Habitat
During the breeding season, Yellow-headed Blackbirds can be found in large wetlands with emergent vegetation. Their habitat requirements are similar to those of Red-winged Blackbirds, but Yellow-headed Blackbirds require larger wetlands with deeper water. Red-winged Blackbirds are often seen with Yellow-headed Blackbirds. When the two do occur together, the larger Yellow-headed Blackbirds generally inhabit the center of the marsh, and the smaller Red-winged Blackbirds inhabit the periphery. In the non-breeding season, they can be found in open fields.
Behavior
Outside of the breeding season, Yellow-headed Blackbirds are commonly found in flocks, often mixed with other blackbird species. In winter, single-species flocks may form, sometimes consisting of all males or all females. Large foraging flocks move in a rolling fashion: birds from the back of the flock fly over the rest of the flock to the front. They forage by walking on the ground or climbing on low marsh vegetation. They often forage beside bodies of water when insects are emerging from aquatic larval stages, and they follow behind farm machinery, taking advantage of food churned up by the equipment.
Diet
In the spring and summer and when feeding young, Yellow-headed Blackbirds eat many insects. The rest of the year they eat seeds and waste grains, and probably two-thirds of their annual diet is seeds and grain.
Nesting
Yellow-headed Blackbirds are polygamous and breed in colonies. Males may have up to 6 mates, but 2 to 3 mates is the norm. Pair bonds last for a single breeding season. The female builds the nest, which is a bulky, open cup made of leaves, stems, and grass, and lashed to cattails, bulrushes, or other plants growing over the water. The female incubates 4 eggs for 12 to 13 days, and broods the young for a few days after hatching. The female provides most of the food for the young, but the male may help feed at one nest. The young leave the nest 9 to 12 days after hatching, but stay nearby, close to the water, until they can fly, about 9 to 12 days later. The female feeds the young for a few days after they fledge. Females typically raise one brood each season but may raise two.