Precocial and altricial chicks
Several birdspecies, including the famous cuckoo (e.g. Cuculus carnosus; Cuculidae) have escaped the labour of incubation and chick rearing by depositing their eggs in the nest of other birds and allowing foster parents to rear its young. Adaptations for such parasitism include close mimicry of eggs between the cuckoo and its host, and the more rapid development of the cuckoo chicks so that they hatch first and can dispose the legitimate eggs of its foster parents. However, all hatchling birdspecies do have a small egg-tooth at the tip of their beak which they use to break the egg. The egg has provided a small air sac at the end of the egg to provide the first air for the chick. Hatchlings can be divided into two categories. Chicks that can run away almost immediately from the nest and are fully covered with down feathers and can feed on their own but still have parental supervision are said to be precocial. This type of hatchling is most common to birds that do not build nests, but lay their eggs in the open such as the plover group. Chicks that at birth are naked and helpless and need to be fed by the parents are said to be altricial and restricted to birdspecies that construct nests.