4 , [Chap. IX, 



^'^ PARA. 28.] 



Why should the echelou flight formation be usual 

 in flocks of some species and not in others (among 

 the Duck tribe, the Pochards prefer to fly en ma^f-e) ; 

 and how are the former (as they evidently are, or the 

 formation would not be employed) aided in their flight 

 by such formation ? These questions are asked and 

 answered by Major Magrath in the Bombay Natural His- 

 tory Society's Journal of 1909. A little consideration, he 

 says, will call to mind those species (see above) in which 

 the echelon is in vogue, and also the flocks in wliich it is 

 not seen or not noticeable, such as those of t]i(> smaller 

 passerine species, the Gulls, Kooks and Lapwings and in 

 coveys of Partridges or packs of Grrouse, etc. If we bear 

 in mind how species in both of tlie above categories fly 

 in flocks, the suggestion will occur that the formation is 

 intimately connected with uniformity and compactness of 

 flocks, with weiglit and size of the component individuals, 

 with velocity, with the powers of sustained flight required 

 by migrant species and with capacity, in spite of size, for 

 making headway in adverse winds; conversely, that it 

 confers no advantage on flocks in which the individnab 

 do not fly in very close proximity to one another, in which 

 the formation is a loose and straggling one, in whicli the 

 individuals, though fast fliers, have little power of sustain- 

 ed flight, and in those composed of birds whose small size 

 offers comparatively little resistance to the air. The 

 answer to the first half of the question must then, I 

 think, lie in the characteristics of the different species of 

 gregarious birds and the necessities of their existence To 

 the second half the answer must, I think, be sought for in 

 the behaviour of the atmosphere on the passage at speed of 

 a heavy body through it. A large or heavy bird in rapid 

 flight in a still or moving atmosphere must continually 

 displace a volume of air equal to its bulk and tlius be the 

 cause of a powerful indraught of air in its immediate rear. 

 Behind the same bird flying against a high wind, if there is 

 no indraught, there is yet a certain space in which the air is 

 comparatively still. In tlie former case a bird immediately 

 behind would be in a strong following wind, a state of 

 things birds dislike intensely as it upsets their equilibrium. 

 In the latter the bird would be to some extent in still air, 

 necessitating a difference of effort of wing stroke. In both 

 cases this would quickly lead to disintegration of tlie flock, 

 accurate keeping of station and an equality of speed becom- 

 ing impossible. It is probaldy the case therefore that 

 echelon formation renders important aid to large birds in 

 keeping the flocks intact when on the wing liy enabling the 

 individuals composing the same, while keeping close to each 

 otliei-, to avoid the air disturbance's set up by those im- 

 mediately in front, at the same time ensuring to each an 

 unifoim air-pressure and outlook to the front. 



