BLACK SWIFT. C19 



others that it is caused by the excitement of electricity ; but these 

 conjectures are destitute alike of ingenuity and truth. It is not 

 in thundery weather alone that Swifts scream, but often in the 

 clear, dry, and sunny skies, that exhibit no phenomena indica- 

 tive of a want of electric equilibrium. And as to the other 

 theory, it suffices to reflect that Swifts scream as frequently 

 over the open fields at the distance of a mile or more from their 

 nesting places, as when wheeling near steeples or towers. The 

 cry of the jackals and hounds when in full chase seems to be 

 analogous to the scream of Swifts under similar circumstan- 

 ces ; but the cause and use of cither is not satisfactorily ascer- 

 tained. I have observed however that single birds very sel- 

 dom scream, and that the loudest and most frequent cries are 

 heard when birds are evidently in active and successful pur- 

 suit. It is so with Terns, Gulls, and even Gannets ; and 

 when you see these birds hovering over the sea, and hear their 

 mingling cries, you may be sure that they have discovered a 

 shoal of fishes, and are enjoying their good fortune. They 

 seem to scream or cry out from pleasure, and thus give inti- 

 mation to their fellows of the ])lentiful existence of food. As 

 to the organ of this loud and shrill scream in the Swift, name- 

 ly, the trachea, it is short, remarkably flattened, gradually di- 

 minishes in diameter to the bifurcation, and is destitute of 

 inferior laryngeal muscles. It has no song, or twitter, like 

 the Swallows. 



Now, having described the organization and habits of this 

 bird, I may offer some explanations as to the action of the for- 

 mer. If we suppose that the Swift is destined to feed exclu- 

 sively on insects as they flutter in the air, we can be at no loss 

 to trace the reason of its peculiar form. Its body is light, but 

 moderately stout, and its pectoral muscles are large, otherwise 

 it could not move its wings with the necessary strength and 

 rapidity. The wings are extremely elongated and narrow, be- 

 cause great rapidity of flight is required in the pursuit of ani- 

 mals which themselves fly with speed, and because sudden turns 

 require to be executed in seizing them. A short, broad, con- 

 cave wing, as that of a Partridge, on being rapidly moved pro- 

 duces considerable rapidity, but is not fitted for either buoyant 



