_Tribonyx ventralis_ may be cited as another species whose movements are influenced by the same law. This bird visited the colony of Swan River in 1833, and that of South Australia in 1840, in such countless myriads, that whole fields of corn were trodden down and destroyed in a single night; and even the streets and gardens of Adelaide were, according to Captain Sturt, alive with them. If we compare the ornithology of Australia with that of any other country in similar latitudes and of the same extent, we shall find that it fully equals, if it does not exceed them all, in the number of species it comprises; and those parts of the country that are still unexplored doubtless contain many yet to be added to the list of its Fauna. In the course of the present work it will be found that I have given a wide range of habitat to some of the species, and that I have at the same time pointed out slight variations, not amounting to a specific difference, in individuals from different localities. This difference I am unable to account for. I do not believe the birds to be distinct species, but am inclined to regard them as varieties or races of the same species, modified by the character of the situations they frequent. I may mention some curious instances in point: the _Artamus sordidus_ is a migratory bird in Van Diemen’s Land, and is partially stationary in New South Wales, yet all the examples procured in the former country are the largest and most vigorous, which we should naturally attribute to the excess of food afforded by its more humid climate; but precisely the reverse of this occurs with regard to the _Graucalus parvirostris_, which is also a migratory bird in Van Diemen’s Land, and examples of which, killed in that island, are much more feeble and diminutive than the _Graucali_ obtained in New South Wales. The _Halcyon sanctus_, again, whose distribution is universal in Australia, varies somewhat in size in every colony, still not sufficiently so as to afford any tangible specific characters. Upon taking a general view of the Australian ornithology, we find no species of Vulture, only one typical Eagle, and indeed a remarkable deficiency in the number of the species of its birds of prey, with the exception of the nocturnal Owls, among which the members of the restricted genus _Strix_ are more numerous than in any other part of the world; a circumstance which is probably attributable to the great abundance of small nocturnal quadrupeds.