254 BRITISH BIRDS. 



arrival of Short-eared Owls in one case, and their subsequent 

 disappearance ; and in the case of Rough-legged Buzzards 

 and Snowy Owls in Scandinavia, following upon great 

 lemming years, as shown by the writings principally of 

 Professor Robt. Collett. 



G. Migration, or rather Dispersal forced by circumstances 

 as detailed under a — f. A phase which may be demonstrated 

 both under Dispersal and Increase of Distribution, and under 

 wider waves — so to speak — and more force during the bi- 

 annual '* Emigration " and " Immigration." 



The above conclusions may fitly be assigned to a simple 

 and plain Law in Nature, namely, that as Nature abhors 

 a vacuum so, during extension and expansion, does animal 

 life fill spaces which are suitable to requirements of life. 



J. A. Harvie-Brown. 



I am naturally very reluctant to differ with so great an 

 authority as Professor Collett, but I should like to remind 

 the Rev. F. Jourdain and Dr. N. F. r Ticehurst that apparently 

 some doubt existed in Professor Collett's mind in regard to 

 the effect of the abundance of the lemming on the fecundity 

 of the Rough- legged Buzzard, Snowy Owl, etc., for he 

 says, " Now it is a fact that many birds breed more 

 abundantly when food is plentiful than under ordinary 

 circumstances. This, for instance, has been shown to be the 

 case with several species of the Owls that prey principally on 

 small rodents, which, in certain years, are exceptionally 

 numerous, but whether such increase in the procreative 

 power is owing to the abundant supply of food, or is to be 

 traced rather to the cause (whatever it be) which renders the 

 small rodents in that very year so much more prolific than 

 common, is still an open question " (Robert Collett, " Orni- 

 thology of Northern Norway," p. 38). I am in entire accord 

 with the opening remark * of the above quotation, which is 

 supported by the following words of the late Professor Newton : 

 " The lemming migrations . . . appear to draw all the birds of 

 prey in the north into one focus" (" Ootheca Wolleyana," Part 

 I., p. 180). To my mind herein lies a possible explanation. If 

 I am to be guided by what I have read on the subject, it 

 would appear that the Eaptores in the districts covered by the 



* We think Mr. Bunyard has mistaken the meaning of the 

 passage quoted. By " many birds breed more abundantly," 

 Prof. Collett meant, we take it, that many individuals reared more 

 young and not that more individuals bred, since he refers later to 

 " such increase of procreative power " [italics are ours] as an ascertained 

 fact. — Eds. 



