Feasible locations of suitable host nests, the hosts nest-building acumen, and general parenting skills are observed by the parasitic birds and compared to other potential parents in the area. After a cuckoo or a cowbird is satisfied that her egg will receive the loving care it needs with a particular pair, she lays the egg. After that, the cuckoo or a cowbird has no regular contact with its chick, leaving this responsibility to the chosen stepparents.

The foster parenting arrangement is not an entirely friendly process. The chicks of cuckoos and cowbirds, once hatched, grow much bigger and much hungrier than their stepsiblings. In order to get a bigger share of the food that their stepparents bring home, cuckoos resort to pushing their nestmates (either as eggs or as nestlings) out of the nest. Cuckoo chicks grow so big, that they often barely fit in the nest and are twice the size of the stepparent.

Caring for another bird's chick is a heavy burden. The parasitic chicks eat more than their share which results in their stepsiblings getting less and growing smaller than their usual size. Some of the hosts developed ways to protect their young from being taken advantage of by the brood parasitic birds. Some, like chaffinches in Europe and American robins in North America, immediately recognize eggs that are not theirs and destroy them by grasping them, puncturing them with their beaks, or pushing them out of their nests. Others, like eastern phoebes nesting under the eaves of many homes in eastern North America, are more tolerant toward the intruder eggs, and typically sit on the eggs and raise the stepchild (or multiple stepchildren) as their own.

One might wonder how it happened that some birds exploit others and why the host birds are allowing the exploitation. The host birds do suffer and, in evolutionary terms, lose fitness, because of the actions of parasites: hosts' own chicks are smaller, weaker, fewer, and have less chance for survival. In some cases, parasitic birds, by installing their eggs in the hosts' nests, contribute to the decline of rare and endangered host species.

Yet, both host birds and brood parasites are an important part of the ecosystem and biodiversity. Brood parasitic birds, including brown-headed cowbirds, are native to much of North America, and represent a unique component of the local biodiversity through their unusual reproductive strategies and one of the few instances among birds where brood parasitism has evolved. Migratory cowbirds are also protected under the Federal Migratory Bird Act, and so their eggs, young, and adults must not be destroyed.