154 THE BIEDS OF HELIGOLAND 



the feathers, finally comes to be spread over their whole surface. 

 The whole change of colouring at the particular part of the 

 body likewise proceeds in an upward direction, so that transi- 

 tional stages of the chansre are met with during the whole course 

 of its progress. 



In the Guillemots and Auks the change of colour proceeds in a 

 different manner ; here the shafts of the feathers of the head and 

 neck, which are white in the winter, but blackish brown in the 

 summer plumage, are the first portions of the feathers in which 

 the black colour makes its appearance ; almost simultaneously the 

 blackish brown colour appears at the lower third of the feathers in 

 the form of very fine specks, which, coalescing, soon give rise to 

 crescentic markings ; these latter, advancing from this point in an 

 upward direction, finally cover the whole surface of the feathers. 

 In this case, however, the whole change of colour does not proceed 

 from below upwards in the regular manner, as described in the case 

 of the "Wagtails and the Little Gull, but both begins and terminates 

 in separate scattered feathers of the plumage, so that at its com- 

 mencement the parts affected appear light, with dark spots, and 

 towards its termination dark, with light spots. 



We ought, moreover, to remark that, in the case of the Little 

 Gull, the change of colour of the bluish grey feathers of the crown 

 of the head proceeds in the same manner as in the portion of the 

 heads and necks of the Guillemots and Auks just described ; here, 

 too, the shafts are the first to acquire the black colour, which after- 

 wards spreads itself over the web of the feathers. Hence we are 

 presented with the singular phenomenon of a change to black, pro- 

 ceedmg in one and the same individual, in the upper portions of 

 the head, in quite a different manner from what it does in the lower 

 portions of the same part of the body. 



The black markings of the heads and necks in the summer 

 plumage of old birds of the Plover species, such as the Grey 

 Plover, Golden Plover, and Asiatic Golden Plover {Charadrius 

 fnlviis), are likewise attained by an alteration of colour, whereas the 

 black breasts of these birds consist of newly-moulted feathers. A 

 peculiar fact in regard to the Golden Plover is that in stuffed examples 

 of this species which have been exposed to the light for a considerable 

 time, the black colouring, which has been formed by mere change of 

 colour, fades to a pale brownish grey, whereas the moulted feathers 

 in the same specimen retain all their former glossy black colour. 



We have seen that in the case of the Little Gull the change of 

 the colour of the plumage may proceed in a different manner in 

 different portions of the body ; a still more surprising result, how- 

 ever, is presented by the birds of the genus Tringa, in many of 



