Vlll PREFACE. 



found in Mr. Newman's edition of ' Montagu's Dictionary,' 

 under the title " Grouse, Sand, Pallas'." 



Of the second kind of irregular migration the Common 

 Crossbills may be taken as an instance, as they are very 

 curious in their migratory movements, being apparently 

 quite regardless of all consideration of time or regularity, 

 making their appearance sometimes at one time of the 

 year and sometimes at another, and sometimes only after 

 a lapse of several years, occasionally remaining to breed 

 in their temporary home. 



Another peculiarity incidental to a good many different 

 species is their various changes of plumage : this change of 

 plumage, which I have referred to more particularly in my 

 notice of each species, has often puzzled the ornithologist, 

 and led in many ways to the multiplication of species. The 

 difference between the young birds and the old has been a 

 fruitful source of error: in some instances this difference 

 disappears after the first moult ; in others, as in the Gulls, 

 many years elapse before the full adult plumage is attained : 

 the difficulty of identification has no doubt in many in- 

 stances been increased by the likeness of the young birds to 

 the mature females ; but these difficulties have now been 

 overcome by observations made at the breeding stations, by 

 specimens having been obtained in various states of change, 

 and in many instances by the birds having been kept tame 

 till they had gone through all their various changes. 



