The famous cuckoo bird never builds its own nest. It flies around until it sees another nest with eggs in it and no mother bird around. The cuckoo quickly lands, lays its eggs there, and flies away. The thrush, whose nest has been invaded, comes back. Not being very good at arithmetic, she gets to work hatching the eggs.

What happens? Four little thrushes hatch, but one large cuckoo hatches. The cuckoo is two or three times the size of the thrushes. When Mrs. Thrush brings to the nest one large, juicy worm, she finds four petite thrush mouths, one cavernous cuckoo mouth. Guess who gets the worm? A full-sized thrush ends up feeding a baby cuckoo that is three times as big as it is.

Over time, the bigger cuckoo gets bigger and bigger, and the smaller thrushes get smaller and smaller. When I was a kid, you could always find a baby cuckoo's nest. You walked along a hedgerow until you found dead little thrushes, which the cuckoo throws out one at a time. Paul teaches in (Romans 8:5-8) that spiritually speaking, you've got two natures in one nest. The nature that you go on feeding will grow, and the nature that you go on starving will diminish.