An Experiment. 151 



birds tried to enlarge, while others boldly labored in 

 places where no such slight openings existed. It was 

 interesting to watch their patient efforts as they clung 

 to the perpendicular wall like bats. Now, two or 

 three flew off and described a few circles in the air, as 

 if to rest themselves, and then again returned to work. 

 At last, convinced of the impossibility of penetrating 

 the mortar, which was much harder beneath the 

 surface, they went away in a body with a general 

 twitter leaving distinct marks of their shallow exca- 

 vations. The circumstance was the more interesting 

 because the road was much frequented (for a rural 

 district) and many people stopped to look at them ; 

 but the birds did not seem in the least alarmed, and 

 evidently only left because they found the wall im- 

 penetrable. Instinct, infallible instinct, certainly 

 would not direct these birds to such an unsuitable 

 spot. Neither was there any peculiar advantage to 

 attract them ; it was not quiet or retired, but the 

 reverse. The incident was clearly an experiment, 

 and when they found it unsuccessful they desisted. 



If we suppose this flight of martins to represent 

 a party emigrating from a sand quarry (there were 

 three such quarries within a mile radius), where the 

 population- had overflowed, it seems possible to trace 

 the motive which animated them. I imagine that the 

 old birds drive the young ones away, when the young 

 return to this country with their parents after the 

 annual migration. This is particularly the case after 

 a very favorable breeding season, when more than 

 the usual proportion of young birds survive. After 

 such a season, upon returning next year to the sand 

 quarry, the older birds drive off the younger ; and if 



