There have been a few swifts seen this year in
England as late as September 1st, the ordinary time of their departure being about August 20th, while it is exceedingly rare to find any of them still in this country in September. Gilbert White says that when a family of young swifts is hatched late, the elders sometimes defer migrating till after the departure of the main body, though very seldom so long after it as September. But if they stay behind the main body at all, it seems plain that it is not the gregarious instinct which compels them to travel together, but rather the simultaneousness of the desire to migrate which makes the migration so gregarious, and that when family motives overpower the wish to be off, that wish can be repressed without any danger that the spectacle of all their neighbours on the wing will induce them to start before family convenience suits. It is not apparently sociable feeling which renders migration so collective, but rather the punctuality of the individual instinct, repeating itself in great multitudes on
the same day. Perhaps imitativeness belongs to a higher plane of intelligence than that on which the migrating birds for the most part live ;—though there is nothing very noble or inspiring in imitativeness itself.