Indigo buntings are socially monogamous. However, pairs only associate until incubation begins, and may switch partners within a single breeding season. Fertilizations outside of a breeding pair are not uncommon and approximately 15% of males have more than one mate.
Males do not sing often in courtship, but they do follow their mate around during the nest building and laying periods, often chasing other males away.
Mating System: monogamous
Indigo buntings breed between May and September, with most activity occurring June through August. They may raise more than one brood per season, and may switch nests or mates between broods. The female chooses the nest site and builds the nest, which may take up to eight days. Nests are built in shrubs in fields or at the edges of woods, roadsides and railways. They are constructed of leaves, grasses, stems and strips of bark. After the nest is complete, the female lays 1 to 4 (usually 3 or 4) white eggs. One egg is laid each day, soon after sunrise. The female begins incubating after the last egg is laid. Incubation lasts for 11 to 14 (usually 12 to 13) days.
The female broods the altricial chicks for the first few days after they hatch. She also feeds the chicks insects and removes their fecal sacs from the nest. The chicks leave the nest 8 to 14 days after hatching, and become independent about 3 weeks after fledging. Indigo buntings are sexually mature at one year old.
Breeding interval: Indigo buntings breed between May and September, with most activity occurring June through August.
Breeding season: Indigo buntings may raise more than one brood per season, and may switch nests or mates between broods.
Range eggs per season: 1 to 4.
Average eggs per season: 3.5.
Range time to hatching: 11 to 14 days.
Average time to hatching: 12-13 days.
Range fledging age: 8 to 14 days.
Average time to independence: 3 weeks.
Range age at sexual or reproductive maturity (female): 1 (low) years.
Range age at sexual or reproductive maturity (male): 1 (low) years.
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate)
Average eggs per season: 4.
The male does not generally help with incubation or raising the chicks. The female chooses the nest site and builds the nest. She broods the altricial chicks for the first few days after they hatch, feeds them insects and removes their fecal sacs from the nest. The chicks leave the nest 8 to 14 days after hatching, and become independent about 3 weeks after fledging.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female)