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 Falconidæ 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,t 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.'
'Montagu's Omithological Dictionary,' art. Little Horned Owl.
