Experimental study of social signaling through delayed plumage maturation in a colony-nesting seabird

Delayed development is a widespread evolutionary strategy that can reduce
competition among social animals. Many seabirds exhibit delayed plumage
maturation, in which young birds spend years in visually distinct
predefinitive plumages before attaining the definitive plumage of adults.
Previous work hypothesized that predefinitive plumages may function to
reduce aggression towards young seabirds at breeding colonies, an idea
known from other lineages of birds as the status signaling hypothesis. We
tested this hypothesis with visual stimulus experiments at a breeding
colony of American Herring Gulls (Larus smithsonianus). We presented
painted models of four different plumage classes (first-cycle
predefinitive gull plumage, third-cycle predefinitive gull plumage,
definitive gull plumage, and a Canada Goose as a control) and measured the
aggressive responses of breeding adults at their nests. Breeding gulls
responded with significantly less frequent, lower, and slower aggression
toward the first-cycle plumage and control stimuli compared to the
definitive plumage. There were no significant differences in response
towards the third-cycle plumage compared to definitive plumage. These
results provide evidence for a status signaling function of the extensive
brown plumage characteristic of first-cycle gulls, which may reduce
aggression in foraging flocks, on the wintering ground, or during young
birds’ brief visits to colonies. Given the lack of differences in
aggressive responses to third-cycle versus definitive plumage, our data do
not settle the issue of why third-cycle gulls, which spend more time at
colonies than younger birds, also molt in brown, predefinitive plumages.
Future research can investigate how immature seabirds—including
third-cycle gulls—may combine plumage, posture, and behavior to shift the
dynamics of social behavior at breeding colonies.

延迟发育是一种广泛存在的进化策略，可降低社会性动物间的竞争。许多海鸟表现出延迟羽毛成熟（delayed plumage maturation）现象：幼鸟会历经数年具有鲜明视觉特征的前成体羽（predefinitive plumage）阶段，之后才获得成鸟的终成体羽（definitive plumage）。既往研究提出假说，认为前成体羽的功能可能是降低繁殖群落中对幼年海鸟的攻击行为，这一假说在其他鸟类类群中被称为地位信号假说（status signaling hypothesis）。
我们以美洲银鸥（Larus smithsonianus）的繁殖群落为研究对象，通过视觉刺激实验对该假说进行验证。我们制作了四种不同羽型类别的涂色模型：第一龄期前成体银鸥羽、第三龄期前成体银鸥羽、终成体银鸥羽，以及作为对照的加拿大雁模型，并记录了繁殖成鸟在巢中的攻击反应。
相较于终成体羽模型，繁殖银鸥对第一龄期羽型及对照模型的攻击行为频率显著更低、强度更弱、速度更慢。而第三龄期羽型与终成体羽型所引发的攻击反应无显著差异。
本研究结果为第一龄期银鸥标志性的大面积棕色前成体羽的地位信号功能提供了证据，该羽型或可在觅食群、越冬地，或幼鸟短暂造访繁殖群落时降低攻击行为。鉴于第三龄期羽型与终成体羽型引发的攻击反应无显著差异，我们的研究尚未能解释为何相较于更年轻的个体，在繁殖群落停留时间更长的第三龄期银鸥仍会换羽为棕色的前成体羽。未来研究可探索未成熟海鸟——包括第三龄期银鸥——如何结合羽型、姿态与行为，改变繁殖群落中的社会行为动态。