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.
Killdeer
The killdeer lays a clutch of four to six eggs that are buff to beige in colour, with brown markings and black speckles. The eggs are about in size, and laid at intervals of 24 to 48 hours. The energy expenditure of both sexes is at its highest during egg-laying; the female needs to produce eggs, and the male needs to defend its territory. Both of the sexes are closer to the nest site than usual during egg-laying and incubation, although the male is generally closer than the female during all stages of breeding. This latter fact is likely due to the male's increased investment in nest-site defence. Up to five replacement clutches can be laid, and there are occasionally two broods. Second broods are usually laid in nesting territory of the first brood. The eggs are incubated for 22 to 28 days by both the male and the female, with the former typically incubating at night. The time dedicated to incubation is related to temperature, with one study recording that killdeer incubated eggs 99% of the time when the temperature was about , 76% of the time around , and 87% of the time at about . When it is hot (above at least ), incubation cools the eggs, generally through shading by one of the parents.
About 53% of eggs are lost, mainly to predators. The young are , starting to walk within the first days of their life. After they hatch, both parents lead them out of the nest, generally to a feeding territory with dense vegetation the chicks can hide under when a predator is near. The chicks are raised, at least in single-brood pairs, by both parents, likely because of the high failure rate of nests and the need for both parents to be present to successfully raise the young. In these broods, the young are usually attended by one parent at a time (generally the female) until about two weeks of age, after which both parents are occasionally seen together with the chicks. Otherwise, the non-attentive adult is at least about away from the chicks. Periods of attentiveness for each parent generally last from about one to one and a half hours. When the chicks are young, this is mainly spent standing; as the chicks get older, less time is dedicated to standing. When the young are below two weeks of age, the attending adult spends little time feeding; foraging time increases as the chicks grow. The non-attentive adult defends the young most of the time when they are less than a week old, but this task steadily shifts onto the attentive adult, until about three weeks of age, when the attending parent does almost all of the defence. One parent at a time broods the chicks, and does so frequently until they are two days old. The young are brooded during rain until about 15 days after hatching and during night for about 18 days after hatching. The only time when they are not in the presence of a parent is when the parents are mating or responding to a predator or aggressive conspecific.