126 THE ANIMAL ' CHAPTER OF ACCIDENTS' 



ground. The only objects against which collision is 

 possible are other birds ; and this possibility is reduced 

 to a minimum because they are not limited to any one 

 plane, or even to one deep ' layer/ of the air for flight. 

 Compared with the case of the terrestrial animals, all 

 moving on the single level of the land surface, just as 

 ships move on the one plane of the sea surface, the 

 birds ought not to be liable to collision at all ; and it is 

 their theoretical freedom from this danger which makes 

 the high rate of bird-speed possible, a speed denied to 

 other animals, if for no other reason, because, moving 

 as they do on a single plane, they would be as liable to 

 disabling collision as autocars running at express speed 

 on Southsea Common. The sole risk of collision is 

 when flocks are travelling together. As the direction 

 is then usually the same, and the birds take most careful 

 precautions to avoid danger by maintaining regular 

 distance, an even speed, and often a kind of military 

 order, such mishaps are rare. They chiefly occur when 

 birds which * get up steam ' at once are rising from the 

 ground. Partridges and grouse are most commonly 

 liable to this accident, and instances are recorded every 

 season ; but even small birds are occasionally * in 

 collision/ the most unusual instance recently noted 

 being that of a pair of greenfinches, one of which flew 

 against the other and broke a wing. The windows of 

 lighthouses and telegraph-wires, though causing very 



