Five white eggs had been laid when a flock of English sparrows arrived. One tree swallow was in the nest and the other on guard outside. A fierce but one-sided battle took place, and the nest and eggs might have been thrown to the ground, had not Duncan protected his tenants by driving the sparrows away.

But the bird house was not long unoccupied. The bluebirds had reared their first brood, and about a week after the swallows had left, Duncan saw new tenants inspecting the little dwelling. A few busy days of nest building, and then the male, in sky-blue coat and reddish vest, perched on the roof, singing softly to his hidden mate, who sat on four blue eggs in a simple nest of hay. Still busier times followed when the eggs were hatched, and every day the parents made scores of trips to the fields near by for caterpillars and other insects to feed four gaping mouths.