V1U PREFACE. 



foregoing causes, — viz., physical geography, mineralogical struc- 

 ture, climate, and absence of old timber. It should be borne in 

 mind that in all the preceding remarks the mere absence or pre- 

 sence of species is considered ; consequently, nothing is said of 

 birds from different causes being less frequently met with in 

 Ireland, than in particular parts of England or Scotland. Such 

 points will be fully treated of under the respective species. 



Although in their polar and equatorial migrations, the crossing 

 of a sea, — as the Mediterranean * for instance, — offers no obstacle 

 to birds, yet is it different when they are spreading latitudinally ; 

 either to the east or to the west, in which case the migration of 

 many species terminates at the margin of the sea. Were Ireland 

 therefore geographically joined to Great Britain, some species 

 that are not now found would certainly inhabit it, but the junc- 

 tion would make no difference with respect to others : — resident 

 as well as migratory birds. In that event, we should in the east of 

 Ireland at least, have those species which are found throughout 

 the most western portion of Great Britain in the same parallel of 

 latitude ; but not those whose range of distribution does not extend 

 to the most western counties of England and to Wales. The 

 species which Ireland would and would not have, under such circum- 

 stances, may be inferred from an examination of the summary ap- 

 pended to the end of each Order of Birds, where the distribution 

 over Great Britain of the species not known as Irish, is pointed out. 

 We should, for example, if the country suited them, have as resi- 



* A paper on birds seen crossing the Mediterranean in spring, by the author, 

 will be fully given in the appendix to the last volume ; but it was considered de- 

 sirable to notice each bird included there when treating of its species. This was 

 done that a person might be enabled on referring to any species to have the whole 

 information respecting it before him at once. The author, however, feels that a 

 reader of the matter continuously, may accuse him of too frequent allusion to the 

 subject. 



