Birds. 6761 



number of birds markedly pushing in one direction, and we can under- 

 stand why it tends to move slowly in one direction, unless something 

 occur to stop it, because the whole flock has got that direction also. 

 Slow gyration is precisely the movement with which the watching 

 birds can progress slowest with least fatigue to themselves, and thus 

 give time to the searching host below ; it is also precisely the move- 

 ment best calculated for the exercise of their telescopic vision and 

 ensure their immediate detection of the discovery of prey, important 

 to the whole flock. And thus we have an instance of a great swift 

 approaching raptorial birds in habit and structure the moment its 

 wants are similar. The mistakes of Cathartes I have before alluded 

 to are accounted for — than whom no bird is less given to frolic — in 

 oddly joining strangers so far removed from him in wants and habits, 

 but whose manners in this one particular so strangely agree with his 

 own. I would add there is no certain proportion between the column 

 and hunting birds. Individual birds are continually passing from one 

 to the other j they resort to the column possibly for rest. A bird in 

 the column sees a locality or object that it thinks betokens prey ; it 

 darts out, is followed by a screaming party, only one or two of which 

 return ; the rest deploy and become hunting birds. 



" I trust I have thus made clear the mode in which I believe these 

 curious birds are enabled to take a survey of an immense district, and 

 wherever they detect insects in such numbers that the little colony of 

 Hirundines stationed on the spot would be unable to cope with them, 

 suddenly throw into the scale their numbers, great size, activity and 

 unusual voracity. 



" You will, I fear, be alarmed at letters of such length as this, and 

 all about a single species, but I found the subject so interesting I 

 knew not where to break off. I trust it will prove interesting enough 

 to you to repay the perusal. 



" Yours very faithfully, 



" W. Osburn. 

 "To P. H.Gosse, Esq., F.R.S." 



Autumn Notes on Birds. — A small flock of the tufted duck appeared in our river as 

 early as the 27th of July, and towards the middle of August I obtained a specimen of 

 a young mallard. A far greater number of terns have haunted our sand-flats this 

 autumn than for several recent seasons. The arctic, common and black terns have 

 been plentiful ; of the latter beautiful little species I have on several occasions seen 

 large flocks fishing assiduously at low water. While the terns were about the river one 

 of my brothers one day shot and brought home to me a specimen of the gullbilled tern 

 XVII. 3 M 



