True gulls attacked decoys with first-year colors less often than those with adult colors.
While many bird species mature from egg to adult in a matter of months, some seabirds spend years in a sort of awkward adolescent phase, sporting darker, duller plumage than adults.
In American herring gulls, this immature coloring may function as a social signalhelping young people avoid aggression from breeding adults, researchers report June 4 in Animal behavior. Using plastic models painted to resemble gulls of different ages, the researchers found that adult gulls were less aggressive toward the gray and brown models of year-old gulls than the bright white, gray and black models of adults.
The experiment “aims to understand why so many seabirds maintain this non-adult plumage for so long,” says Gavin Leighton, an evolutionary biologist at Buffalo State University who was not involved in the research.
Carrying out the study was not an easy task. Breeding colonies of American herring gulls (Larus Smithsonianus) are not a place for the faint of heart. “In these colonies, there is sometimes a nest every two to three feet. They can be incredibly densely populated,” says Molly Hill, an undergraduate student at Yale University at the time the research was conducted on Canada’s Kent Island, New Brunswick. “It’s just chaos when you walk – seagulls flying everywhere, screaming, fighting with each other.”
Among breeding adults and their chicks, researchers also spotted immature gulls in these chaotic colonies. It’s not entirely clear why the young gulls are here, given that they no longer need parental care and don’t appear to be trying to mate. This is a risky move for young birds who may not yet have the skills to safely navigate these crowded and complicated social environments. The researchers hypothesized that the distinct plumage of young birds might signal their immaturity to breeding birds, helping to protect the young from adult aggression until they learn the rules of the colony.
To test this idea, researchers placed fake gulls painted with first-year or adult colors near real nesting gulls and recorded their behaviors. Sometimes the nesting gulls barely reacted; other times, they made loud, territorial trumpet calls or performed grass-pulling demonstrations. The gesture essentially conveys “this is my territory, this is my grass, I’m building a nest here, you should leave,” Hill says.
Nesting gulls acted aggressively toward younger models in about 30% of encounters, but were nearly 1.5 times more likely to be hostile toward adult models. Additionally, they were slightly slower to react to the young, waiting about seven seconds longer before becoming aggressive. The results indicate that this immature coloration could help reduce conflicts with adult birds, the researchers say.
“I think this opens the door to a lot of research on other seabirds,” Hill says. “There are many species of seabirds that exhibit very similar delayed plumage maturation, even in completely unrelated lineages, such as the albatross and the gannet.” She says future research could explore the social or environmental factors that push these disparate species toward similar developmental patterns.
