Complex sentence: The book's title is therefore a reference the practice of certain species of cuckoo birds in which they lay their eggs in other birds' nests; the unwitting hosts then expend their energy hatching the cuckoo's eggs.
Split into simpler sentences: The book's title is therefore a reference the practice of brood parasitism among certain species of cuckoo birds. In this practice, the cuckoos lay their eggs in other birds' nests and the unwitting hosts then expend their energy hatching the cuckoo's eggs.