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.
Verreaux's eagle-owl
On average, the female lays two white eggs, which typically measure , with a range in height of and a range in width of . The eggs weigh from , the upper weight being the mean mass of the first egg and the lower weight being the mean mass of the second egg. The eggs are reportedly laid at up to 7 day intervals and may take up nearly seven days as well between hatching. Most nest reportedly contain two eggs, but some may contain only one, and no more than two has been recorded in this species. The adult female incubates the eggs for 33 to 39 days, the incubation stage being slightly longer than those of most other eagle-owls, at least the more northern species. On average at hatching, the young weigh about . The weight of the nestling can triple within five days after hatching. Due to the extreme interval between the hatching of the first and the second egg, the older owlet is always considerably larger than the second. As is widely reported in different kinds of raptorial birds, the smaller chick usually dies in the nest. This may be due to starvation upon being outcompeted for food by the older chick or the smaller chick may be being attacked and killed by its older sibling. Usually the smaller chick is gone within two weeks after hatching in this species. In rare cases, both chicks are reared and survive to leave the nest, although there are no known cases of two fledglings resulting from a Verreaux's eagle-owl nest in southern Africa. The young are covered in off-white down from hatching on and the pink eyelids may become apparent within the first week of life. By three weeks of age, the chicks down will thicken and darken to a greyish colour with some barring present. By six weeks, the young eagle-owl will start to somewhat resemble an adult, replete with the blackish brackets on the facial disc of the adult but still being fairly downy, particularly about the head. Only a week later, almost all the down is likely to be moulted.