The Protective Power of Drab Feathers
Immature American herring gulls use their distinct, drab plumage as a social signal to reduce aggression from breeding adults, according to a study published June 4 in the journal Animal Behavior. Researchers found that adult gulls were less hostile toward models resembling one-year-old birds than those painted to look like mature adults, suggesting that juvenile coloring acts as a protective mechanism in dense nesting colonies.
Field Tests on Kent Island
To determine if plumage serves as a communicative signal, researchers conducted field experiments on Kent Island in New Brunswick, Canada. They placed custom-painted plastic gull models near active nests to observe the reactions of breeding adults.
The results showed that nesting gulls responded aggressively to the youngest-looking models in approximately 30 percent of encounters. In contrast, the adults were nearly 1.5 times more likely to display hostility toward models painted with adult plumage. Furthermore, the breeding gulls exhibited a delay of about seven seconds before reacting to the juvenile models, indicating that the drab coloring may help reduce conflict with adult birds.
Navigating High-Stakes Colonies
American herring gulls (Larus smithsonianus) spend years in a sort of awkward adolescent phase, characterized by darker, drabber plumage, before reaching full sexual maturity. While these young birds do not participate in mating, they frequently enter densely packed breeding colonies where nests can be situated just two to three feet apart.
According to researchers, this behavior is inherently risky. The colonies are environments defined by frequent conflict, where adults perform territorial displays—such as grass-pulling or loud, trumpet-like vocalizations—to defend their space. The study suggests that the immature plumage signals immaturity to the breeding birds, effectively shielding the younger birds from adult aggression until they learn the rules of the colony.
Solving a Long-Standing Mystery
This study works toward answering why so many seabirds have this nonadult plumage retained for so long. Gavin Leighton, an evolutionary biologist at Buffalo State University who was not involved in the research, noted that the findings help explain why so many seabirds have this nonadult plumage retained for so long.
The phenomenon is not unique to herring gulls. Similar patterns of delayed maturation appear in unrelated lineages, including albatross and gannets. Future research may focus on identifying the specific social or environmental pressures that have driven these different species to evolve such similar developmental strategies.
Evidence of Social Signaling
- Reduced Aggression: Adult gulls are nearly 1.5 times more likely to be hostile toward adult-plumaged models than juvenile-plumaged models.
- Delayed Response: Nesting gulls hesitate approximately seven seconds longer before engaging with juvenile-colored intruders.
- Social Signaling: Immature plumage acts as a visual cue, potentially helping reduce conflict with adult birds.
- Broad Relevance: The study suggests that plumage-based social signaling may be a widespread strategy among seabirds with extended adolescent phases.