Christine,
I love Carolina Wrens, they have the prettiest song and the juveniles are such cuties. Wrens sure do choose unusual places to build their nests but that propane tank is the most unusual I've ever seen! She must feel like it's a safe place from predators but I hope a snake doesn't find the nest. My sister used to have Carolina Wrens build nest in hanging baskets off her back porch and we had them build nests in the purple martin houses in our backyard; one year a black snake slithered up the tall pole and got to the nests.
GrammaChar, Those Brown-headed Cowbirds must be everywhere now. I never, ever saw a single one until about ten years ago and now they are prolific and appear in large flocks Pretty birds but being brood parasites unfortunately depletes the survival rate of other birds.
Love the Chickadee!