for ringing (Boyes, Cade and Green), a disproportionately high percentage compared with other rare species and an almost identical figure to that shown by de By in his national study. Records range over the period from the 26th July to the 1st November; the dates of first occurrence of all birds are shown below.
The concentration of records in the narrow period from mid-August to early September is self-evident. In his analysis of the records throughout western Europe, de By (1990) showed a second but smaller peak about 6 weeks after the first, which is clearly not visible here. This was considered to refer to individuals from a second brood, although as this peak was more evident from the Bodeness area of the Netherlands and Switzerland he postulated that later individuals took a more direct south-westerly migration route than the first brood.
Unusually for a scarce migrant most records refer to one date only. Of 80 birds ringed by SRG none have been re-trapped in subsequent days. During the period in question 5 birds ringed by Portland Bird Observatory or at Radipole have been retrapped, 2 the following day, and singles after 2, 3 and 8 days after ringing. Although it is impossible to prove that a sight-only record over a number of days refers to a single bird, there are several occurrences of what appears to be the same bird remaining in a particular area for several days and being seen by numerous observers. One such bird remained on Lodmoor from 18th September until the 2nd of October 1992. However in general it is clear that most Aquatic Warblers pass on rapidly, possibly to stop-over sites in Iberia.
De By (1990) considered the concentration of British ringing records in August and early September to be an artefact caused by the reduction in ringing effort in reed beds outside this period, citing the lack of a mismatch between sighting and ringing records in the Netherlands as evidence.
Whilst it is true that many ringers avoid reed beds prior to late July to avoid any disturbance to breeding birds, at certain sites ringing is continued well into October (although this is not true at Keysworth) and few Aquatic Warblers are trapped at this time. However this early cessation of ringing must have some effect on the temporal distribution.
An alternative explanation for the bias towards later sight records may result from the fact that Aquatic Warblers are seldom observed in the habitat in which they are trapped. Birds arriving before first light may land in the reed-beds that flank rivers and other areas of water (which may be visible from the air in poor light) and so are easily trapped by mist nets set in this habitat. If Aquatic Warblers preferred habitat is sedge beds rather than reed-beds, after dawn birds may leave reed- beds and seek out sedge and other rank vegetation, explaining both the lack of ringing records after the first couple of hours of the day and the very low retrap rates. In this habitat they are more likely to be discovered by birders. In general, birds (of a wide variety of species) that arrive late in that species migration period tend to remain for longer than those that arrive in the peak of the period in question. This may be due to the fact that they originated in a distant part of the species range and require time to replenish fat reserves. Thus a late arriving individual remaining in sedge etc. stands a greater chance of discovery simply due to its protracted stay, so late individuals may represent a small proportion of the population numerically but a relatively large proportion of the sight records, explaining at least in part the discrepancy noted by de By.
The identification, moult, population, distribution and migration of the Aquatic Warbler is discussed. Autumn migration takes the bulk of the population to the Channel coast of Europe, prior to migrating to Iberia and then West Africa. The number of birds occurring in Britain is dependant on suitable wind conditions as well as population density. The apparent westerly bias to the records is probably real, although the fact that most records from some sites originate from observations and at others from netting (with or without tape lures) makes comparisons difficult to interpret. A possible explanation for the lack of retraps and for the fact that sight records often occur later than ringing records, is that the birds preferred feeding habitat is not the same as the habitat in which the vast majority are mist-netted.
I would like to thank Martin Cade, Lee Evans and Steve Rumsey for providing some of the data.
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