864 The Slwhj of Nestling Birds. [Ibis. 



Titmice (i.e. Great, INIarsli, and Blue Tit), althou;ih the 

 liunieial and spinal tracts are represented, these are con- 

 sideral)lv al)l)rcviatcd, and the latter is nsnalh^ reduced to 

 a few tufts in the centre of tlie l)ack (fig. 7 *) . 



As I pointed out in 1907 f. it is usual with Passerine 

 birds nesting- in holes, or in dark places, to find that the 

 flanges of the gape are much enlarg(>d. These have a con- 

 spicnous swollen and wax-like appearance, and their purpose 



Text-fi-iires 7, 8, and 9. 





Vh^. 7. — Marsh - Ti" 

 ( P(ii-//s ]Hilii.^tris) 

 about •') dav,s old. 



Fig-. S. — NN'iUow- 

 Wavhler (Pln/Uo- 

 scojjus troch iltia) . 



iu. !).— Yellow Wag- 

 tail {Motacilla f. r(iii), 

 about 2 days old. 



is almost certainly to guide the feeding parent. An interest- 

 ing example; oC the development of this feature maybe found 

 in the Jackdaw (^Corcus monedula), which is tlie only member 

 of the British Corvidae that nests in deep holes. In most 

 other (.'rows — the Rook and Carrion-Crow, tor example — the 

 gape is very much smallef and quite inconspicuous, for tlie 

 flanges become darkly pigmented soon after hatching and 

 long before the swollen process is absoi bed (see fig. 10). 



* Compare tills with the dowu pterylosi.'^ ol the Carrion-Crow, Wau- 

 tails, etc., viz., species that are not usually crowded into a confined 

 space when hatched. 



t "Ton2-ae-;narks in Young Birds.'' Ihis, 1907,p. oTG. This paper, by 

 the way, was in the hands of the editor about seven months before it 

 was published, and it was written some eight montlis before the 

 appearance of Pyeraft's iiist paper en " Yonug Birds," in Drit. I'ds. Mul"-. 

 (vol, i.). 



