The cuckoos are small to medium-sized birds. There are two basic body forms that include arboreal species (like the Squirrell Cuckoo), which are slender and have short tarsi, and terrestrial species (like the Rufous-vented Ground-Cuckoo), which are more heavy set and have long tarsi. One of the most important distinguishing features of the family Cuculidade are the feet, which are zygodactyl, meaning that the two inner toes point forward and the two outer backward. Almost all species have long tails which are used for steering in terrestrial species and as a rudder during flight in the arboreal species.
The wing shape also varies with lifestyle with the more migratory species like the Black-billed Cuckoo possessing long narrow wings and the more terrestrial and sedentary cuckoos having shorter rounded wings for gliding flight. The majority of cuckoo species build their own nests. Only three species pheasant, pavonine, and striped cuckoss are brood parasites, laying their eggs in the nests of other birds. These species are obligate brood parasites, meaning that they only reproduce in this fashion. There are 7 genera and 16 species of cuckoos known to occur in Peru. Photo: Squirrell Cuckoo.