LOCOMOTION OF BIRDS. 



113 



■vancement of the right and left leg ; the body is supported and 

 pushed forward by one as the other leg is raised and swuno- 

 forward to the step in advance ; the centre of gravity oscillatino- 

 laterally, in a degree corresponding with the breadth of the pelvis 

 and shortness of the legs, and which is such as to cause the 'waddle' 

 of the Duck. The forces acting on the centre of gravity are 

 preserved in equilibrio, during the w^alk, by movements of the 

 neck and head, conspicuous in poultry ; and, in Kails and Coots, 

 also by movements of the tail. Most Cantor es advance both legs 

 at once, and progress by leaps or hops, the joints being first 

 flexed and the body propelled by their sudden extension. In 

 Volitores the legs merely support or suspend the body, and loco- 

 motion is wholly performed by the wings. Some birds derive 

 assistance in rapid terrestrial progression by the flapping of the 

 wings, and this is especially the case with the Ostrich, which 

 runs by the alternate advancement of its legs. 



The act of climbing is performed by means of a peculiar dis- 

 position of the toes, the fourth usually being bent back like the 

 first ; but sometimes, as in Trogons, the first and second toe are 

 opj)osed to the third and fourth. The grasp of such ' scansorial ' 

 foot may be aided by prehension mth the beak, as in the Mac- 

 caws and Parrots ; or by the prop formed by the stifl* tail-feathers, 

 as in the Woodpeckers. 



Birds float by the specific levity of their body, arising from 

 the extension of the air-cells and the lightness of the plumage ; 

 but to swim requires an expanse of sole, either by marginal 

 membranes of the toes (Water-rails, Coots), or by the extension 

 of webs between and uniting the toes. In such true swimmers 

 the under side of the trunk is boat-shaped, the down is thick and 

 covered by closely imbricated well-oiled feathers, the bulk of the 

 bird being enlarged and its specific gravity diminished by the 

 air intercepted in the plumage. Much of the body is thus sus- 

 tained above the w^ater by hydrostatic pressure, and muscular 

 action is needed solely for the horizontal movements. The broad 

 oars, acting at the end of a long lever, strike the w^ater backward 

 wdth great force, the webs being fully expanded ; but they 

 collapse, the toes coming together, in the forward movement, and 

 in some of the best swimmers {Colymbus e.g.) the metatarsal is 

 compressed to further diminish the resistance in preparing for the 

 next effective stroke. The oar-like action of the hinder legs is 

 still further favoured by their backward position in Nntatores ; 

 and by the metatarse and toes being placed almost on the same 

 perpendicular line with the tibia, an arrangement, however^ which 



VOL. II. I 



