BIRD ADAPTATIONS 



but we do not see any evidence in any of our curves that there is 

 any effective increased insulation. 



HANNON: This goes on in most animals, does it not? We grow 

 more skin continuously. 



IRVING: If you look at the plumage of birds from tropic or 

 arctic locations, you have a hard time convincing yourself by that 

 examination that one is arctic and one is tropic. Among the jays 

 you might think that there is a little thinner body plumage on the 

 tropical than on the arctic form, but the quantity of feathers does 

 not seem to vary very much with the climate where the specimen 

 originated. Of course feathers are not for insulation alone; they 

 serve an aerodynamic function in which the dimensions of a bird 

 that would be affected by increasing its feathers would quite des- 

 troy its aerodynamic qualities, although a mammal can carry fur 

 ten times as long if he does not trip over it. 



PITELKA; Your question, Dr. Prosser, is aggravated by a 

 circumstance which is not yet well documented in the literature, 

 which is that some tropical species, if they are not breeding, are 

 molting. Dr. Irving a moment ago mentioned tropical jays; I have 

 some data shortly to be published for a 50-gram species of tropical 

 jay. The breeding season is March through June or July and other- 

 wise the population as a whole is molting, starting its molt in late 

 May and continuing into February, so that in effect the birds are 

 either breeding or molting. And when we get situations like this, 

 contrasting with tree sparrows or longspurs which molt in a very 

 short time, then the whole business of budgeting of energy and 

 the advantage of the molt is so delicately adjusted as it is, becomes 

 more interesting and intriguing. 



I would like to comment on a couple of other things about plum- 

 age which are relevant to Dr. West's remarks about insulative 

 problems and also relevant to something Dr. Irving said a moment 

 ago. There is one kind ofdifference between high latitude and tropi- 

 cal birds which, to the best of my knowledge, has gone unnoticed in 

 the literature, and which as you will see, must obviously bear a 

 great deal on the capacities of birds thatdeal with low temperatures 



329 



