The little mother saw the fox before he caught sight of her.

The Child saw her stiffen herself suddenly, with a low _chit_ of warning which sounded as if it might have come from anywhere.

On the instant every chick had vanished.

The Child realized that it was impossible for even such active creatures as they were to have run away so quickly as all that.

So he knew that they had just made themselves invisible by squatting absolutely motionless among the twigs and moss which they so exactly resembled in coloring.
The fox, meanwhile, had been gazing around in every direction but the right one, to try and see where that partridge cry had come from.