Live fast, die young: Personality matters when you’re a fairy-wren
Most mornings, I watch a group of superb fairy-wrens flit across my backyard, chasing insects through the grass. The birds are easy to identify from a distance: even when males aren’t wearing their vibrant blue-and-black breeding plumage, there’s no mistaking that upright tail and vocal repertoire.
Fairy-wrens are already a popular bird, but there’s so much more to them than meets the eye. They live in cooperative family groups (in which sons – but not daughters – hang around to help raise their younger siblings), learn vocalisations from their mother while still inside the egg, and boast perhaps the highest rate of adultery in the bird kingdom.
I have a greater interest in the species than most, having dedicated my two-year Masters degree to studying them. Now, new research I helped publish in Frontiers in Ecology and Evolution adds yet another fascinating puzzle-piece to their complex worlds: fairy-wrens also have personalities.
Animals with personality
For decades, biologists tended to assume that all individuals of a species behave in basically the same way. But this view began to shift in the 1970s, when ethologist Felicity Anne Huntingford noted that some stickleback fish were consistently more aggressive than others. Such behavioural differences between individuals are known as temperament or personality.
In recent years, the research field has exploded in all directions: scientists have reported personality differences in a whole range of species, from birds to lizards to aphids. These personality traits are usually consistent over time and genetically heritable, so an aggressive mother will be more likely to have aggressive offspring.
Some standard measures of personality include boldness (an animal’s response to a risky situation), exploration (response to a novel environment), activity (rate of movement) and aggressiveness (response towards same-species threats). These measures are often correlated with each other, so a bold animal is also more likely to be more exploratory, active and aggressive.
Measuring fairy-wren personality
Over several years, we established a wild study population of about 400 superb fairy-wrens at Serendip Sanctuary near Geelong, Victoria. Birds were fitted with plastic coloured leg-bands so we could keep track of who’s who over their lifetime.
To determine personality at a young age, we briefly removed 8-day-old nestlings from their nests, and measured their response to the stress of handling. Nestlings differed, for example, in their amount of movement, their breathing rate, and how much they struggled when held gently on their backs.
We wanted to know whether these differences were still measurable once the nestlings had grown into adults. We quantified adult personality using what’s called a novel environment test, for which we released birds into an experimental room with artificial lighting, concrete floors, and fake wooden “trees”.
Some birds were quick to enter and explore this new environment – we called them fast-explorers. Others, the slow-explorers, were more cautious in the room, moving slowly or even staying huddled in the same corner for the entire length of the test. We tested birds on multiple occasions, months or even years apart, and they tended to behave similarly each time.
Our study showed that differences in fairy-wren personality were consistent over time. In particular, birds that struggled more when handled as nestlings were later more exploratory in the experimental room.
Intriguingly, being more exploratory also affected a bird’s survival chances. Fast-explorers were less likely to be present in the population a year later, mostly likely because they had died. We don’t know why this is the case, but we can speculate that fast-explorers are perhaps less wary of potential threats and more likely to stray into the sights of predators.
But if being more exploratory increases your chances of dying, then why does this variation exist at all? Shouldn’t natural selection quickly weed out all the fast-explorers? This is a question we’re now working on. One possible explanation is that birds with different personalities facilitate each other by performing different social roles (known as social niche specialisation). Fast-explorers may be filling a separate social niche that achieves equal reproductive fitness despite their shorter lifespan.
Another possibility is that different personality types are useful in different contexts. Fast-exploring animals tend to form routines more quickly, and therefore thrive in consistent environments. Slow-exploring animals are better able to respond to changes in their environment, which may be beneficial in a fluctuating habitat. One study showed exactly this, in that selection favoured either fast– or slow-exploring great tits, depending on food availability in a given year.
So now we know that fairy-wrens have personalities, and that certain personality types survive better than others. The next step is to explore other ways in which a bird’s personality can influence its life history. For example, how does it affect mating success? Fairy-wrens are especially fascinating because they are cooperative breeders – so does personality have any bearing on this? Are slow-exploring birds more likely to stay home and assist their parents, while fast-explorers disperse into their own territories?
For my part, I am particularly interested in how personality affects a fairy-wren’s vocalisations. My preliminary findings suggest there is a relationship between a bird’s personality and what its contact calls sound like. Given the importance of vocal communication in songbirds, such variation could have massive survival or reproductive consequences. But that’s a story for another day, and the noisy fairy-wrens in my backyard are calling me back to the window.
[Header image: A male superb fairy-wren. Photo provided by author.]