1392 Birds. 



of the same age, as was very apparent in the specimens killed in 1841, 

 which were in almost all the stages of immature plumage. It is, 

 therefore probable, that the irregular movements of certain species, 

 both of the Falconidse and other families, are totally independent of 

 the cause which we have suggested in the Catalogue, in speaking of 

 the white-tailed eagle. 



Most of our rare herons and sandpipers are included in this class of 

 migrants, but they appear in most cases singly, though commonly 

 with little variation as to time. 



The appearance of the eared grebe, which is mentioned in the Cata- 

 logue as having lately occurred, is another example of those irregular 

 movements, the immediate cause of which can be only conjectured. 

 That they are sometimes, as in the case of the little auk referred to in 

 the Catalogue, and (more often) as the storm petrel, produced by sudden 

 storms and similar accidents, is very probable ; but we are inclined to 

 consider this rather as the exception than the rule. It has been more 

 than once recorded, and has probably happened much more fre- 

 quently than has been noticed, that the appearance of some species of 

 birds, at other times rare, has been simultaneous with an unusual 

 abundance of the particular food by which those species are main- 

 tained. Four remarkable instances of this may be mentioned, viz. 

 the appearance of the crossbill in the years 1254 and 1593,* and of 

 an owl, supposed by Montagu to be the scops-eared owl in 1580 and 

 1648,f and it is not improbable that to a similar cause, might 

 be traced the migration of the nutcracker into Belgium in the au- 

 tumn of 1844, and many other irregular and (so-called) accidental 

 migrations. It may be objected that examples such as we have men- 

 tioned are both few and far between, but it is to be remembered that 

 such occurrences, even at the present time, are seldom noticed, and 

 that they were so in these instances probably, only because the arrival 

 of the owls proved the destruction of a great nuisance and prevented 

 much damage, and because in both cases, the unusual appearance 

 and colour of the visiters, would especially make the circumstance to 

 be remembered. 



In conclusion, it may be supposed that many birds which thus ap- 

 pear amongst us, are either travelling towards certain localities where 

 a temporary superabundance of that which is their natural food, would 

 unless thus controlled, destroy the balance of some other part of the 



* ' Yarrell's British Birds.' 

 f ' Montagu's Ornithological Dictionary,' art. Little Horned Owl. 



