1920.] Study of Nestling Birds. 869 



head. Tlie disappearance of down from this area is no 

 d()ui)r an advantage to a ginttonous feeder lilvc the young 

 Hook, and I think the s>ame remark may also he a|)plie(l to 

 the nestling Jackdaw*. The fact that the Jackdaw's eggs 

 are still coloured, and that the down persists in the nestlings 

 is certainly evidence that the bird has taken to breeding in 

 dark places at a comparatively recent date. This being so, 

 the development of such large gape-flanges is very inter- 

 esting, and illustrates, I think, the importance of these 

 guiding processes to species nesting in dark or semi-obscure 

 situations. The Magpie builds a nest completely screened 

 from above; a procryptie plumage for its helpless young- 

 is therefore unnecessary, and the nestlings are consequently 



'J'ext-fia'ure 12. 



Nightjar (Cf/^jyiwm/f/ws europceus), -i days old; compare with figure of 

 the embryo (p. 870) and note appreciable reduction of the nasai 

 excrescences. 



quite naked when hatched. This explanation does not 

 apply, however, to the Jay, whose young are hatched in an 

 open nest and who are also without any downy covering. 

 Possibly the keen vigilance and boldness of the i)arents, 

 and the fact that the nestlings are almost continuously 

 brooded during the earlier stages, in a measure obviates 

 the necessity for a procryptie neossoptile plumage. But 

 with the Jay, and to a lesser degree with the Magpie, the 

 young are no (loui)t [lartly camouflaged by the curious 

 greenish colour of the skin, the upper parts of the nestling 

 -Jay being of a distinct olive-green hue. 



In 1915 I discovered a four-day-old nestling Nightjar 

 [Capriiiiulgus ei(ropceus) and made notes and sketches of 

 the peculiar cu[)-shaped form of its nostrils (lig. 12). This 



*. Cf. reduction of down on facial area in Shearwaters, Petrels, etc. 

 (p. 874> 



