O ne o f the problem s o f b iologists is to iden­ tify individual anim als in the population they are studying. In the case o f the B ew ick's Sw an C ygnus colu m bianu s b ew ick ii the first diagram s o f the variability o f the black and yellow areas o f the bill were published by A cland (1 923). G eroudet (1 9 6 2) observed three Bew ick's Sw ans on L ake G eneva, Sw itzerland, and drew their bill patterns. The follow ing winter he found a pair there, and believed them to be tw o from

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... previous year (G eroudet 1963). Interestingly enough, Sermet (1 9 6 3) drew four B ew ick's wintering 50 km aw ay on Lake N euchâtel, and one could be the third o f G eroudet's birds. Scott (1 9 6 6) conclu ded that 13 o f the Bew ick's Sw ans that flew daily from the Severn estuary, G loucestershire, England, into the grounds o f the W ildfow l Trust in the winter o f 1 9 6 4-1 9 6 5 had bill patterns iden­ tical to 13 o f the 18 he had carefully drawn the previous winter. M oreover the birds were obviou sly familiar with the rather artificial surroundings, several were in the sam e pairs as in 1 9 6 3-1 9 6 4 , and one had been ringed. The Bew ick's Sw an num bers at Slim ­ bridge increased annually with the protection and food offered, until by 1 9 6 9-1 9 7 0 over 5 0 0 Bew ick's Sw ans o f secon d winter or older were recorded during the winter (Evans 1970). A s m uch research has been based on th is in d ivid u al id e n tific a tio n , it seem ed necessary, scientifically, to assess the m ethod and its reliability. Development of the bill pattern up to the third winter W h e n fo u r-m o n th-o ld B e w ic k 's S w a n cygnets arrive at Slim bridge their bills are usually coloured cream next the feathering, then pink, with black appearing around the nail and nostrils. Feathering usually extends partially d o w n th e c u lm en , so th a t no browline is obvious. By February, when the birds leave, pale yellow and dark grey patterns, often ill-defined, have em erged, es­ pecially on the sides. T hese can be recorded on printed form s used for all the sw ans, giving outlines o f the tw o profiles and a front view o f the head. T hese patterns m ay be used the follow ing winter to su ggest the identity o f a se c o n d-w in ter bird w ith rem n a n ts o f juvenile grey feathering on the head and neck) w h ose confident behaviour indicates som e Slim bridge experience. This identity m ay be confirm ed if the parents arrive, and are acknow ledged by the second winter bird, but the evidence is som ew h at tenuous. U n less ringed as a cygnet, therefore, the life record o f an individual B ew ick's Sw an can n ot generally start until its second winter, when it is given a reference nam e, and recorded by bill pattern. Som e 2 ,5 0 0 sw ans have now been so recorded and nam ed. The bill is now virtually the adult black and yellow , although there m ay be pink in a patch in the centre o f the culm en, or in tw o parallel lines extending tow ards the nostrils from the edge o f the black (term ed 'post nareal a r ea '-F igu re 1). T h is pink is replaced by black during the winter. A little juvenile feathering m ay still rem ain in the centre o f the brow line. Figure 1. Post nareal area on Bewick's Swans' bill pattern. T o check on any subsequent changes, 23 sw ans were photographed by Philippa Scott from both sides and head-on in their secon d winter and again in a later one. O nly ringed birds were used so that their identity w as confirm ed independently. E ach photograph measured 8 x 13 cm , and m any were very detailed, being taken w hen the bird w as in the hand.