BRITAIN'S BIRDS AND THEIR NESTS. 53 



other parts of the plumage. Full plumage is attained at 

 the second moult, and the birds breed when two years 

 old. 



It has always been a matter for remark that this 

 species does not nest on the Fame Islands. There the 

 Common, Arctic, and Sandwich Terns all nest in abund- 

 ance, and even a few pairs of Roseate Terns are to be 

 found ; but Little Terns are altogether absent. An attempt 

 to introduce them by placing their eggs in the nests of 

 other Terns was successful so far in that the young Little 

 Terns were reared, and in due course departed with the 

 others. But none of them ever returned. This raises an 

 interesting point. Can strictly migratory birds be intro- 

 duced by this method ? We are familiar with it only for 

 more or less sedentary species, and know of no successful 

 results with migrants. An attempt, for instance, to intro- 

 duce the Nightingale into parts of Scotland was a complete 

 failure. Although we have as yet very little knowledge of 

 the migration routes and winter-quarters of our summer 

 visitors, from what we do know it seems that these are 

 sufficiently fixed and definite for us to be able to assume 

 that there must be either hereditary knowledge or guidance 

 in the case of young birds performing the journey for 

 the first time. Guidance seems out of the question, and 

 we are practically compelled to fall back on hereditary 

 knowledge, although that is but an explanation itself 

 requiring to be explained. It would be an interesting 

 experiment to introduce and carefully mark a sufficiently 

 large number of Little Terns or other migrants in the 

 way described, and to discover the subsequent history of 

 some of the birds. One can readily imagine cases arising 

 which would go far to prove or disprove the hereditary 

 knowledge theory, for it is often by disturbing the normal 

 course of affairs that light is thrown on the ordinary 



