Primary moult in black terns and common terns

Abstract

In July‐September 1984 the primary moult of Black Terns Chlidonias niger and Common Terns Sterna hirundo was studied in the southwestern region of the Ijsselmeer near Amsterdam, The Netherlands. The study was undertaken using the method of collecting and identifying shed primaries. Primaries collected were identified by their lengths. Both species have a similar timing and speed of the postnuptial primary moult. In the second half of August the progress of this moult, expressed in the unit “average primary per collecting date,” slows down gradually to practically nil by mid September. It is shown that this is caused by earlier departure of more advanced moulters out of the area, whereas less advanced moulters continue to contribute to the overall moult pattern. The stages at which the birds arrest the prenuptial primary moult, as reflected by shed primaries collected, are compared with data in the literature and from personal observations on whole wings. The differences found between the two groups point ...