Moulting sandwich terns Sterna sandvicensis in the Öresund area, S. Sweden in september

For readers from other continents: The Sandwich Tern Sterna sandvicensis has a patchy distribution in Europe, North and South America. The adults start moulting in the breeding colony or when conducting young in lkate summer.

In July, when the breeding is finished, adult Sandwich Terns - accompanied by birds of the year if the breeding has been successful - will assemble at favourable sites in the South Baltic area (Gothland, Rügen etc.) and stay there for at least two months: from 15 July to 15 September, and in many cases till early October. The species puts a premium on a combination of good feeding-grounds and safe sleeping-grounds; in S. Sweden this combination can be found on Falsterbo Peninsula (the extreme SW corner of Sweden), and 100 - 400 birds will be present here in late summer, their roaming in the south Öresund area often being recorded as "migration". During this period a series (Stresemann & Stresemann 1966) of primary moult starts in adult birds, a fact that can be readily observed in the field with binoculars.

Catching Sandwich Terns in late summer is no easy task, the Nordic night is depressingly bright (for bird-ringers) even at our low latitude, and the adult birds are wary, so there are no moult records from the opening moult phase. (In 1999, Ulf Lundwall and Peter Olsson ringed 66 and controlled 14 Sandwich Terns in SW Scania between 14 August and 11 September; they emphasize that moonlight matters little by this time of the year). The species may be readily caught from early September, however, and in 1997 and 1998 seven moult-cards were obtained from four successful catches at the spits of Skanör ("Skanörs revlar", 55.26 N, 12.51 E). These data are presented in full here, as a starting-point for a deeper look into the moult process of Baltic Sandwich Terns. The progress of growing feathers is given as rate of full length, there are 10 full-size primaries and a confusing full score of secondaries. All birds were investigated in the field with forehead torches as light-source. In chronological order:

  1. Skanör, 970925. P1 - 5 fresh, P6 0.9, from P7 old. S1 - 3 n, S4 0.8, S5, 6 0.7, from S7 old. Pscore: 29.
  2. ibid., 970925. P1 - 5 fresh, P6 0.9, from P7 old. S1 - 5 fresh, from S6 old. Pscore: 29
  3. ibid., 970926. P1 - 6 fresh, P7 0.7, from P8 old. S1 - 6 fresh, S7 - 9 c.0.8, from S10 old. Pscore: 34
  4. ibid., 970926. P1 - 5 fresh, P6 growing, from P7 old. S1 fresh, S2 - 3 0.9, S4 shed, from S5 old. Pscore: c.28
  5. ibid. 970928. P1 - 6 fresh, P7 0.6, from P8 old. S1 - 6 fresh, S7 0.9, from S8 old. Pscore: 33
  6. ibid. 980920. P1 - 5 fresh, P6 0.9, from P7 old. S1 - 3 fresh, S4 0.9, S5 0.8, from S6 old. Pscore: 29.
  7. ibid. 980920. P1 - 3 fresh, P4 0.8, P5 shed, from P6 old. S1 0.4, from S2 old. Pscore: 20.

The "goal" of Sandwich Terns obviously is to exchange six or seven primaries and two to six secondaries in the South Baltic area in late summer, but we do not know if this first phase is followed by a true suspension, or if the birds migrate with new growing remiges. At any rate most terns have left the area by October 15th, so a suspension from P7 or P8 and from S5, 6 or 7 seems probable in the birds involved here. The scores are c.10 points higher than UK scores from mid-August (14 - 24), while two birds from September 11th had scores 23 and 24 (Ginn & Melville 1983); the convergence indicating that all North European Sandwich Terns moult synchronously. So far we have not been able to discern a second (or third) moult series, possibly due to poor light conditions. (In 1999 we were theoretically better prepared on this point, and three simultaneously present moult series were discerned).

Two birds, one 1 c(alendar), one 5c, have carried foreign rings, shedding light upon the area of origin:


The Swedish check-list (SOF 1990) assumes that Sandwich Terns visiting Gothland in late summer derive from across the Baltic (Estonia, Lithuania, Latvia), so there is nothing remarkable about these recoveries. The Sandwich Tern is a pronounced marine species, and another assumption seems reasonable as well: that all breeding birds from the Baltic are forced to migrate NW (by way of Öresund and the Belts) in order to reach the North Sea.

Literature

Ginn, H.B. & Melville D.S. (1983): Moult in Birds. BTO Guide 19. - SOF (1990): Sveriges fåglar. 2:a uppl. Stockholm. - Stresemann, E. & V. (1966): Die Mauser der Vögel. J. Orn. 107: 1 - 448, Sonderheft.

The moult cards were recorded by Peter Olsson and Christer Persson, first version of this note published on the web 28.10.98.

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