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.
Eurasian eagle-owl
The eggs are normally laid at intervals of three days and are incubated only by the female. Laying generally begins in late winter but may be later in the year in colder habitats. During the incubation period, the female is brought food at the nest by her mate. A single clutch of white eggs is laid; each egg can measure anything from long by in width and will usually weigh about . In Central Europe, eggs average , and in Siberia, eggs average . Their eggs are only slightly larger than those of snowy owls and the nominate subspecies of great horned owl, while similar in size to those of spot-bellied eagle-owls and Blakiston's fish owls. The Eurasian eagle-owl’s eggs are noticeably larger than those of Indian and pharaoh eagle-owls. Usually clutch size is 1 to 2, rarely 3 to 4 and exceptionally to 6. The average number of eggs laid varies with latitude in Europe. Mean clutch size averages 2.02–2.14 in Spain and the massifs of France, 1.82 to 1.89 in central Europe and the eastern Alps; in Sweden and Finland the clutch size averages 1.56 and 1.87, respectively. While variation based on climate is not unusual for different wide-ranging palearctic species, the higher clutch size of western Mediterranean eagle-owls is also probably driven by the presence of lagomorphs in the diet, which provide high nutritional value than most other regular prey. The average clutch size, attributed as 2.7, was the lowest of any European owl per one study. One species was attributed with an even lower clutch size in North America, the great grey owl with a mean of 2.6, but the mean clutch size was much higher for the same species in Europe, at 4.05.