66 Gross, The YcUow-billed Tropic-bird. [f^^_ 



brownish tinge. The skin about the base of the upper mandible 

 and the lores remains jet black and is devoid of down or feathers 

 at this age. 



From the 20th to the 35th day marked changes in the plumage 

 occur. At the end of the 25th day the developing wing feathers 

 are so heavily charged with blood that the nestling is unable to 

 support its wings continuously (Plate IX, fig. 16). 



By the end of the 35th day the wing feathers, including the 

 coverts, are well expanded and now form with the scapulars, 

 which were the first feathers to appear, a continuous band. The 

 down gradually frays away as the feathers replace this first pro- 

 tective covering. 



The tail feathers are well advanced during this period, and by 

 the 35th day the two central tail feathers, destined to become the 

 long feathers in the tail of the adult, attain a length of 4 cm. The 

 feathers of the head and rump, although sprouted, do not show 

 through the coat of down; so, when the bird is viewed from the 

 side it displays alternate areas of down and feathers (Plate X, 

 fig. 17). The feathers of the breast become well expanded at the 

 37th day and with the down form a veritable feather mattress, on 

 which the bird rests or crawls about the nest (Plate X, fig. 18). 

 The feathers about the base of the beak have appeared and now 

 completely conceal the dark skin. The black loral spot, so con- 

 spicuous in the adult, is now well differentiated. 



The adult bird spends less time with the young as the latter 

 becomes older, at least this is true of the birds which were under 

 continual observation. After the 20th or 25th day the .young 

 bird on Agar's island was visited by the parent bird only during 

 the morning at feeding time. The adult bird usually appeared 

 about 6 o'clock and after 2 or 3 visits was seen no more until the 

 next day. After being fed the young bird settled down for a pro- 

 found sleep, but at irregular intervals, from some unknown impulse, 

 it would suddenly arise, stretch its wings, preen its feathers, 

 and then settle down again for another nap. In the afternoon the 

 young bird was usually awake and would amuse itself by picking 

 at the stones about the nest, or perhaps would snatch at the flies 

 or gnats which were often abundant in the vicinity of the nest. 

 At other times it would spend much time oiling and preening its 



