The female red-winged blackbird also will defend the territory from other females. They build nests to hold three or four pale bluish green eggs with dark patches. Grasses are woven into a basket-shaped nest. The nests are often attached to reeds or cattails within about two metres of the water. They also nest in dense grass or in nearby fields.
Marshes supply the red-winged blackbird with aquatic insects as a source of food.
The red-winged blackbird also feeds on weed seeds in the nearby fields. Its diet includes flies, mosquitoes, termites, dandelions, milkweeds and cattails. It inserts their closed bill into a rotting log, or soil and then opens their bill. This exposes insects that are not visible on the surface. The red-winged blackbird is classified as an omnivore since it feeds on both plants and insects.