Red-winged blackbirds are reportedly the most abundant bird in North America. Their colonies are highly social. The majority of the males are polygynous, with up to 15 females in the territory of one male. Switching from one alarm call to another, males act as sentinels to provide information about predators. (Introduction to Birds of the Southern California Coast by Joan Lentz). The Red-Winged blackbird photographed below was doing just that when I appeared on the scene.
The Yellow-headed Blackbird often nests in the same marsh as the Red-winged Blackbird. The larger Yellow-headed Blackbird is dominant to the Red-winged Blackbird, and displaces the smaller blackbird from the prime nesting spots. The Yellow-headed Blackbird is strongly aggressive toward Marsh Wrens too, probably because of the egg-destroying habits of the wrens. When the Yellow-headed Blackbird finishes breeding and leaves the marsh, Marsh Wrens expand into former blackbird territories (AllAboutBirds.org).