126 Mr W. Macgillivray on the Covering of Birds. 



the plumules, unlike the barbules of the feather, come off in ge- 

 neral at right angles to the filaments. 



The uses of the down are not well understood. As it is well 

 known, however, to be a bad conductor of caloric, it i-s presum- 

 ed that it serves in the aquatic birds, and particularly in those 

 of cold climates, ""to retain the heat generated in their bodies. 

 In birds which are not furnished with down, but which yet in- 

 habit cold countries, the deficiency might be supposed to be sup- 

 phed by the downy feathers which we observe in these birds, as 

 in Strix bubo, Strix nyctea, Falco albicilla, and Falco chrysaetos. 

 In the gallinaceous birds, the accessory feather might, in like 

 manner, be imagined to be subservient to this purpose. But 

 when we reflect that the eagles, owls, and gallinaceous birds of 

 cold climates, are at least not much better furnished with down 

 or downy feathers than species of the same genera inhabiting 

 warm climates, we naturally look for some other reason for which 

 birds are furnished with down ; and. when we observe that the 

 Alca impennis of the arctic seas is not more plentifully supplied 

 with plumage than the penguins of the pacific ocean, nor the 

 Sturnus vulgaris of Europe than the Sturnus capensis of Africa, 

 we suspect that other principles than heat have been employed 

 in modifying the nature and quantity of the plumage. 



In the gallinaceous birds, the omnivorous, and many others, 

 in fact, in land birds in general, there is no general layer of 

 down immediately covering the skin. 



In the genus Falco, and many others, and especially in the 

 larger species, F. albicilla, for example, F. chrysaetos, and F. 

 pere<Trinus, besides down of the above description, there exist 

 plumules of the following structure. From the upper part of 

 a short tube, there issue two filamentary shafts, which are flat- 

 tened, and exceedingly delicate. From these there branches 

 out on either side a series of extremely delicate filaments, having 

 each two lateral series of filamentules. The whole has the ap- 

 pearance of a single tuft of extreme fineness, and silky texture. 

 The filaments have a very considerable degree of elasticity. 

 The tube is open above, between the two shafts, there being a 

 direct continuation of it on either side into the shafts ; and at 

 this opening the pith comes out and terminates. These plu- 

 mules being largest on the belly, may be best seen there ; they 



