He could not even go duck hunting.

Mateo hunted the ducks, using Johnny's gun or Cliff's, and seldom failing to bring back game.

It would be ducks shot by Mateo which would furnish the circumstantial evidence which Cliff mentioned that morning.
Johnny went out to the Thunder Bird, shooed three kids from under the wings, and began to fuss with the motor.

One advantage of being idle most of the time was the easy life the Thunder Bird was leading.

The motor was not being worn out on this job, at any rate.
So far he had not spent a hundred dollars of his salary on the upkeep of his machine.