Why do young seagulls look ‘plain’?

Scientists tested this on American silver gulls (Larus smithsonianus) in a colony on Kent Island in Canada. They placed plastic models of birds with different plumage near the nests and observed how adult gulls defending their territory reacted to them.

The study was published in the journal *Animal Behaviour*.

The results were striking: adult gulls were 48 per cent less likely to react aggressively to a model of a young brown gull than to a model of an adult bird with white and grey plumage. This supports the idea that ‘adolescent’ plumage helps young birds avoid provoking attacks in a dense and noisy colony.

Details

In many birds, maturation is evident from their plumage. Some quickly shed their juvenile plumage for adult plumage, whilst others remain in an intermediate stage for a long time. In gulls, this process can take several years: young birds gradually transition from brown, mottled plumage to the more recognisable adult appearance — a white head, grey back and wings with dark tips.

For a long time, scientists assumed that this delay in the development of adult plumage might serve a social purpose. A young bird that clearly does not look ‘adult’ signals to those around it that it is not yet a competitor for a mate or a nesting site. Consequently, there is no point in adult birds expending energy on intense aggression.

To test this idea, the researchers conducted an experiment in a colony of American silver gulls. They selected 120 nests where adult birds were incubating eggs and placed various models of ‘intruders’ next to them. These were not live birds, but plastic models, so that the scientists could precisely control the appearance of each object.

There were four models: a first-year juvenile gull with brown, mottled plumage; a more mature ‘subadult’ gull of about three years’ age; a fully adult gull; and a Canada goose as a control. The Canada goose was needed for comparison: gulls do not usually perceive it as a direct rival for a mate or a nest.

Adult gulls reacted differently to the models. They attacked the youngest brown-spotted ‘gull’ less frequently, with less intensity and more slowly than they did the adult model. The study report states that aggression towards the first-year model was significantly lower than towards the adult model.

When adult birds did display aggression, their reaction to the young model developed more slowly. According to Science News, it took around 44 seconds to reach the peak of aggression, compared with approximately 37 seconds in the case of the adult model.

However, the protective effect does not appear to apply to all ‘adolescent’ stages. Adult birds perceived the three-year-old seagull model almost in the same way as an adult. This suggests that the distinctive brown plumage of the very youngest birds may be particularly important, whilst later on this signal weakens or ceases to provide protection.

What this means in simple terms

Adult gulls are highly territorial during the breeding season. They defend their nests, eggs, mates and the space around them. If another adult gull appears nearby, this can be seen as a threat: a rival has arrived to claim territory, food or the chance to breed.

A young brown gull looks different. Its mottled plumage may signal to adults that they are not facing a mature rival, but a bird of low social status. It is therefore less advantageous to attack it.

Why this is important

The research helps to explain why, in some seabirds, adult plumage does not appear immediately. At first glance, the brown, mottled plumage may seem simply like an incomplete stage of development. But it may play a practical role: reducing conflict between young and adult birds.

This is particularly important for life in a colony. Gulls nest in dense groups, where dozens, hundreds or thousands of birds are constantly in close proximity. In such an environment, any misinterpreted movement can trigger an attack. Visual signals help to reduce the number of unnecessary conflicts.

The research also shows that a bird’s plumage serves purposes beyond attracting a mate or providing camouflage. Sometimes feathers act as a social signal: they help others understand the bird’s age, status and level of threat.

Background

Scientists have long known that a bird’s appearance can be an important signal. Bright plumage is often linked to mate choice, health or competitiveness. But the situation is different for young birds: looking too ‘adult’ can be dangerous if it provokes aggression from territorial individuals.

This idea is known as the status signal hypothesis. The basic idea is simple: a young bird’s appearance indicates that it is not among the adult competitors. Such signals have previously been discussed in relation to small songbirds, but for colonial seabirds, the situation remained less clear.

The new experiment is significant in that it specifically tested the role of plumage. The researchers used dummies rather than live birds, so the reaction of the adult gulls depended primarily on the appearance of the model, rather than on the behaviour of a real ‘intruder’.

Source

Study: Molly M. Hill, Sarah L. Dobney, Liliane K. Fanburg, Daniel J. Mennill, Liam U. Taylor, “Experimental study of social signalling through delayed plumage maturation in a colony-nesting seabird”, Animal Behaviour, 2026.