https://phys.org/news/2018-05-birds-species-cooperate.html

"Finding that these two species associate was not surprising, as mixed species flocks of birds are observed all over the world," said Allison Johnson, Ph.D., a postdoctoral scholar at the University of Nebraska who conducted the study as part of her dissertation research at UChicago. "But when we realized they were sharing territories with specific individuals and responding aggressively only to unknown individuals, we knew this was really unique. It completely changed our research and we knew we had to investigate it."

Variegated fairy-wrens and splendid fairy-wrens are two small songbirds that live in Australia. The males of each species have striking, bright blue feathers that make them popular with bird watchers. Their behavior also makes them an appealing subject for biologists. Both species feed on insects, live in large family groups, and breed during the same time of year. They are also non-migratory, meaning they live in one area for their entire lives, occupying the same eucalyptus scrublands that provide plenty of bushes and trees for cover.