The Cuculiforme order includes about 140 species. The cuckoos are of unusual  ethological interest because about 50 species are brood parasites, they lay their eggs in the nests of other species, which then rear the young parasites.

Brood parasitism In western Europe common cuckoos lay their eggs in the nests of many species, mostly song birds (Passerines); more than 140 host species have been reported, including some that could hardly rear a young cuckoo successfully. A given female normally restricts her parasitism to the single host species by which she herself was reared, and therefore the common cuckoo population is composed of numerous clans.

Non parasitic cuckoos Most of the nonparasitic cuckoos form stable pair bonds and defend territories, within which they build their nests and rear their own young. The guiras and the anis are exceptional in that they live in flocks of five to 20 individuals, each flock defending a territory within which its members feed and nest. Several birds of the flock may cooperate in building a nest and in feeding the young. The coucals are unusual in that they build sizable domed nests of grass and twigs, on or near the ground, with side entrances.

cuculiform. (2011). Encyclopædia Britannica Ultimate Reference Suite.  Chicago: Encyclopædia Britannica.

Hackett SJ etal (2008): Phylogenomic Study of Birds (useful for section on behaviour and ecology, but does not mention recent phylogenomic data.)