Black-billed cuckoo
Many cuckoos are obligate brood parasites, meaning they only lay eggs in other birds' nests and never take care of their own young. Birds that do this, such as the common cuckoo or the brown-headed cowbird, lay relatively small eggs because their expected hosts are usually smaller birds. Cuckoos in the genus "Coccyzus", lay relatively large eggs even though they still parasitize smaller birds. Yellow warblers are the smallest birds recorded caring for black-billed cuckoo eggs. In experiments, these wood warblers were found to accept the cuckoo eggs 63% of the time even though their own eggs are only a quarter of the size. Even with the size difference, the warbler parent is often still able to raise the cuckoo as long as it can provide the chick with sufficient nutrition and incubation.
The black-billed cuckoo was originally placed in genus "Cuculus". This genus comprises Old World cuckoos that are all obligate brood parasites. Upon further genetic testing, it was placed into genus "Coccyzus" that comprises nine species of New World cuckoos. The black-billed and yellow-billed cuckoos are the only two that migrate into North America while the other seven are permanent residents of either Central or South America. Even though these two cuckoos overlap in range, they are not sister taxa. This suggests that the two species invaded North America separately. Although family Cuculidae is monophyletic, the genus "Coccyzus" was found to be polyphyletic.
Pitta
The clutch size varies by species. Typically three to five eggs are laid, but two is typical for the garnet pitta, whereas six is more common for the blue-winged pitta and the Indian pitta. It is thought that species with higher levels of predation tend to have smaller clutches, as smaller clutches involve fewer provisioning trips that might alert a predator to the presence of a nest, and smaller clutches are easier to replace if lost. Clutch size may vary within a species depending on latitude. A study of noisy pittas found that birds in the tropics had smaller clutch sizes than those in more temperate environments. The eggs of pittas are slightly pointed at one end, and generally smooth (the deeply pitted eggs of the superb pitta being the exception to this). The size of eggs varies by species, with smaller-sized species laying smaller eggs. There is also some variation in egg size within a species in species with large ranges. For example, the eggs of noisy pittas are smaller closer to the tropics. Eggs are typically white or creamy, and usually slightly glossy.
Both parents incubate the clutch, the period between laying and hatching being between 14 and 18 days (14 to 16 being more typical). The chicks usually hatch asynchronously, over a number of days, but in some species the hatching is synchronous. On hatching the Gurney's pitta parents are reported to consume the eggshells. This behaviour ensures that the calcium used to create the eggs is not lost. It is unknown if other species do this, but it is a common behaviour among birds. As with the incubation, both parents are involved in rearing the chicks. The chicks of pittas are entirely altricial, hatching both naked and blind, and dependent upon their parents for warmth, food and nest sanitation. Young chicks are brooded continuously, with the female brooding alone in some species and sharing responsibilities with the male in others. The males and females make regular feeding trips to the chicks; one study of Gurney's pittas found a pair made 2300 feeding visits to the nest, traveling an estimated over the nestling stage. Earthworms are important food items for many species, and the dominant item in the nestling diet of some. 73% of the parental visits of fairy pittas, 63% of rainbow pitta's, up to 79% of Gurney's pitta's visits involved bringing earthworms. Parents can and do carry more than one item in their bills during visits; in a study of breeding fairy pittas, as many as six items were observed being brought in a single visit, although less than four was typical. When the chicks are small. prey may be broken up prior to being fed to the chicks, and larger prey items like skinks and snakes are only fed to the older chicks able to manage them.