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ON THE SUPPOSED "COLOUR-CHANGE" AND 



THE SPRING MOULT OF THE BLACK-HEADED 



GULL. 



BY 

 P. H. BAHR, M.A., M.B., M.B.O.U., F.Z.S. 



A small investigation which I undertook for the purpose 

 of attempting to settle the vexed question with regard 

 to the supposed vernal colour-change in the Black-headed 

 Gull (Larus ridibundus) is perhaps worth recording. 



In the fourth edition of " Yarrell" (III., p. 603) we find it 

 stated that an individual of this species in the Zoological 

 Gardens of London assumed the black head of the spring 

 plumage in five days, not, it is said, by a process of 

 moulting, but by a complete colour-change. Now, 

 I do not suppose there are any who at the present day 

 believe that the vernal plumage is assumed in this 

 manner, but it is asserted by some, notably by Mr. J. L. 

 Bonhote,* that previous to the spring moult there is a 

 distinct colour-change in the feathers of the head over 

 the area covered later by the brownish-black feathers of 

 that moult. 



In the case of the North American Laughing Gull 

 {Larus atricttla)^ Mr. Beebe proved that the black head 

 was assumed by a moult, the white feathers of the head 

 being gradually displaced by new grey feathers with 

 white tips, which latter were afterwards worn off. 



In January, 1908, I purchased five adult Gulls of this 

 species, out of a large collection of which the majority 

 wore the white cap with the post-auricular stripe of the 

 winter plumage. Amongst them, however, were two 

 individuals with darker heads. I labelled three of them 

 by means of nicks cut into the primaries of their wings, 

 1, 2, and 3, so that they could be identified with certainty 

 whenever examined. The other two adult specimens 

 were kept as controls, and besides these I had a young 

 bird of the year caught wild, which I labelled No. 4, and 



* " Avicultural Magazine." Jan., 1907, p. 103. 

 t " Auk," Vol. XXII., No. 4, Oct., 1906. 



