DUCKS 217 



mode of life of its primitive wild ancestor, for, like the latter, it is a true 

 aquatic bird, which spends hours and whole days in the water. There 

 we may meet it in the winter, too, swimming about even among drifting 

 ice, for 



A. It is able to defy the Cold of the Water. 



1. The plumage of the duck on the breast and belly is distinguished by 

 the great quantity of down-feathers, and by the possession of rigid 

 contour-feathers (compare, on the other hand, the owl), which fit closely 

 over the under-garment of down. These contour-feathers are arched, 

 in order to form a more effective covering for the down-feathers. By 

 these arrangements a large number of firmly-closed air-spaces are 

 formed between the feathers, which markedly dimmish the loss of body- 

 heat on the one side, and on the other prevent the access of cold water 

 to the body-surface. The cooling of the latter is further prevented 



2. By careful oiling or preening of the contour-feathers. By the aid of 

 its broad beak the bird squeezes an oily fluid from the ducts of the 

 uropygial gland, which open between the shafts of the tail-feathers, and 

 then, oiling each of the contour-feathers by drawing it through its beak, 

 makes them waterproof. Why has this process to be frequently re- 

 peated ? 



3. The duck possesses a second means of protection against cold in 

 the presence of a layer of fat below the integument, with which we are 

 all familiar in the bird when served on the table (as also in the goose ; 

 compare with seal). 



4. The feet, which are naked, contain so little blood that no con- 

 siderable cooling of the total quantity of the blood results from their 

 immersion. (Imagine, on the other hand, a man wading in winter 

 through ice-cold water.) 



B. The Duck ploughs its Way with Ease along the Surface of the Water 



(Swimming-bird) . 

 i. Why it does not Sink. 



A swimming mammal sinks in the water to such a depth as only to 

 leave its head exposed. The duck, on the other hand, swims on the 

 surface of the water, only the lower portion of its body being immersed ; 

 indeed, it can remain in this position while at rest; this is due to the 

 fact that 



1. The body- weight is much reduced, owing to the presence of air- 

 sacs (see p. 145), the holloivness of the bones and feathers, the air-spaces 

 between the feathers (compare with a cork), and the subcutaneous fat layer. 

 (Compare with seal.) 



