ARCTIC METABOLIC ECONOMY 345 



feathers alone were the reason for the gull's superior insulation. In 

 these two examples we must evidently look for other factors which 

 can modify the escape of heat. 



It seems to be a common opinion that the feathers of arctic birds 

 are much thicker than those of tropical birds, for the effectiveness of 

 a physical insulator varies with its thickness. By inspection of sys- 

 tematically related arctic and tropical birds in the museum, I could 

 not find much difference in the thickness of contour feathers in the 

 prepared skins from related land birds of the warmest and coldest 

 regions or seasons. 



In winter small arctic birds at rest erect their feathers until they 

 appear much enlarged and nearly spherical. It would not be possible 

 for a bird to keep this shape in flight, but I find that the individual 

 contour feathers of birds from the arctic are distinguishable from 

 those of a warm climate in having less rigid terminal barbs with softer 

 barbules containing extended fine processes. William Eowan remarked 

 to me upon this distinction of arctic feathers that a given mass of 

 them would retain more air than those from warm climates. When 

 erected on the bird, the arctic feathers retain air among the soft in- 

 terlocking barbules, whereas the feathers from warm climates readily 

 separate. 



Taking this characteristic of feathers as a criterion for arctic birds, 

 I numbered several contour feathers from birds of twelve species of 

 migratory and resident Fringillidae taken at Anaktuvuk, and arranged 

 them in order of apparent usefulness for insulation as follows : 



1. Pinicola enucleator alascen- Resident in winter 



sis 



2. Acanthis homemanni exili- Resident in winter 



pes 



3. Acanthis flammea flammea Resident of interior Alaska in winter 



4. Plectrophenax nivalis ni- Resident of interior Alaska in winter 



valis 



5. Calcarius lapponicus alas- First arrival date April 29 



censis 



6. Spizella arborea ochracea First arrival date May 8 



7. Junco hyemalis hyemalis First arrival date May 9 



8. Zonotriehia leueophrys gam- First arrival date May 13 



belli 



9. Passerculus sandwichensis First arrival date May 13 



anthinus 



10. Passerella illiaca zaboria First arrival date May 19 



11. Leucosticte tephrocotis teph* First arrival date May 22 



rocotis 



12. Calcarius pictus First arrival date May 27 



I found that the winter resident birds ranked first and that the mi- 

 grants followed about in the sequence of their arrival dates in spring. 



469496—60 24 



