Common grackles are usually monogamous, though polygyny occasionally occurs. Pair formation begins in flocks in early spring. Formation of pairs is indicated by flights and mutual displays between a single female and multiple males. A male and a female show preference for one another by flying together, usually with the female in the lead. As the pair-bond is established, the pair leaves the flock to fly and sing together.
The female of a pair typically chooses the nest site. Though this is usually done after pair formation, females sometimes chosen sites several weeks before pairing with a male. From pair formation through incubation, the male remains in close association with his mate by perching near her, following her, and engaging in mutual displays. This pattern exhibited by the male probably functions to guard against extra-pair copulations. Once incubation has begun, his attentiveness decreases steadily.
Adult common grackles sometimes function as helpers to other birds of the species. In one recorded case, two males frequently showed up at the same nest to feed the young, and there was no antagonistic behavior between them. It is assumed that one of the males was the father of the offspring.
Mating System: monogamous ; cooperative breeder
Common grackle nests are built by the female, usually in coniferous trees, though more unusual sites have been documented. These include woodpecker holes, on rafters, under the eaves of barns, in the crannies of ospreys' large nests, and in clumps of cattails. The nests are large and bulky, constructed of woody stems, leaves and fine grasses. Other materials may be used, including fishing line, feathers, manure and tape. The nest cup is lined with mud, and finally fine grasses and horsehair.
Copulation begins soon after the female has completed the nest. She lays 1 to 7 eggs (usually 5 to 6). The eggs are smooth-textured, and highly variable in color. They are typically light blue to pearl gray, though they range from nearly white to dark brown. Some are scrawled with blackish brown, especially at the larger end, and others are spotless. The female incubates the eggs for 12 to 14 days. At this time, about half of common grackle males desert the female and the nest. Those that remain participate in parental care after hatching.
During incubation, various displays and calls are given by both sexes. Parental care, including brooding and feeding, is performed mainly by the female, although males have been observed feeding young. The food supply is monopolized by more aggressive nestlings. The young leave the nest about 12 to 15 days after hatching, and remain near the nest for another 1 to 2 days. The adults continue to feed the young for several weeks.
Common grackle nests are sometimes parasitized by brown-headed cowbirds, but the cowbird eggs in these nests are largely unsuccessful. Common grackles are usually single-brooded, but can double-brood in some areas. Common grackles breed between March and July.
Breeding interval: Common grackles breed once yearly. Common grackles are usually single-brooded, but can double-brood in some areas.
Breeding season: Common grackles breed between March and July.
Range eggs per season: 1 to 7.
Average eggs per season: 5.
Range time to hatching: 12 to 14 days.
Range fledging age: 12 to 15 days.
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate)
Average birth mass: 105 g.
Average eggs per season: 5.
Brooding and feeding of the altricial chicks are performed mainly by females, although there have been reports of males assisting in feeding the young. The food supply is monopolized by more aggressive nestlings. The young leave the nest about 12 to 17 days after hatching, though they remain near the nest for another 1 to 2 days. Adults continue to feed the young for several weeks. About half of all grackle males remain with the female through hatching and help in the parental care of the young.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Protecting: Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Female); pre-independence (Provisioning: Male, Female)