Common grackles are medium-sized blackbirds. Their plumage is black, and has a sheen that is glossy and iridescent. Generally, their heads, necks and breasts are glossy purplish-blue or bluish-green. However, common grackles in different parts of North America have somewhat different colored plumage. In New England and in the West, the subspecies has a brassy bronze body coloration. East of the Allegheny Mountains, the body is purple, and in the southeast the feathers have a greenish hue. Common grackles have long, sharp, black bills and yellow eyes. Their tails are long and keel-shaped.
Adult common grackles are 28 to 34 cm long. Females are smaller and duller than males and have a shorter tail. Males usually weigh about 122 g while females weigh around 94 g. Young common grackles look similar to adults, but have brown plumage and brown eyes. (Peer and Bollinger, 1997; Terres, 1980)
Common grackles range over almost all of eastern North America east of the Rockies, extending far into Canada in the summer breeding season. (Terres, 1980)
Common grackles are found in open areas with scattered trees, including around human habitation. Because of this habitat preference, common grackles are very common in agricultural and suburban areas, including parks, cemeteries, fields, and orchards. Human alteration of forested habitats for agriculture has resulted in an expansion of the range of common grackles and an increase in their numbers. (Peer and Bollinger, 1997; Terres, 1980)
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. (Peer and Bollinger, 1997; Skutch, 1996; Terres, 1980)
Common grackles are usually monogamous. Males and females form breeding pairs in early spring. Potential mates fly together and perform displays for each other. Once a male and female have formed a breeding pair, they leave the flock to fly and sing together.
The female usually chooses the nest site. She usually does this after she has found a mate, though she may chose a site beforehand. Once a breeding pair is formed, the male stays very close to his mate. He is always nearby, perching near her, following her, and displaying with her. Males probably do this to prevent other males from mating with their partner. However, once the eggs have been laid, males usually stop guarding their mates. (Peer and Bollinger, 1997; Terres, 1980)
Common grackle nests are built by the female. They are usually built in coniferous trees, though they have also been found in woodpecker holes, on rafters, under the eaves of barns, inside osprey nests, and in clumps of cattails. The nests are large and bulky. They are made of woody stems, leaves and fine grasses and are lined with mud and fine grasses and horsehair.
The male and female begin copulating as soon as the nest is complete. The female lays 1 to 7 eggs (usually 5 to 6). The eggs are smooth and are usually light blue to pearl gray. Some have blackish brown marks, especially at the larger end, and others are spotless. The female incubates the eggs for 12-14 days. During incubation, the male and female of a pair communicate by calling to each other and performing displays. Many males abandon their mate during incubation, and do not return to help to raise the chicks.
While some male common grackles help to raise their chicks, most females raise the chicks alone. The female broods the chicks when they are young, and brings food to them. The chicks leave the nest about 12 to 15 days after they hatch, and they stay near the nest for another 1 to 2 days. The parents continue to feed the chicks for several weeks after they have left the nest.
About half of all grackle males leave the female during incubation, and do not return to help care for the chicks. The other half of the males remain with the female and help care for the chicks.
The chicks are helpless (altricial) and have their eyes closed when they hatch. The female does most of the brooding and feeding of chicks. However, males have been seen helping to feed the young. The chicks leave the nest about 12 to 17 days after they hatch. They stay near the nest for another 1 to 2 days. The adults continue to feed the chicks for several weeks after they leave the nest until they become independent. (Peer and Bollinger, 1997; Skutch, 1996; Terres, 1980)
The maximum lifespan recorded is just over 22 years, although most do not live that long. About half of all common grackles reach adulthood. (Terres, 1980)
Common grackles are very social. Even during the breeding season, common grackles that are not incubating eggs roost together at night. These roosts can be very large, containing thousands of individuals. They often include other species of birds, such as red-winged blackbirds, European starlings, and brown-headed cowbirds. Breeding pairs may build a nest away from other birds, or they may nest in a colony of up to 200 nests.
Even though they are highly social, common grackles do sometimes attack other grackles and other species of birds. They attack others by biting, pecking, scratching, and flying toward them. Common Grackles eat other birds' eggs and nestlings, and sometimes kill and eat other adult birds. They commonly eat adult house sparrows. Common grackles defend a territory around their nest. The breeding pair defends the nest by mobbing, chasing or diving at predators, including humans.
Common grackles migrate in mixed-species flocks with red-winged blackbirds, European starlings, and brown-headed cowbirds. Most common grackles migrate between breeding and wintering sites. However, many grackles that breed in the Gulf Coast states stay in the same area year-round. Those common grackles that do migrate do not usually migrate far. Common grackles have a magnetic mineral called magnetite in their brains. This mineral may allow them to use the earth's geomagnetic fields to navigate. (Peer and Bollinger, 1997; Terres, 1980)
We have no information regarding the home range of common grackles. (Terres, 1980)
Common grackles use body signals and songs to communicate. Each common grackle has sings one song that is different from other common grackles, and probably helps to identify that individual. The songs of common grackles are said to sound like a squeaking, rusty gate opening and closing. Males sing most often around the time of copulation, and sing less often during incubation. Females sing less often than males. They seem to sing most often when responding to their mate.
During breeding, common grackles' diets consist mainly of insects and other invertebrates. The diet may also include goldfish, minnows, crayfish, small frogs, salamanders, mice, and small bats, which are caught from the air. During migration and winter, common grackles eat mostly grains from farm fields and seeds, particularly corn and acorns. They also eat some fruits.
Common grackles are opportunistic foragers - they take advantage of whatever kind of food they can find. They often follow plows, picking up the grubs that are plowed up, and they even eat human garbage. Adults sometimes steal earthworms from robins. Grackles forage mostly on the ground, though they may also search for food in trees and shrubs. They feed in large flocks, especially in winter. Grackles use their bill, not their feet, to search for food on the ground.
Humans kill large numbers of common grackles to control populations in areas where they destroy crops. Fox squirrels, eastern chipmunks, rat snakes, domestic cats, gray squirrels, bullsnakes, and racoons eat the eggs and nestlings of common grackles. Red-tailed hawks, northern harriers, Cooper's hawks, short-eared owls, and great horned owls are known to kill and eat adult common grackles. (Peer and Bollinger, 1997)
Common Grackles are food for several bird and small animal species. As predators, they help to control populations of insects and other prey. They also disperse plant seeds through their droppings during the parts of the year when seeds make up most of their diet.
Brown-headed cowbirds occasionally lay their eggs in the nests of common grackles. If the common grackles care for the brown-headed cowbird chicks along with their own chicks, they help the brown-headed cowbird population.
Common grackles are a serious crop pest. They cause millions of dollars worth of damage to sprouting corn each year.
The roosting sites of common grackles and other blackbirds may hold a fungus that causes a very serious human disease called histoplasmosis. (Peer and Bollinger, 1997)
Common grackles may help to control populations of crop pests.
Common grackles are one of the most successful and wide-spread species in North America. There are about 97,000,000 individuals. Eastern forests cut down for farms in the 1700s and 1800s. This created additional nesting habitat and food for common grackles. Shelterbelts of trees in the western United States also helped this species spread. Common grackles are very common. In fact, they are killed as an agricultural pest in many parts of their range. (Peer and Bollinger, 1997)
There are three recognized regional kinds of common grackles: bronzed grackles, Florida grackles, and purple grackles. In areas where bronzed and purple grackles overlap, a small amount of intermediate forms have been discovered and show strong barring on the backs.
Tanya Dewey (author), Animal Diversity Web.
Kari Kirschbaum (editor), Animal Diversity Web.
Alicia Ivory (author), University of Michigan-Ann Arbor.
Peer, B., E. Bollinger. 1997. Common Grackle (Quiscalus quiscula). Pp. 1-20 in A Poole, F Gill, eds. The Birds of North America, Vol. 271. Philadelphia, PA: The Academy of Natural Sciences and Washington, DC: The American Ornithologist's Union.
Skutch, A. 1996. Orioles, blackbirds and their kin: A natural history. Tuscon: University of Arizona Press.
Terres, J. 1980. The Audobon Society Encyclopedia of North American Birds. New York: Alfred A. Knopf.