I saw that hypothesis when I was looking for the picture, but it doesn't apply to the particular picture, where the cuckoo is the only chick in the nest, in fact, too big for even the mother to perch on the rim. That was way beyond the pictures I'd seen before, where the cuckoo is merely bigger than the mother. Actually, the picture doesn't make sense me: how can the mother provide enough food for this gigantic chick, much bigger than her whole brood?