A great musical interlude on the patio announced an airlift of construction materials eventually producing this pile inside the top cover of the propane tank:
The male Carolina Wren switched from the Tweedle of Great Nestbuilding to the less musical Mighty Chirr of Disapproval, presumably because he noticed a mouse (or, perhaps, chipmunk) occupying the lower ring of the tank. Rodents and birds do not coexist well at all; I have no doubt a mouse would climb right up the tank for a supply of breakfast eggs.
I must blow the crud off the tank before the next fill.
The Smell of Molten Projects in the Morning 
Carolina Wrens do love cozy little places like that for nests. My propane tank has had wasps and mice nests but no birds due to the lid being pretty tight except for a small gap. You’d think that the scent of mercaptan would keep away wildlife but I apparently not. Hadn’t ever thought about mice getting to birds eggs and what that challenges that might provide, however if they can chew through walnut shells, then an egg shouldn’t be much of an issue.
I spotted a wren flying out of a port and thought “Wrens discover armor!”
One tried starting a nest in Mary’s bike helmet on the bike seat, whereupon we closed the garage door. Doesn’t take ’em long to make a decision when they find a nice spot!