Avian brood parasitism, or the laying of one's eggs in the nest of another individual, is a reproductive strategy whereby parasites foist the cost of rearing their offspring onto another individual, the host (Davies 2000). Brood parasitism may be facultative at the species or individual levels, with some eggs incubated by the mother and others laid in foreign nests, or obligate. Brood parasitism may also be intraspecific, with eggs laid in other nests of the parasite's own species, or interspecific, with all eggs laid in the nests of other species. Cowbirds and cuckoos are the most commonly studied avian brood parasites (Davies 2000), although obligate interspecific brood parasitism has evolved at least 7 separate times among various avian clades, including cowbirds (Icteridae), honeyguides (Indicatoridae), Old World cuckoos (Cuculinae), twice in the New World cuckoos (Neomorphinae), indigobirds and their allies (Ploceidae), and the Black-headed duck (Anatidae).

For the parasite, benefits include increased fecundity due to greater allocation of resources toward mating and producing more eggs rather than defending nests, incubating eggs, and feeding young. For hosts of brood parasitic birds, the costs of parasitism range from diminished nestling growth rate, due to competition with larger and more competitive parasitic offspring (cowbirds, whydahs), to total loss of breeding by the abandonment of parasitized broods (cowbirds, cuckoos), the eviction of all host eggs by the early-hatching parasites (cuckoos), or the killing of host hatchlings by parasitic hatchlings (cuckoos, honeyguides) (Kilner 2005; Servedio & Hauber 2006). These costs exert reciprocal natural selection on parasites and hosts, such that in many cases host-parasite interactions result in escalating coevolution between intimately tied and interdependent species (Langmore et al. 2003). In turn, many hosts are able to discriminate against and reject foreign eggs or chicks based on visual, acoustic, or multimodal sensory cues (Cassey et al, 2008). The eggs of many brood parasites, for example, mimic those of their hosts (to deceive hosts to accept), have harder shells (to impede rejection by puncture), and require slightly shorter incubation times (causing a size advantage for parasitic nestlings) (Davies 2000) (Figure 1).