390 Lieut.-Col. R. Meiuertzhagen on [Ibis, 



3. Sporadic migration, invasion, or extensive 

 ivanderings. 



The well-known invasions of Syrrhaptes need no comment. 

 That they would lead to eventual permanent colonization is 

 almost certain, but so far the species has never had a fair 

 chance. There is no reason, however, why the wide dis- 

 tributions of Pterocles arenarius or P. senegallus should not 

 have been initiated b}'- colonization after sudden invasion, 

 for the Sand-Grouse as a group are essentially wanderers in 

 search of suitable and rather specialized food, seeming to 

 pride themselves on erratic movement arul ignoring any 

 seasonal lines of flight, which, generally speaking, constitutes 

 migration ; though in some spots they are particularly regular 

 on migration, as is the case with P. arenarius of northern 

 India. 



The Rose-coloured Starling, aptly described as a veritable 

 gipsy among birds, gives us a further illustration of coloni- 

 zation (in Italy and elsewhere) after invasion ; and the 

 various subspecific colonies of the Crossbill (Loxia curvi- 

 rostrd) in the Mediterranean region might equally be due to 

 colonization after irruption at some remote date, as opposed 

 to either gradual expansion or regular migration, tiiough 

 the accuracy of such a theory to account for their present 

 distribution is by no means certain. 



4. Human agency, direct or indirect. 



The introduction of such species as the Pheasant, Goldfinch, 

 and Starling to various parts of the world will suffice to 

 illustrate expansion of range due to direct human agency. 

 In the case of the Goldfinch, we find in the Bermudas 

 that the bird has already established for itself a differen- 

 tiation entitling it to subspecific rank. In the case of the 

 introduction of the Starling to Cape Town, it is interesting 

 to note that the species has abandoned the migratory habit 

 and has become a pure resident, not even congregating into 

 flocks in winter. 



Contraction of distribution under this heading is the sad 

 story of extermination, being generally confined to species 



