230 THE REASON WHY: 



&quot; The breezy call of incense breathing morn, 



The swallow t\vitterig from the straw-built shed, 

 The cock s shrill clarion, or the echoing horn, 

 No more shall rouse them from their lowly bed.&quot; QUAY. 



in their appointed track there are more insects to be met with. The 

 house martin, flying slower still, catches those insects which, except 

 when rain is near, move along at a height of from fifteen to forty 

 feet above the ground. The sand martin has his post still lower 

 down, for he usually skims after gnats and other flies just over 

 low flat grounds. 



Thus, each bird accomplishing its work at a different altitude, 

 their united efforts clear the air of insects which would otherwise 

 be injurious to vegetation, and annoying to animals. 



713. fHiy do the habits of swallows peculiarly adapt those 

 birds to migration? 



Because their exercise renders 

 long flights endurable ; and the 

 capability of sustaining them 

 selves in the air for fourteen or 

 sixteen hours together in search 

 of food, renders these distant 

 journeys less fatiguing than 

 they would be to birds of 

 inactive habits. 



714. Why are the holes made by swallows nearly as circular as 

 though they had been planned with a pair of compasses ? 



Because, when the swallow is constructing its nest, it perches on 

 the circumference with its claws, and works with its bill from the 

 centre outwards ; a radius is thus formed by a part of the bird s 

 v &amp;gt;ody, whieh is subject to little variation. 



715. Why do the mandibles of swallows open further back than 

 in most other birds ? 



Because, with this bird, the food is caught when flying, which 

 requires a peculiar construction of the mouth, producing a large 

 gape, in which the insects are caught, as in a net. 



