Social birds live longer
Collaboration in paradise
It may be one of the most beautiful places on earth, but the smell is really, really awful. RUG biologist Martijn Hammers remembers the first time he travelled to Cousin, a small, uninhabited island that is part of the Seychelles archipelago in the Indian Ocean.
The first thing he saw was its white sandy beach with a leafy, verdant jungle in the background. Here and there, a coconut tree towered over the other plants, the remnants of a fifty year old coconut plantation. ‘It looked so peaceful’, says Hammers. ‘Peaceful and empty.’
But just before their boat landed on the beach, he suddenly realised the island was full of birds: white terns, shearwaters, frigate birds, tropic birds, and the Seychelles blue pigeon. He saw them all, and he could smell them as well. 300,000 birds pooing in unison on an island barely a quarter of a square kilometre big.
Martijn Hammers and his colleague ringing a Seychelles warbler / Photo by Charlie Davies
An ideal non-migratory bird
Hammers visits Cousin Island at least once a year. He sleeps in a small, simple hut set up by researchers. He spends his time watching sea turtles on the beach, and listens to the Seychelles warbler, a small, remarkable bird that he has been studying for approximately a decade.
‘It used to be the rarest bird in the world’, he says. ‘The island where they had their habitat had all been turned into coconut plantations so there were only twenty-five birds left.’ The remnants withdrew to the small mangrove forest on Cousin, which was largely untouched because coconut trees couldn’t grow there.
Fortunately, those twenty-five birds managed to restore the warbler population after a conservation agency bought the island to get rid of the plantations in 1968. There are currently 320 warblers on Cousin Island, and a few thousand birds in total.
That’s a good thing according to biologists, who love the little warbler. For one, it’s a non-migratory bird, which means it almost never leaves its own island. ‘We’ve only seen two birds leave their habitats in the past thirty years.’
The fact that the birds don’t move means it’s easy to study a population. ‘With great tits, for example, if you can’t see them, you don’t where they are. Are they dead? Are in the next forest over? But Seychelles warblers just stay in the same place all the time!’ says Hammers.
Even more interesting is the birds’ complex paternal care, which takes place in a strictly defined territory. Hammers made a remarkable discovery while studying this behaviour, and he will be publishing his results in the prestigious journal Nature Communications.
He discovered that older birds that are helped by their children who still live ‘at home’ don’t age as quickly as other birds.
His findings could also provide insight into another matter: why do social animals, including people, live longer than less social species?
Seychelles warbler on Cousin / Photo Charlie Davies
Birds helping their mother
‘Territory is the most important thing for Seychelles warblers’, Hammers explains. ‘If they lose it, they can’t find enough food and they’ll die.’
On an island as small as Cousin, this can present a problem. There’s only room for approximately a hundred separate territories. Even if they wanted to, young birds can’t just fly the nest. They’ll get attacked by angry neighbours whenever they move close to the territorial boundaries.
This means the young often stay with their parents, waiting for neighbouring birds to get weak and die. ‘Any available real estate gets snapped up in just a few days’, says Hammers.
This limited space also means Seychelles warblers don’t lay eggs often. Raising chicks takes time and effort, and that lowers the birds’ life expectancy. ‘Seychelles warblers would rather have one really good chick than several weak ones who’ll never conquer their own territory. They eat a lot and would just keep bothering their parents.’ says Hammers.
Since nature is fickle, Seychelles warblers developed a hierarchy of dominant birds and helper birds. The dominant bird allows younger birds to live in her territory, and they’re even allowed to lay eggs in her nest. In exchange for this, the helper birds, who are almost always female, have to help raise the dominant bird’s chick.
Hammers questioned whether this behaviour influenced how the birds age in any way. He also wondered if ageing had any effect on the birds’ social behaviour.
He’s since been able to answer his questions and it’s a resounding yes.
A young Seychelles warbler being fed by a older bird / Photo by Charlie Davies
Slowing down the ageing
In general, older birds have a smaller chance of survival. But when an animal has help raising its chicks and works less itself, the ageing process slows down. ‘The help not only compensates for the other bird’s ageing, it even slows down severe ageing.’
The birds using this strategy live longer and show less damage to their telomeres (DNA sequences at each end of a chromosome, which protect the genetic information from deteriorating). It’s important to note that the older birds benefit from the helpers; the latter don’t appear to gain anything from the relationship.
So, surely all the older Seychelles warblers use helper birds? Not necessarily says Hammers.
‘Compare it to a village in which there are no vacant homes’, Hammers explains. ‘Once your child is grown up, you’ll probably allow them to stay, but on the condition that they help out around the house. Even then you might not be all that happy about it.
When warblers manage to capture another flock’s territory, they immediately kick out the previous tenants. That means they won’t have anyone to help them, and they have to fend for themselves. ‘It’s a trade-off’, says Hammers. ‘They have to decide which option is most advantageous.’
He is the first to prove that collaboration slows down the ageing process. Now biologists might finally be able to answer a question that has puzzled them for a while.
‘We know that social animals tend to live longer, but we haven’t yet been able to determine the mechanism behind it’, says Hammers. ‘We’ve observed that the older birds tend to be more social in their behaviour. There’s a good chance that social behaviour and the ageing process have a positive influence on each other.’