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.
European goldfinch
The nest is built entirely by the female and is generally completed within a week. The male accompanies the female but does not contribute. The nest is neat and compact and is generally located several meters above the ground, hidden by leaves in the twigs at the end of a swaying branch. It is constructed of mosses and lichens and lined with plant down such as that from thistles. It is attached to the twigs of the tree with spider silk. A deep cup prevents the loss of eggs in windy weather. Beginning within a couple of days after the completion of the nest, the eggs are laid in early morning at daily intervals. The clutch is typically 4-6 eggs which are whitish with reddish-brown speckles. They have a smooth surface and are slightly glossy. The average size is with a calculated weight of . The eggs are incubated for 11–13 days by the female, who is fed by the male. The chicks are fed by both parents. Initially they receive a mixture of seeds and insects but as they grow the proportion of insect material decreases. For the first 7–9 days the young are brooded by the female. The nestlings fledge 13–18 days after hatching. The young birds are fed by both parents for a further 7–9 days. The parents typically raise two broods each year and occasionally three.