62 THE FLIGHT OF BIRDS 



all the heads turned one way, the object being to 

 follow with the eye the movements of the leader or 

 leaders of the flock ? 



Good Steerers and Bad. 



There are good steerers and bad steerers. 

 Contrast the Swallow, the Lapwing, or the Sparrow- 

 Hawk, those adepts at sudden swerving and 

 doubling, with the Duck ! The Duck becomes the 

 slave of his own ponderous momentum, and changes 

 his course slowly and with effort. The Swallow, 

 the Lapwing and the Sparrow-Hawk have all of 

 them a fine expanse of tail, while the Duck's is small 

 in area and, what there is of it, feeble. Efficiency 

 or inefficiency of tail, no doubt, accounts for a 

 great deal. But it is not the only factor to be 

 considered. The Duck, though very strong, is 

 lacking in agility ; the three other birds have far 

 more suppleness, I believe, notably at one im- 

 portant point. Though birds' backbones are, 

 below the neck, very stiff, yet they allow of a good 

 deal of bend, either up and down or sideways, at the 

 waist. I have measured the amount of sideways 

 bend in some few species and found that, as far as 

 my far from complete evidence goes, good steerers 



Angle formed by backbone 

 bending sideways at waist. 



Kestrel 141 5° 



Swallow 150° 



Swift 153-5° 



Common Tern . . . . 155° 



Kestrel (another specimen) . . 156° 



Domestic Duck . . . . 165° 



