On a swaying tree branch, a massive gray-brown “big bird” gapes open its blood-red beak, while the reed warbler beside it—less than one-third its size—tiptoes and even steps on its head, desperately stuffing insects into that abyss-like mouth. It looks like a small bird is feeding the “eagle.” But this “giant baby” is actually a juvenile great-spotted cuckoo. The reed warbler, working tirelessly, is its deceived foster parent. The cuckoo lays its eggs in someone else’s nest; once the young cuckoo hatches, it kicks out the reed warbler’s eggs one by one. When only its single “fake” egg remains, the deception truly begins. It wildly demands, grows rapidly, and stretches the delicate little nest until it becomes deformed. Yet the reed warbler is completely unaware—like it has been controlled—constantly catching insects from morning to night, feeding everything into that ever-unfillable mouth.

Why does this happen? Because cuckoos don’t need to “look like” anything; they only need to “outwit/override the instinct.” An intensely crimson beak and exaggerated, over-the-top begging cries—just one chick—can simulate the hungry calls of an entire brood of nestlings. These signals precisely hit the reed warbler’s instinctive switchboard, leaving it unable to refuse or think, only to keep paying the cost. Even when the other’s body is already three or four times its size, even when something is clearly “off,” it still chooses to continue feeding. It no longer judges; it only reacts.

In the end, once the great-spotted cuckoo’s wings are fully grown, it flies away without a second thought, bound for another land. Meanwhile, the reed warbler exhausts everything, misses the breeding season, and leaves nothing behind.