Circannual phenology of Dunlin Calidris alpina on Isle of Fehmarn, the German Baltic: an attempt to use field observations
by Klaus Hein and Christer Persson
Klaus Hein, Lessingplatz 8, D-24116
Kiel, Deutschland.
Christer Persson, Ljungsätersvägen 43, S-236 41
Höllviken, Sweden. E-mail cp.hollviken'at'swipnet.se
On Isle of Fehmarn, the German Baltic, the maximum number of Dunlin observed in a decade culminates in the second decade of November. The treatment of the material is described and discussed. It is suggested that birds belonging to the wintering population and birds staging before departing for the Mediterranean are involved. Proximate resting-grounds before the occurrence on Fehmarn should be the South Baltic, the Belt Sea and Kattegat, but possibly the Waddensea as well.
Introduction
Germany has an ornithological tradition of old, and there is a wealth of observations in archives. This communication is an attempt to encircle the all-year-round occurrence of Dunlin Calidris alpina on Isle of Fehmarn (appr. 54.26 N, 11.10 E) in the German part of the south Baltic. Fehmarn has a strategic position in the southbound stream of migrating birds from Denmark and Sweden, the Danish isles Lolland - Falster - Mön following in a sequence to the north, and - even more important in this context - a strategic position in the east-to-west movement of waders along the southern border of the Baltic. Geographically it marks the divide between the Bay of Mecklenburg (with towns Lübeck, Wismar, Rostock) to the east and the Bay of Kiel (with the mouth of the Kiel Canal) to the west. The long-standing ringing station Langenwerder (the island in itself a nature reserve) is situated north of Wismar, from here Brenning 1987, 1989 has presented basics for Dunlin migration in the German part of the Baltic, furthermore Kube et al. 1994 have contributed a survey of Dunlin migration in the whole of the south Baltic area; these three papers and the perspective on Baltic conditions in Goede, Nieboer & Zegers 1990 constitute the background of this paper.
Material
The basic material is a variety of field observations; regular, irregular, occasional, combined from mainly the following five sources:
- Early data obtained between 1920 and 1952, mainly collected in diaries by K.-H. Andresen, C. Lunau, H. Olderoog and G. A. J. Schmidt.
- Data from archives and published papers by Faunistische Arbeitsgemeinschaft Schleswig-Holstein, Hansestadt Hamburg and Lübeck (1948 - 64), OAG (Ornithological Working-Group) Schleswig-Holstein and Hamburg (successor of Faunistische Arbeitsgem. from 1965) and VAG ("Vogelkundliche Arbeitsgruppe") Schleswig-Holstein, the latter founded in a split from OAG in 1968.
- NABU (German Society for Nature Conservancy) weekly counts from the reserves Wallnau (W. coast) and Grüner Brink (N. coast, 3 kms W of the ferry harbour), 1982 - 1997. Wallnau is the major reserve, presently the overall counts are organized by M. Altemüller.
- All midwinter counts, mid-January 1972 - 1997 (J. Dien et al.), mid-February (R. K. Berndt et al.).
- October counts 1970 - 1981 (diaries of K. Hein) and 1982 - 97 (K. Hein, S. Lunk et al.)
In general the coverage presented by the above compilation is excellent, there are no pockets of scant or no observations. After the compilation there followed a "subjective" effort; one of the authors (KH) subdivided the material into decades and estimated the highest number of birds simultaneously present on Fehmarn, excluding all possible double counts. Finally the most likely maximum number was achieved by considering other sites and days within the same decade and, if called for, adding weighted values from them. (The birds move in relation to disturbance, predation, food availability, wind and water levels, sometimes a particular congregation may go unnoticed for a day or two). The waviness of Dunlin migration (e.g. Brenning 1987, Gromadzka 1986) favours such a procedure, even if the turnover within a decade of peak migration will involve a succession of "shifts" of adult Dunlin present. Still, one single day will practically always be outstanding; the information contained in this maximum number was the goal of the whole project. The total material of the selected "best decades" among all years available comprises observations of some 65.000 individuals, the total of the selected sums is 14.948.
The procedure was invented by KH, initially arriving at the resulting annual distribution of maximum presence without knowledge of similar results obtained in other ways by Persson 2003. Following this, both authors (KH and CP) have discussed the result in correspondence; the achievement of our joint paper is the simple statement of the distribution and a discussion of its possible bias and faults.
Fig. 1. Maximum number of Dunlin present per decade, Isle of Fehmarn, Germany (S. Baltic). Material: observations from 1920 to 1997.
Discussion
Wind flats Most important wader grounds in the Baltic are wind flats (Ge.: Windwatten), accessible to waders only with favourable air pressure, currents and wind directions. Fehmarn has 3 - 4 wind flats of importance to the birds at the south side and one to the north (Grüner Brink), in addition a few lagoons with salt water at the NW bend. The Wallnau reserve is centered around an inland pond. The importance of wind flats makes the occurrence of Dunlin somewhat erratic even during peak migration; when wind flats are unavailable migrants may overfly an area within hours, while available flats will attract thousands of birds for at least a day or two. So, absence of observations in a single year doesn't mean, that Dunlin didn't pass the area, it only means that resting conditions were unfavourable. (Or too favourable; Henning Behmann noted square kilometers of wind flats and only a handful of visible Dunlin at Fehmarn in the second half of November 2003, in the Sound area and Denmark there were square miles of wind flats by the same time, this could be called "the curse of too much", and it is as common as "too little".) Furthermore November is the particular "gale-month" in North Europe, with winds blowing from northerly directions if the low has passed sufficiently close or south of Scandinavia, this will inevitably raise water levels in the South Baltic and initiate some movement among Dunlin present. In Denmark and Scania all waders enter pasture, the sheltered inner parts of firths etc. during such conditions; these congregations are very spectacular and easily spotted.
Variations in juvenile share Second, the diagram is based on a mixture of age classes: adult breeders (June - Aug), adults and juveniles together (Aug - mid-October), almost exclusively juveniles (mid-October - mid-November) and again a mixture of age-classes (mid-November onwards, cf. Persson 2003). In most years Rösner 1997 recorded 10 - 30 % 2nd calendar-year Dunlin in spring catches from the Waddensea area; he speculates that there is a 3-year-cycle, similar to that among Brent Geese Branta bernicla (Summers 1986, Summers & Underhill 1987), but the only site showing significant numbers in this respect was Isle Langenwerder. At any rate we could expect at least a variation 1 : 3 between years in the number of juveniles over, say, a 10-year-period, while the actual totals of adults passing the South Baltic area - whether recorded or not - could be expected to show less abrupt fluctuations.
Duration of adult stay Third, adults migrate faster than juveniles, and have less extended rests. On the Falsterbo peninsula surprisingly many birds - the vast majority - arrive with fat-free weights at 2h or 3h and have left the area completely before 8h; their stay does not exceed 6 hours. The average stay of adult Dunlin caught twice calculated by Brenning 1987 - 2,9 days - grossly overestimates the resting time if all adults are considered; on the Falsterbo peninsula the estimated stay of the majority of adults is 4 - 6 hours. When adults feel the proximity of the Waddensea their strategy is to fly on as soon as possible, even in a rather poor condition. Rightly, all days within a decade of major adult migration should therefore be added in order to approximate the number of visitors. On the other hand, when there is headwind or rain, even adults will rest for a day or two, and where populations heading for the Mediterranean (or just France and Iberia) are involved, longer stays with substantial fattening may take place (see controls, Persson 2003). In order not to disturb the general pattern of maximum numbers, we have left totals from the period of adult migration untouched by this consideration, but it should be stressed: the flux, the rate of transmigration is highest as long as juveniles have not entered the migration. On the other hand, some juvenile cohorts move as quickly and as purposefully as adults, there is no way of telling in advance.
Accumulations at other sites Brenning 1987 calculated a mean stay of 11.4 days in juveniles caught twice at Langenwerder. Some juveniles linger behind in the Baltic, and this brings another complication; in spite of juvenile numerical inferiority it may come to slightly "artificial" accumulations at certain sites in autumn. The Bock area (Rügen), the Spits of Skanör (Falsterbo peninsula, Sweden) and maybe a few Danish sites are noted for such accumulations, order of magnitude 1,000 - 10,000 Dunlin (e.g. Müller 1985). Conditions on Fehmarn do not promote growth of flocks to quite the same level. Still, since there will be large flocks present not too far away in some years, Fehmarn may get sudden visitors from such flocks when there is a sudden change of water levels. What we see is not always the "normal" Fehmarn spacing share; the day before the same birds may have foraged at Ulvshale, Mön or Hyllekrog, Lolland or the Bock, Rügen and were forced to abandon these areas by sudden rise of the water levels - or desiccation of the wind flat surface during a high with lasting drought.
With these reservations in mind we want to give prominence to the following features of the diagram:
1. The relatively stable share of numbers from the last decade of November till the end of May; during this half-year-period an average maximum decade number of some 275 Dunlin can be counted, this in a sense approximates the carrying capacity of the island. The peak in mid-January is based on a lot of information over 30 years; the focus on the mid-month, midwinter counts may have been unfair to the surrounding decades.
2. The low Fehmarn numbers in late June and early July are not artefacts, caused e.g. by field workers keeping away from breeding habitats; in particular there have been regular counts at Wallnau. In contrast the Öresund area will see substantial Dunlin migration in late June in at least one year out of ten (and possibly unseen nocturnal migration on a regular basis). The course angle of female migration at this time of year - possibly due west, heading for some northern part of the Waddensea - may disfavour the whole of the German Baltic area. These early migrants have a preference for wind flats and barely visit coastal breeding habitats.
3. At least three peaks (and neither of us excludes: four) are indicated in the autumn material, and the last one represents the relative and absolute maximum of the annual cycle. The closest approximation to these findings can be found in Brenning 1987; the whole thing should serve as an additional reminder to Goede, Nieboer & Zegers 1990 (also see Persson 2003). The notion of a fairly early culmination of juvenile migration has always had some foothold in Sweden as well; in "Fåglar i Skåne 1988" (Anser, suppl. 24) there is a significant passage: One month later saw the culmination of juvenile Dunlin migration with 1,000 resting and 320 migrating at Nabben (Falsterbo; my remark, CP) 3.9. As a matter of fact early September has always belonged to the opening phase of juvenile Dunlin migration in Scania! [CP]
Still, the late Fehmarn culmination is puzzling, considering that the Polish ringing stations end their activities by the end of September (Gromadzka 1986), considering that Martin-Löf 1958 studied presumed centralis transmigrants at Ottenby in late September, and considering that the amounts counted at Ottenby were quite insignificant after October 1st (Edelstam 1972) - at least in the 1940's, which is by now quite some time ago! There is little published evidence of an influx from east or north in October. The sudden occurrence of large migrating flocks, including hundreds of adults in the last phase of moult (with conspicuous belly-patches) before mid-October, on Fehmarn as well as in the Öresund area, must be connected with these directions of origin, however; among newly arrived October adults the most exhausted ones (males) weigh 37 g, light juveniles 40 g, the same as July migrants after a long flight [CP; unpublished data]. In contrast the November birds are heavy (a few weigh twice as much as adult males one month earlier), and their extra weights are caused by substantial fat-loads (Persson 2003). Because of their immobility (high predation risk), the fat birds shouldn't linger, and they don't; the rate of turnover is a matter of a few days. (Still, there will be a succession of birds belonging to the same population, the passage lasting at least a fortnight; 20th October - 5 November in one year, 1 November - 15 November in some other, practically nothing - breeding failure! - in a third and fourth). The obvious reason for the "blank" in our knowledge on this point is the rate of turnover and the fact that the birds arrive in darkness and depart in darkness; counts from migration sites are not likely to reveal the actual level of movement, connected with migration or just: redistribution by this time of year.
Against the background of our reservations (fluctuation in juvenile numbers and wind flat availability, short resting-time of birds with migration fat), and against the background of ostensible phasing out of Dunlin migration to the east and north (Ottenby, Poland) in October, we still believe that the November peak is real, that the counts reflect an increase in the number of birds present, and that this in turn should reflect a maximum in overall numbers. Rise of water levels would not produce 2000 Dunlin at Fehmarn if there were not tens of thousands present in the overall South Baltic area, at least in years of good reproduction, and this massive presence has a cause. There can only be one explanation: the sudden increase is recruited from the Waddensea or the Belt Sea or Kattegat, and the birds arriving are by all likelihood wintering adults arriving from moulting-grounds, but maybe even more important, considering the geographical position of Fehmarn: juveniles resting here before their Continent crossing for the Mediterranean (Persson 2003). (There are two recoveries illustrating such crossings: 1y Bottsand 19.12.76 (Weight: 52 g!) - Salin de Giraud, Bouches-du-Rhone 25.12.76 and 1y Foteviken 15.10.95 - Campos del Puerto, Mallorca 2.12.95. But we want and need direct recoveries further to the east! Italian ringers should be more active in November-December, there would be a reward for that. And how about the Evros delta, this remote corner of the world: does anything happen with Dunlin numbers there by mid-November?)
Suggestions: What could be done in order to try to solve the problem?
KH: Ring Dunlin with flags at the five to eight most important ringing stations: one colour for each station, adults over ring on the right tarsus, juveniles over ring on the left tarsus. It would be an easy task to search 200 Dunlin for flags at each visit to Grüner Brink or other Fehmarn sites.
CP: The state of stations and schemes must be considered: in recent time Gdansk had no funding, and Ottenby is a strange station (things are ordered done, there is little initiative, no real dialogue between field workers and "cultivators" of material, peculiarities in recent papers probably arose out of this lack of ground contact). The goal of any station should be autonomy and initiative, today it is economy and academic conformism in most places, and nothing really valuable ever came from that. But there is a problem, I admit it; Gdansk had driving initiative, but no economy. Hiddensee seems to be on the verge of the same problem: one of the better European centres, short of money. Likewise the Copenhagen centre is under constant economic pressure. This is our hard reality when it comes to Dunlin studies: dwindling resources, stations working like zombies, ill-conceived and ill-prepared boy-scout expeditions to Arctic areas.
I could add: are we served by Ottenby large-scale flaggings of adults, going to Great Britain? No. Indiscriminate ringings in the South Baltic area would result in such dilution, that not a single flag is likely to be sighted at Fehmarn. Bottsand has 25 recovery connections with Ottenby based on 6,736 Dunlin ringed up till and including 2003, and Roos 1984 lists 44 recoveries between Skanör (harbour on the W coast-line, guiding-line N - S) and Ottenby based on 17,211 ringed; at my preferred ringing-site a few kms east of Skanör, where all migrants arrive from the east following guiding-lines E - W, I barely take Ottenby birds and Ottenby even more barely takes mine (the last one in 1992, eleven years ago, one single retrap from 5,418 Dunlin ringed up till and including 2003, and this should be measured against the general recovery rate of other schemes from this site and my own two dozen connections with Gdansk/Hiddensee during the same period). By all likelihood most of my summer birds come by way of Latvia, Lithuania, Poland; somewhere close to the mid-axis of the Baltic there is an intersection zone of two major migration flows, and these flows in turn are not unaffected by each other or by prevalent wind directions. (Cf. the suggestions by Nørrevang 1955, second paragraph from the end of the summary, and the short and untransparent note by Leslie & Lessells 1978). It could be added, that the two flows do not have identical phenological patterns; one very important task in the future would be to map these two, slightly differing time patterns - are differences caused by differences in departure time, or simply by weather events on one Eismeer, one more or less inland route? (And: do Meissner rankings differ much between the routes? I believe they do.)
The situation is worse still with the heavy juveniles staging in November, with some overstatement I could say: they haven't left a single trace, they are never ringed, they are not recovered. Their proximate origin in late October/ early November should be the Waddensea (or Kattegat/the Belt Sea) - but before that? Their longer wings and larger bodies suggest that they routinely fly longer stages in a Knot manner, maybe crossing 4000 km of inland Russia when arriving from their ultimate origin. (They are often referred to as "small Knots"; I had a similar comment from you, Klaus. At least they equal Curlew Sandpipers in all respects. The same "small Knots" may have been spotted already by the Finnish ornithologist Ivan Hortling in the late 1920's: Hortling 1927, 1928). I find this particularly interesting: size in Dunlin must be determined by the migration mode; the particular arrangement of moulting-, wintering- and staging-grounds occupied by a population, and the way of reaching them, the length of stages etc. I have a hunch - and this is important - that the birds we are after shouldn't be measured by normal Dunlin standards; the suggestion: Curlew Sandpiper-large Dunlin come from Curlew Sandpiper grounds and migrate much like Curlew Sandpipers doesn't seem too far-fetched to me. We shouldn't expect them to concur with patterns of e.g. Kola Peninsula Dunlin; their wintering-grounds might be very distant. But all suspicions, hunches, guesses must be proven, and we are deplorably short of hard facts... Surprisingly late culmination, startling fat-loads, long wings, long bills, probably adult buff medians in the first breeding plumage. In addition: adults probably moult completely on breeding-grounds and migrate directly from there to the unknown wintering area, I doubt that many of them - if any - ever see W. Europe again after the first autumn. The "small Knots" are probably the Dunlin referred to as "sakhalina" for fifty years now; a marginal centralis population from Gydan or Taimyr. Juveniles leak into the Waddensea from August, here their flocks are refined by some subtle mechanism, I suggest: based on voice. There are differences in Dunlin voices; schinzii birds do not call like standard alpina, nor do staging November Dunlin. There is no mystery here: an experienced field ornithologist with musical ear can tell Icelandic Tringa totanus robusta from Scandinavian totanus simply from voice, bird populations have "dialects". This is utterly speculative, but we need speculation in this case, to be confirmed or rejected - or we won't take one single step forward. (Let us say: Taimyr - Waddensea - Persian Gulf. It could as well be Gydan - Waddensea - Egypt. Or both).
So, where should we ring, and when? Nobody wants to put on five-thousand flags and have no sightings where he hoped for them... Wouldn't it be more effective to ring on Fehmarn, as long as we do not know the exact routes? (And Schlei, Westerhever in September/October, Sylt, Römö, Fanö; nocturnal netting with some sort of electronic lure, flags only on Dunlin weighing more than, say, 65 or 70 g, or with wing-lengths > 125 mm. Resulting in some sex bias, but we can live with that). The recent ringing in the Belts should be involved - who lies behind there? - and if Nidingen (Swedish west coast) could be talked into some dedicated and purposeful Dunlin work, their position would be ideal. But, as I see it, the odds for a flag ringing project are not overwhelmingly good, as long as we barely know when and where to ring. Maybe the best thing would be to make it a WSG project and discuss it there to start with. (To tell the truth I believe more in a "Hanseatic" project, close cooperation between German, Danish and Swedish workers around the Baltic - Gdansk belongs in this context, too - with the World Wide Web as uniting resource, das ist zukunftsträchtig (to speak with my favourite German philosopher). Today, the ornithological journals of the past represent a side-track, they are not really productive; what I have published on the web is more valuable than the joint production on Dunlins in recent time by Ardea, Ibis and Journal für Ornithologie. You, Klaus, should get yourself a computer and an e-mail address, then I will construct a Little Ringed Plover home-page for you, or better still: you learn on your own. Next, the whole thing could be united in some all-embracing Limikolen-Netz... It takes handy people, computer freaks, but also steady and persevering people, capable of keeping steam up for decades. Volponi's vital Cormorant page is an interesting example; it still hasn't imploded, but maybe lost some steam. The ideal thing would be: new webmaster, new programming and layout ideas every two years, but the same old field work addicts - with new ones gradually added - behind the scene...)
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About "adult buff" coverts
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First published 31.10.03, last additions 23.1.04.