SWAMPS AND SHORES 



473 



and a few members of the crane and plover tribes. The waders, char- 

 acterized by elongate legs, are found in the stork tribe, the cranes, and 

 the plovers and their allies. Both swimmers and waders tend to be 

 gregarious, sometimes nesting in vast colonies. This gregariousness may 

 transcend the usual specific limits — a flock of various kinds of sand- 

 pipers may be led by a large yellowlegs or a godwit. 4 



The swimmers have a dense oily under-plumage, which does not 

 become wet. In the best-adapted forms, such as the clucks, these feathers 

 extend over the wings, protecting them also from wetting. Numerous 

 swimmers, derived from various groups, 

 are skilled divers. The divers have in com- 

 mon close and dense feathering and mar- 

 row-filled bones; their greater specific 

 gravity causes them to sink more deeply 

 into the water. The non-diving swimmers 

 float almost on top of the water (Fig. 121) . 

 Some of these divers swim under water 

 with their wings, notably the auks, loons, 

 and penguins. The penguins, indeed, have 

 the wings completely transformed into 

 flippers. The swimmers are more closely 

 associated with open water than the 

 waders, and many of the diving forms 

 can wander far at sea. The fact that none 

 of them have become wholly aquatic, like 

 the whales or sea snakes, is plainly due to 

 the fixity of the egg-laying habit in birds 



as a class. Among birds the extinct toothed and wingless Ichthyornis of 

 the Cretaceous, and the penguins, have advanced as far as is possible 

 in aquatic adaptation, in completely different directions. 



The waders have long legs, the elongation mainly in the tarsal 

 bones, though the tibia may also be involved. This feature enables 

 them to wade into the water without wetting the plumage. Though 

 essentially semi-aquatic, they are less dependent on these habitats than 

 the swimmers, and may leave the water entirely to take advantage of 

 a food supply on land. Storks feed on grasshoppers in the African 

 savanna, and the demoiselle crane in central Asia may take to the 

 desert in pursuit of lizards. 



A number of other birds, of varied origin, have been attracted to 

 the neighborhood of water in various degrees and for a variety of 

 reasons. The kingfishers, incapable of either swimming or wading, have 



Fig. 121. — Upper, a swim- 

 ming bird incapable of diving 

 floating high on the water 

 (phalarope) ; lower, a diving 

 bird, much deeper in the water 

 (grebe). After Brehm's Tier- 

 leben. 



