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.
Asian desert warbler
It is a small bird (the second-smallest in the genus after African desert warbler), 11.5–12.5 cm long. The sexes are almost identical in colour, pale grey-brown above with browner wings and tail, and whitish below; the bill and legs are yellowish, and the eye has a yellow iris. Like its relatives, it is insectivorous, but will also take small berries; unlike most warblers, it commonly feeds on the ground. The song is a distinctive jingle often given in an advertisement flight, with a mix of clear and harsher notes. It breeds in semi-desert and dry steppe environments, as long as some bushes for nesting occur. The nest is built in low shrub, and 4–6 eggs are laid.