Their natural independence might then come into play.

But these are effects rather than causes, besides which I think birds and animals often act from custom rather than for advantage.

Among men customs survive for centuries after the original meaning has been lost.

I had always been told by country people that the missel-thrush was a solitary bird, and when I first observed a pack and mentioned it some incredulity was expressed.

Very naturally in summer people do not see much but hay and wheat.