Ornithology term
Brood parasitism
Also: nest parasitism
Definition
A reproductive strategy in which a bird lays eggs in another bird’s nest, leaving the host to provide care.
Why it matters in the field
This strategy explains why an unfamiliar egg or unusually large nestling may appear in an otherwise ordinary nest. Identification still requires documented host and parasite evidence, not color alone.
Examples
- A Brown-headed Cowbird may place an egg in another songbird’s nest, whose adults then incubate and feed the young.
- Some ducks lay eggs in nests belonging to other females of their own or another species, another form of brood parasitism.
Common confusion
Brood parasitism is different from predation or nest takeover. Its defining act is placing an egg where another bird provides some or all parental care.
Observation notes
Document a suspected case with dates, host identity, egg or nestling differences, and adult care behavior. A single unusual egg is not decisive because eggs vary within species and can look different under changing light. Avoid handling eggs or lingering near the nest; photographs taken from an established observation point provide a safer record for later comparison.
Host responses range from accepting an egg to abandoning or removing it, and those responses cannot be inferred from one brief visit. Keep separate notes for what you directly observed and what a field guide says is possible. This distinction prevents a plausible brood-parasitism story from becoming an unsupported record about a particular nest.
When sharing a record, name the suspected host and parasite only when photographs, vocalizations, or later development support them. Local breeding atlases and specialist nest-monitoring groups can help evaluate unusual observations. Do not remove an egg or young bird: wildlife laws and ethical monitoring standards apply, and intervention can harm both host and parasite without improving the record.