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Scientists studying winter flocks of golden-crowned sparrows at the UC Santa Cruz Arboretum have discovered surprisingly complex social behavior in these small migratory birds. In this new study published in July, 2018, they reveal that the sparrows have different ways to assess dominance status depending on whether the interaction is with a familiar bird or a stranger.

The Bruce Lyon laboratory has been studying golden-crowned sparrows for 15 years. The plumage studies were conducted by Alexis Chaine, a former Bruce Lyon student who is now a CNRS researcher at the SETE station in Moulis (and therefore within TULIP). Previous studies have shown that the yellow and black markings of the head of these birds serve as "status badges", and are signals correlated with the combat ability and thus at the rank of social dominance avoiding costly fights. Birds with bright crown colors are dominant over those with duller colors, regardless of whether bright colors are natural or artificially modified by researchers. These are the conclusions of two studies published in 2011 and 2013, in which the researchers staged confrontations by placing two birds captured in different locations in an aviary.

"To show that birds are using badges of status, you have to pit birds against each other that have never met to make sure they do not have prior information about fighting ability," explained Bruce Lyon, professor of ecology and evolutionary biology at UC Santa Cruz.

In this study, published July 25 in Ecology Letters, the researchers found that badges of status don't matter to golden-crowned sparrows that already know each other. The researchers can paint bold colors on the crown of a dull-colored sparrow and it will still be as subordinate to other birds in its flock as it was before the plumage makeover.

"Plumage manipulations do not fool birds that know each other," Lyon said, noting that the researchers decided to return to the plumage manipulation studies after subsequent studies revealed close affiliations between birds in winter flocks. 

"The idea with badges of status is that they will be useful to animals that live in large or fluid social groups where they wouldn't be able to recognize and remember the fighting abilities of all the opponents they might encounter," he said. "We realized that the previous experiments we did with unfamiliar birds did not represent the social context in which these sparrows spend most of their time in winter."

The new findings show that golden-crowned sparrows use crown plumage to resolve dominance status with strangers, while social recognition governs dominance relations with familiar birds. The researchers have found that all yearlings have dull crown colors, but the plumage changes and becomes fixed in a bird's second year. At that point, there is tremendous variation in the crown plumage of different birds, but an individual's coloring remains the same for the rest of its life.

"Some of that is likely due to genetic variation, but it could also be affected by early experiences or developmental conditions. At this point, we just don't know," Lyon said.

The roles of badges of status and individual recognition in dominance behavior raise many questions that have led to long-running debates among evolutionary biologists, he said. A key question for badges of status has been, what keeps them honest? The whole system would break down if, in the case of golden-crowned sparrows, crown patches did not correlate with fighting ability, allowing a "cheater" with bold crown coloring but poor fighting ability to dominate the seed pile.

In their new paper, however, the researchers suggest that the use of both badges of status and individual recognition in different contexts can diminish the value of a dishonest badge, because interactions within a social group no longer rely only on badges of status. "Badges of status might be used to settle dominance when a bird initially joins a group, but recognition subsequently modulates interactions," they wrote.