Social needier: Harsh conditions promote complex sociality in superb fairy-wrens

Harmonious cooperation between groups of animals is one of the most fascinating phenomena in nature – while many groups live in a state of conflict with neighbouring groups, sometimes groups can interact peacefully, tolerate each other, or even cooperate.

Such cooperation between stable social groups is found in multilevel societies, the most complex type of sociality in animals.

These societies are characterised by a nested social structure, with individuals forming stable, exclusive groups, and some groups occasionally merging to form stable communities.

This type of society is traditional in humans – for example, in hunter-gatherer societies, where families can at times merge to form higher groups, such as clans and bands.

Recently, we demonstrated that such complex multilevel sociality is also found in a tiny songbird, found in our local parks and gardens – the much-loved superb fairy-wren.

Developing social relationships

Fairy-wrens are not only gorgeous bobbing balls of joy, they also lead highly complex social lives. They live in small social groups formed by a breeding pair and a few additional non-breeding birds that help the pair raise their young.

During the breeding season, social groups inhabit their exclusive breeding territories that are vigorously defended against other groups.

However, during the non-breeding season, these hostilities cease, and some neighbouring breeding groups associate to form stable communities.

As a result, these birds develop social relationships of varying levels of strength, and this gives birds access to a range of beneficial relationships. This social flexibility is an important benefit of complex societies.

Such flexible cooperative relationships might be particularly beneficial when conditions are harsh.

However, this hypothesis, while plausible, had not yet been tested in wild animal populations.

Read more: Fire in northern Australia’s tropical savanna is a threat to endangered fairy-wrens

Here we used the seasonal change in social behaviour of superb fairy-wrens – the harmonious coalescing of previously competitive groups and individuals – to test the hypothesis that multilevel societies emerge to enable individuals to reap the benefits of broader cooperative relationships during more challenging environmental conditions.

To do so, we first collected information on social associations in a local population of free-living superb fairy-wrens, in Lysterfield Park reserve, on the traditional land of Bunurong Boon Wurrung and Wurundjeri Woi Wurrung peoples of the Eastern Kulin Nation.

In this population, all wrens were individually marked with a unique combination of coloured leg rings.

Constructing a social network

PhD student Ettore Camerlenghi observed more than 1600 social associations, and used these to construct a complete social network of the population.

We partitioned the social network into two types of relations – those from the same breeding social group, and those from the same community.

We recorded distress calls from individuals from all breeding groups. These distress calls are emitted in conditions of great danger to solicit aid from other individuals; specifically, they can induce a unique altruistic distraction display whereby an individual approaches very closely (within striking range), simulating a running rodent, to distract a predator.

We tested whether the likelihood of helping an individual in distress, and the type of help, was influenced by group membership and environmental conditions.

To do so, we presented foraging groups of superb fairy-wrens with a simulated threat consisting of a taxidermic predator (a kookaburra) and a simultaneous playback of distress calls.

Distress calls played were from individuals from the same breeding group or from the same community (played in random order).

We then measured the responses of listening superb fairy-wrens, which could include helping behaviour (such as approaching the predator and giving alarm calls, or performing the rodent runs), or, as a measure of aggressive territorial behaviour, singing.

We repeated the experiment during winter, when environmental conditions are harsher, and during spring, when climatic conditions are favourable.

We found a dramatic increase in cooperation in the harsh winter season, which parallels the changing structure of fairy-wren society.

During the winter months, birds never responded aggressively, and usually helped, approaching, alarming or performing “rodent runs”, the latter especially if the individual in distress was from their breeding group.

However, during the breeding season, they often responded aggressively and rarely performed helpful or altruistic behaviours.

Interestingly, individuals increased their help towards breeding group members more than they did towards members of neighbouring groups from the same community, despite the fact breeding groups remain unchanged across the year, highlighting the importance of both social connections and environmental harshness for the expression of risky altruistic behaviour.

Our study reveals that the interplay between social structure and environmental conditions drives the seasonal switch between cooperative and competitive behaviour, and supports the hypothesis that multilevel societies can emerge to increase cooperation during harsh environmental conditions.