BIKDS. 



109 



boldly north on a due course, anxious to reach their perhaps far-distant 

 summer homes, but next day retreating before the renewed blizzard of 

 snow or frost. This is, I know, almost a platitude, but it has a meaning if 

 we can read it aright, which may be found to apply to the sensitive birds, 

 the name of which heads this article. 



What I mean to say is, I think that it is at least more than merely 

 possible that this curious increase of Martins in our wetter and colder 

 summers is due to the fact that there has been a crowding down of 

 the population from sfill weffer and still colder latitudes, whether north 

 or north-east, or from some other direction possibly on the Continent. 

 If I may further place my idea before my readers, I might express it 

 another way. The birds and much tender life in our northern hemi- 

 sphere are in measure — very likely in great measure — dependent upon 

 the variation of the mean of the isothermal lines of the areas affected 

 during the spring and summer seasons across Europe and even across Asia. 

 I will not say more here, as already I consider I have departed from the true 

 objects of this statistical series! Only, it may just be possible the remarks 

 so far will help some who are still groping in darkness as to solutions 

 of several puzzles. And finally, I desire to ask those who read these 

 passages to refer to instances among other sensitive species, which I have 

 already spoken of in preWous volimies of this series {vidp the last volume 

 under Woodcock and ►Snipe), and also refer them to further remarks to 

 follow. 



Hirundo rustics, L. Swallow. 



Summer ^-isitant. Common. Breeds. Not truly gregarious. 



In 1905 I observed distinct scarcity of this species in the cold 

 weather which prevailed between the 19th or 20th Mav and well on 

 into June in Perthshire and the other districts of Tay and Strathmore 

 which I visited, and an equally remarkable abundance of Martins ; 

 while Sand-Martins were again quite as remarkably scarce, as com- 

 pared vriih their usual numbers. 



Later on in the summer, however, both these birds seemed to 

 again become commoner. 



Hirundo urbica, L. Martin. 



Summer visitant. Common. Breeds. 



Mr. Milne instances a Martin's nest fixed directly upon a pane of 

 glass upon the flat, smooth surface, without any support. He had it 



