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 diminish 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 hollowness of the bones and feathers, the air-spaces 

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

 (Compare with seal.) 



