no LETTERS TO MARCO xvn 



commencement of a hard frost, and have no 

 doubt these birds feed in flocks, except during 

 the breeding season, when they separate, and 

 each half acre of cultivated land has its pair 

 of larks and nest upon it. In winter, during 

 frosts, food for the birds is only found in 

 isolated spots, towards which they are 

 naturally drawn together for self-preserva- 

 tion. A freshly ploughed or manured field, 

 a disturbed manure-heap, a flooded meadow, 

 or the water-course of a spring that does not 

 freeze, is sure to attract a flock or pack of 

 birds. 



Then again the advantages of safety are 

 obvious. Surprise from enemies is almost 

 impossible to a large flock of birds, each 

 member of which can feed at leisure in a way 

 that the solitary bird dares not trust itself 

 to do. 



The only difficulty I find is in the case of 

 the many birds that do not apparently pack. 

 In the case of birds of prey it is easily under- 

 stood, as neither beasts nor fish of prey assort 



